Opening the shutter on snapshots (dcNYC 22)

Video: https://www.droidcon.com/2022/09/29/opening-the-shutter-on-snapshots/

Jetpack Compose shows the power of a custom compiler plugin. But not all the magic happens during compilation. A lot of Compose features are based on a runtime library that doesn't require any compiler support: the snapshot system. It might seem like magic at first, but it's just built on top of things you might already know: ThreadLocals, linked lists, and, yes, even regular old callbacks. Once you understand how Compose thinks about state, you might find ways to use its tools in your own code – even outside of Compose.

Transcript

1. Zach Klippenstein (he/him)
   
   
   Google
   Opening the shutter on snapshots
   

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2. @zachklipp
   Pre-watch: A Hitchhiker’s Guide to Compose
   
   
   Post-watch: Using Compose Runtime to create a client library
   not this talk
   this talk
   snapshots
   

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3. @zachklipp
   Pre-watch: A Hitchhiker’s Guide to Compose
   
   
   Post-watch: Using Compose Runtime to create a client library
   these slides my twitter
   not this talk
   this talk
   snapshots
   

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4. @zachklipp
   What for?
   
   
   What do they do?
   
   
   How do they work?
   
   
   What can we do with them?
   

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5. @zachklipp
   

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6. @zachklipp
   

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7. @zachklipp
   

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8. Snapshots are Git for your variables
   @zachklipp
   

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9. Composition is the release manager
   @zachklipp
   

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10. @zachklipp
    Git Snapshots
    branches snapshots
    
    
    main branch global snapshot
    
    
    commits state object writes
    
    
    merge apply snapshot
    
    
    resolve conflicts merge records
    
    
    hooks read, write, apply listeners
    

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11. @zachklipp
    ACID
    “In-memory”
    

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12. @zachklipp
    ACID
    
    
    tomicity
    On-disk
    “a”, Red
    “b”, Red “b”, Blue
    “b”, Blue
    

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13. @zachklipp
    A CID
    
    
    onsistency
    On-disk
    “a”
    “b”
    “c”
    “c” “c”
    tomicity
    

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14. @zachklipp
    AC ID
    
    
    solation
    On-disk
    “a”, Red
    “a”, Blue
    “b”, Red “b”, Blue
    onsistency
    urability
    

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15. @zachklipp
    ACI D
    urability
    On-disk
    solation
    

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16. @zachklipp
    ACI D
    urability
    On-disk
    In-memory
    solation
    

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17. @zachklipp
    ACID
    “In-memory”
    tomicity
    onsistency
    solation
    urability
    

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18. @zachklipp
    Database Snapshots
    ACID ACID
    
    
    transactional snapshots
    
    
    commit/rollback apply/dispose
    
    
    views derived state
    

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19. @zachklipp
    How do they work?
    

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20. @zachklipp
    

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21. @zachklipp
    

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22. No magic
    @zachklipp
    

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23. Compiler plugin?
    @zachklipp
    from A Hitchhiker’s Guide to Jetpack Compose
    

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24. No compiler plugin
    @zachklipp
    

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25. @zachklipp
    (but shipped with it)
    Snapshots ≠ Compose
    

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26. @zachklipp
    (but shipped with it)
    Snapshots ≠ Compose
    snapshots
    the rest of Compose
    ThreadLocal integers
    linked lists
    

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27. @zachklipp
    snapshots
    ThreadLocal integers
    linked lists
    

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28. @zachklipp
    Giant state container?
    (i.e. Redux)
    snapA
    “a”
    “foo”
    Range(0, 100)
    snapB
    “a”
    “bar”
    Range(0, 50)
    snapC
    “b”
    “foo”
    Range(25, 75)
    all
    state
    in app
    

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29. @zachklipp
    mutableStateOf
    mutableStateOf
    Range
    “a”
    “b”
    “foo”
    “bar”
    snapA
    snapB
    snapC
    

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30. @zachklipp
    Snapshots State Objects
    mutableStateOf
    mutableStateOf
    Range
    “a”
    “b”
    “foo”
    “bar”
    snapA
    snapB
    snapC
    

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31. @zachklipp
    Snapshots State Objects
    records
    mutableStateOf
    mutableStateOf
    Range
    “a”
    “b”
    “foo”
    “bar”
    snapA
    snapB
    snapC
    

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32. @zachklipp
    reads:
    96
    snapshots records
    128
    167
    181
    64
    75
    111
    209
    52
    53
    87
    many :
    state
    “foo”
    

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33. @zachklipp
    writes: :
    “bar”
    210
    96
    128
    167
    181
    64
    75
    111
    209
    52
    53
    87
    state
    “foo”
    

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34. @zachklipp
    Snapshots concepts State Objects
    low-level API
    
    
    concrete
    
    
    ID
    
    
    previous IDs
    
    
    invalid set
    primary API
    
    
    abstract
    
    
    map of IDs to values written
    by snapshot with ID
    
    
    i.e. “records”
    

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35. @zachklipp
    Snapshots API State Objects
    takeNestedSnapshot
    
    
    takeNestedMutableSnapshot
    
    
    withMutableSnapshot
    
    
    registerApplyObserver
    val state = mutableStateOf(“”)
    

    mutableStateListOf
    
    
    mutableStateMapOf
    
    
    …whatever!
    

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36. @zachklipp
    Snapshots operations State Objects
    Create:
    
    
    choose new ID
    
    
    Enter:
    
    
    set ThreadLocal
    
    
    restore ThreadLocal
    
    
    Apply:
    
    
    commit ID
    
    
    notify listeners
    Read:
    
    
    find record
    
    
    read from record
    
    
    Write:
    
    
    find or copy record
    
    
    write to record
    

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37. @zachklipp
    Selecting record for read
    message
    id value
    “he”
    “hello asdf” tombstoned
    “hello ” written by 72
    “hello world”
    color
    id value
    Red
    Blue written by 80
    snapshot ID invalid set
    

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38. @zachklipp
    Selecting record for read
    message
    id value
    “he”
    “hello asdf” tombstoned
    “hello ” written by 72
    “hello world”
    color
    id value
    Red
    Blue written by 80
    snapshot ID invalid set
    

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39. @zachklipp
    Selecting record for read
    message
    id value
    “he”
    “hello asdf” tombstoned
    “hello ” written by 72
    “hello world”
    color
    id value
    Red
    Blue written by 80
    snapshot ID invalid set
    

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40. @zachklipp
    Selecting record for read
    message
    id value
    “he”
    “hello asdf” tombstoned
    “hello ” written by 72
    “hello world”
    color
    id value
    Red
    Blue written by 80
    snapshot ID invalid set
    

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41. @zachklipp
    Selecting record for read
    message
    id value
    “he”
    “hello asdf” tombstoned
    “hello ” written by 72
    “hello world”
    color
    id value
    Red
    Blue written by 80
    snapshot ID invalid set
    

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42. @zachklipp
    Selecting record for read
    message
    id value
    “he”
    “hello asdf” tombstoned
    “hello ” written by 72
    “hello world”
    color
    id value
    Red
    Blue written by 80
    snapshot ID invalid set
    

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43. @zachklipp
    Selecting record for read
    message
    id value
    “he”
    “hello asdf” tombstoned
    “hello ” written by 72
    “hello world”
    color
    id value
    Red
    Blue written by 80
    snapshot ID invalid set
    

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44. newest valid record
    @zachklipp
    “mutableState.value” get() = findRecord(SnapshotThreadLocal.get())
    highest ID
    not higher than snapshot ID
    
    
    AND
    
    
    not in snapshot invalid set
    
    
    AND
    
    
    not tombstoned
    

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45. @zachklipp
    Git: commit SHAs
    

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46. @zachklipp
    Snapshot IDs
    

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47. @zachklipp
    Walkthrough: create child, apply
    val message = mutableStateOf("")
    
    
    val snapshot = Snapshot.takeMutableSnapshot()
    
    
    snapshot.enter {
    
    
    message.value = "hello"
    
    
    }
    
    
    snapshot.apply()
    message
    

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48. @zachklipp
    Walkthrough: create child, apply
    message
    val message = mutableStateOf("")
    global snapshot
    

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49. @zachklipp
    Walkthrough: create child, apply
    message
    val message = mutableStateOf("")
    global snapshot
    

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50. @zachklipp
    Walkthrough: create child, apply
    message
    val message = mutableStateOf("")
    “”
    global snapshot
    

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51. @zachklipp
    Walkthrough: create child, apply
    message
    val snapshot = Snapshot.takeMutableSnapshot()
    “”
    global snapshot
    snapshot
    

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52. @zachklipp
    Walkthrough: create child, apply
    message
    val snapshot = Snapshot.takeMutableSnapshot()
    “”
    global snapshot
    snapshot
    

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53. @zachklipp
    Walkthrough: create child, apply
    message
    val snapshot = Snapshot.takeMutableSnapshot()
    “”
    global snapshot
    snapshot
    

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54. @zachklipp
    Walkthrough: create child, apply
    message
    val snapshot = Snapshot.takeMutableSnapshot()
    “”
    global snapshot
    snapshot
    

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55. @zachklipp
    Walkthrough: create child, apply
    message
    val snapshot = Snapshot.takeMutableSnapshot()
    “”
    global snapshot
    snapshot
    

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56. @zachklipp
    Walkthrough: create child, apply
    message
    message.value = "hello"
    “”
    global snapshot
    snapshot
    

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57. @zachklipp
    Walkthrough: create child, apply
    message
    message.value = "hello"
    “hello”
    “”
    global snapshot
    snapshot
    

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58. @zachklipp
    Walkthrough: create child, apply
    message
    snapshot.apply()
    “”
    “hello”
    global snapshot
    snapshot
    

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59. @zachklipp
    Walkthrough: create child, apply
    message
    snapshot.apply()
    “”
    “hello”
    global snapshot
    snapshot
    

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60. @zachklipp
    Walkthrough: create child, apply
    message
    snapshot.apply()
    “”
    “hello”
    global snapshot
    

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61. @zachklipp
    Walkthrough: create child, apply
    message
    snapshot.apply()
    “”
    “hello”
    global snapshot
    

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62. @zachklipp
    Walkthrough: create child, apply
    message
    snapshot.apply()
    “”
    “hello”
    global snapshot
    

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63. @zachklipp
    Walkthrough: create child, apply
    message
    snapshot.apply()
    “”
    “hello”
    global snapshot
    

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64. @zachklipp
    Walkthrough: create child, apply
    message
    snapshot.apply()
    “”
    “hello”
    global snapshot
    

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65. @zachklipp
    Walkthrough: create child, apply
    message
    snapshot.apply()
    “hello”
    “”
    global snapshot
    

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66. @zachklipp
    Walkthrough: create child, apply
    message
    “hello”
    “”
    merged!
    global snapshot
    

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67. @zachklipp
    Snapshot rules:
    Create child
    
    
    Apply
    
    
    Advance
    

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68. @zachklipp
    Snapshot rules:
    Create child:
    
    
    1. Create new snapshot
    
    
    2. Copy parent’s invalid list
    
    
    3. Advance parent – child’s ID will be added to invalid set (see Advance rule 3 below)
    
    
    Apply:
    
    
    1. Advance parent
    
    
    2. Remove child ID from invalid set – parent sees child’s writes
    
    
    3. Add child’s current and previous IDs to parent’s list of previous IDs – parent “absorbs” child
    
    
    Advance:
    
    
    1. New ID is next highest ID
    
    
    2. Add old ID to list of previous IDs
    
    
    3. Add IDs between old and new (exclusive) to invalid set
    
    
    This is a superset of all open snapshots
    
    
    i.e. Unrelated snapshots created since old ID should be ignored
    

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69. @zachklipp
    Walkthrough: create 2 children, apply
    val message = mutableStateOf("")
    
    
    val color = mutableStateOf(Red)
    
    
    val snapshotA = Snapshot.takeMutableSnapshot()
    
    
    val snapshotB = Snapshot.takeMutableSnapshot()
    
    
    snapshotA.enter {
    
    
    message.value = "hello"
    
    
    }
    
    
    snapshotB.enter {
    
    
    color.value = Blue
    
    
    }
    
    
    snapshotA.apply()
    
    
    snapshotB.apply()
    

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70. @zachklipp
    Walkthrough: create 2 children, apply
    val message = mutableStateOf("")
    
    
    val color = mutableStateOf(Red)
    message
    color
    global snapshot
    

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71. @zachklipp
    Walkthrough: create 2 children, apply
    val message = mutableStateOf("")
    
    
    val color = mutableStateOf(Red)
    message
    “”
    color
    Red
    global snapshot
    

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72. @zachklipp
    Walkthrough: create 2 children, apply
    val snapshotA = Snapshot.takeMutableSnapshot()
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    

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73. @zachklipp
    Walkthrough: create 2 children, apply
    val snapshotA = Snapshot.takeMutableSnapshot()
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    

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74. @zachklipp
    Walkthrough: create 2 children, apply
    val snapshotA = Snapshot.takeMutableSnapshot()
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    

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75. @zachklipp
    Walkthrough: create 2 children, apply
    val snapshotA = Snapshot.takeMutableSnapshot()
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    

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76. @zachklipp
    Walkthrough: create 2 children, apply
    val snapshotA = Snapshot.takeMutableSnapshot()
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    

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77. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    val snapshotB = Snapshot.takeMutableSnapshot()
    snapshotB
    

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78. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    val snapshotB = Snapshot.takeMutableSnapshot()
    snapshotB
    

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79. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    val snapshotB = Snapshot.takeMutableSnapshot()
    snapshotB
    

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80. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    val snapshotB = Snapshot.takeMutableSnapshot()
    snapshotB
    

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81. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    val snapshotB = Snapshot.takeMutableSnapshot()
    snapshotB
    

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82. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    val snapshotB = Snapshot.takeMutableSnapshot()
    snapshotB
    

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83. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    snapshotB
    snapshotA.enter { message.value = "hello" }
    

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84. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    snapshotB
    snapshotA.enter { message.value = "hello" }
    

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85. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    snapshotB
    “hello”
    snapshotA.enter { message.value = "hello" }
    

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86. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    snapshotB
    “hello”
    snapshotB.enter { color.value = Blue }
    

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87. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    snapshotB
    “hello”
    snapshotB.enter { color.value = Blue }
    

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88. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    snapshotB
    “hello”
    Blue
    snapshotB.enter { color.value = Blue }
    

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89. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    snapshotB
    “hello”
    Blue
    snapshotA.apply()
    

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90. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    snapshotA
    global snapshot
    snapshotB
    “hello”
    Blue
    snapshotA.apply()
    

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91. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    global snapshot
    snapshotB
    “hello”
    Blue
    snapshotA.apply()
    

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92. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    global snapshot
    snapshotB
    “hello”
    Blue
    snapshotA.apply()
    

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93. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    global snapshot
    snapshotB
    “hello”
    Blue
    snapshotA.apply()
    

    View Slide

94. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    global snapshot
    snapshotA.apply()
    snapshotB
    “hello”
    Blue
    

    View Slide

95. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    global snapshot
    snapshotA.apply()
    snapshotB
    “hello”
    Blue
    

    View Slide

96. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    global snapshot
    snapshotB.apply()
    snapshotB
    “hello”
    Blue
    

    View Slide

97. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    global snapshot
    snapshotB.apply()
    snapshotB
    “hello”
    Blue
    

    View Slide

98. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    global snapshot
    snapshotB.apply()
    “hello”
    Blue
    

    View Slide

99. @zachklipp
    Walkthrough: create 2 children, apply
    message
    “”
    color
    Red
    global snapshot
    snapshotB.apply()
    “hello”
    Blue
    

    View Slide

100. @zachklipp
     Walkthrough: create 2 children, apply
     message
     “”
     color
     Red
     global snapshot
     snapshotB.apply()
     “hello”
     Blue
     

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101. @zachklipp
     Walkthrough: create 2 children, apply
     message
     “”
     color
     Red
     global snapshot
     snapshotB.apply()
     “hello”
     Blue
     

     View Slide

102. @zachklipp
     Walkthrough: create 2 children, apply
     message
     “”
     color
     Red
     global snapshot
     snapshotB.apply()
     “hello”
     Blue
     

     View Slide

103. @zachklipp
     Walkthrough: create 2 children, apply
     message
     “”
     color
     Red
     global snapshot
     “hello”
     Blue
     snapshotB.apply()
     

     View Slide

104. @zachklipp
     Walkthrough: create 2 children, apply
     message
     “”
     color
     Red
     global snapshot
     “hello”
     Blue
     merged!
     

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105. @zachklipp
     Walkthrough: created nested child, apply
     val message = mutableStateOf("")
     
     
     val color = mutableStateOf(Red)
     
     
     val snapshotA = Snapshot.takeMutableSnapshot()
     
     
     snapshotA.enter {
     
     
     message.value = "hello"
     
     
     val snapshotB = Snapshot.takeMutableSnapshot()
     
     
     snapshotB.enter {
     
     
     color.value = Blue
     
     
     }
     
     
     snapshotB.apply()
     
     
     }
     
     
     snapshotA.apply()
     

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106. @zachklipp
     val message = mutableStateOf("")
     
     
     val color = mutableStateOf(Red)
     message
     color
     global snapshot
     Walkthrough: created nested child, apply
     

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107. @zachklipp
     val message = mutableStateOf("")
     
     
     val color = mutableStateOf(Red)
     message
     “”
     color
     Red
     global snapshot
     Walkthrough: created nested child, apply
     

     View Slide

108. @zachklipp
     val snapshotA = Snapshot.takeMutableSnapshot()
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     Walkthrough: created nested child, apply
     

     View Slide

109. @zachklipp
     val snapshotA = Snapshot.takeMutableSnapshot()
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     Walkthrough: created nested child, apply
     

     View Slide

110. @zachklipp
     val snapshotA = Snapshot.takeMutableSnapshot()
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     Walkthrough: created nested child, apply
     

     View Slide

111. @zachklipp
     val snapshotA = Snapshot.takeMutableSnapshot()
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     Walkthrough: created nested child, apply
     

     View Slide

112. @zachklipp
     val snapshotA = Snapshot.takeMutableSnapshot()
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     Walkthrough: created nested child, apply
     

     View Slide

113. @zachklipp
     snapshotA.enter { message.value = "hello"
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     Walkthrough: created nested child, apply
     

     View Slide

114. @zachklipp
     snapshotA.enter { message.value = "hello"
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     Walkthrough: created nested child, apply
     

     View Slide

115. @zachklipp
     snapshotA.enter { message.value = "hello"
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     “hello”
     Walkthrough: created nested child, apply
     

     View Slide

116. @zachklipp
     snapshotA.enter { val snapshotB = Snapshot.takeMutableSnapshot()
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     snapshotB
     “hello”
     Walkthrough: created nested child, apply
     

     View Slide

117. @zachklipp
     snapshotA.enter { val snapshotB = Snapshot.takeMutableSnapshot()
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     snapshotB
     “hello”
     Walkthrough: created nested child, apply
     

     View Slide

118. @zachklipp
     snapshotA.enter { val snapshotB = Snapshot.takeMutableSnapshot()
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     snapshotB
     “hello”
     Walkthrough: created nested child, apply
     

     View Slide

119. @zachklipp
     snapshotA.enter { val snapshotB = Snapshot.takeMutableSnapshot()
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     snapshotB
     “hello”
     Walkthrough: created nested child, apply
     

     View Slide

120. @zachklipp
     snapshotA.enter { snapshotB.enter { color.value = Blue }
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     snapshotB
     “hello”
     Walkthrough: created nested child, apply
     

     View Slide

121. @zachklipp
     snapshotA.enter { snapshotB.enter { color.value = Blue }
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     snapshotB
     “hello”
     Walkthrough: created nested child, apply
     

     View Slide

122. @zachklipp
     snapshotA.enter { snapshotB.enter { color.value = Blue }
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     snapshotB
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

123. @zachklipp
     snapshotA.enter { snapshotB.apply() }
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     snapshotB
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

124. @zachklipp
     snapshotA.enter { snapshotB.apply() }
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     snapshotB
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

125. @zachklipp
     snapshotA.enter { snapshotB.apply() }
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

126. @zachklipp
     snapshotA.enter { snapshotB.apply() }
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

127. @zachklipp
     snapshotA.enter { snapshotB.apply() }
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

128. @zachklipp
     snapshotA.enter { snapshotB.apply() }
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

129. @zachklipp
     snapshotA.enter { snapshotB.apply() }
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

130. @zachklipp
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     “hello”
     Blue
     snapshotA.apply()
     Walkthrough: created nested child, apply
     

     View Slide

131. @zachklipp
     snapshotA.apply()
     message
     “”
     color
     Red
     snapshotA
     global snapshot
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

132. @zachklipp
     snapshotA.apply()
     message
     “”
     color
     Red
     global snapshot
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

133. @zachklipp
     snapshotA.apply()
     message
     “”
     color
     Red
     global snapshot
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

134. @zachklipp
     snapshotA.apply()
     message
     “”
     color
     Red
     global snapshot
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

135. @zachklipp
     snapshotA.apply()
     message
     “”
     color
     Red
     global snapshot
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

136. @zachklipp
     snapshotA.apply()
     message
     “”
     color
     Red
     global snapshot
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

137. @zachklipp
     snapshotA.apply()
     message
     “”
     color
     Red
     global snapshot
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

138. @zachklipp
     snapshotA.apply()
     message
     “”
     color
     Red
     global snapshot
     “hello”
     Blue
     Walkthrough: created nested child, apply
     

     View Slide

139. @zachklipp
     message
     “”
     color
     Red
     global snapshot
     “hello”
     Blue
     merged!
     Walkthrough: created nested child, apply
     

     View Slide

140. @zachklipp
     MutableState
     message
     “”
     “hello”
     interface MutableState : State {
     
     
     

     View Slide

141. @zachklipp
     MutableState
     message
     “”
     “hello”
     interface MutableState : State {
     
     
     override var value: T
     
     
     operator fun component1(): T
     
     
     operator fun component2(): (T) -> Unit
     
     
     }
     

     View Slide

142. @zachklipp
     SnapshotMutableState MutableState
     message
     “”
     “hello”
     interface SnapshotMutableState : MutableState {
     
     
     val policy: SnapshotMutationPolicy
     
     
     }
     interface MutableState : State {
     
     
     

     View Slide

143. @zachklipp
     SnapshotMutableStateImpl SnapshotMutableState
     message
     “”
     “hello”
     internal class SnapshotMutableStateImpl(
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     override var value: T
     
     
     get() = TODO("???")
     
     
     set(value) { TODO("???") }
     
     
     }
     interface SnapshotMutableState : MutableState {
     
     
     

     View Slide

144. @zachklipp
     SnapshotMutableStateImpl StateObject
     message
     “”
     “hello”
     internal class SnapshotMutableStateImpl(
     
     
     val firstStateRecord: StateRecord
     
     
     fun prependStateRecord(value: StateRecord)
     
     
     fun mergeRecords(
     
     
     previous: StateRecord,
     
     
     current: StateRecord,
     
     
     applied: StateRecord
     
     
     ): StateRecord? = null
     
     
     }
     interface SnapshotMutableState : MutableState {
     
     
     

     View Slide

145. @zachklipp
     message
     “”
     “hello”
     interface StateObject {
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     override var value: T
     
     
     get() = TODO("???")
     
     
     set(value) { TODO("???") }
     
     
     }
     SnapshotMutableStateImpl StateObject
     :
     

     View Slide

146. @zachklipp
     message
     “”
     “hello”
     interface StateObject {
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     override var value: T
     
     
     get() = TODO("???")
     
     
     set(value) { TODO("???") }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = TODO("???")
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     TODO("???")
     
     
     }
     
     
     }
     SnapshotMutableStateImpl StateObject
     :
     

     View Slide

147. @zachklipp
     internal class SnapshotMutableStateImpl(
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     override var value: T
     
     
     get() = TODO("???")
     
     
     set(value) { TODO("???") }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = TODO("???")
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     TODO("???")
     
     
     }
     
     
     }
     

     View Slide

148. @zachklipp
     internal class SnapshotMutableStateImpl(
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     override var value: T
     
     
     get() = TODO("???")
     
     
     set(value) { TODO("???") }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = TODO("???")
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     TODO("???")
     
     
     }
     
     
     private class StateStateRecord(myValue: T) : StateRecord() {
     
     
     var value: T = myValue
     
     
     override fun create(): StateRecord = TODO("???")
     
     
     override fun assign(value: StateRecord) {
     
     
     TODO("???")
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

149. @zachklipp
     internal class SnapshotMutableStateImpl(
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     override var value: T
     
     
     get() = TODO("???")
     
     
     set(value) { TODO("???") }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = TODO("???")
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     TODO("???")
     
     
     }
     
     
     private class StateStateRecord(myValue: T) : StateRecord() {
     
     
     var value: T = myValue
     
     
     override fun create(): StateRecord = StateStateRecord(value)
     
     
     override fun assign(value: StateRecord) {
     
     
     TODO("???")
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

150. @zachklipp
     internal class SnapshotMutableStateImpl(
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     override var value: T
     
     
     get() = TODO("???")
     
     
     set(value) { TODO("???") }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = TODO("???")
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     TODO("???")
     
     
     }
     
     
     private class StateStateRecord(myValue: T) : StateRecord() {
     
     
     var value: T = myValue
     
     
     override fun create(): StateRecord = StateStateRecord(value)
     
     
     override fun assign(value: StateRecord) {
     
     
     this.value = (value as StateStateRecord).value
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

151. @zachklipp
     internal class SnapshotMutableStateImpl(
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     private var next: StateStateRecord = StateStateRecord(value)
     
     
     override var value: T
     
     
     get() = TODO("???")
     
     
     set(value) { TODO("???") }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = TODO(“???")
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     TODO("???")
     
     
     }
     
     
     private class StateStateRecord(myValue: T) : StateRecord() {
     
     
     var value: T = myValue
     
     
     override fun create(): StateRecord = StateStateRecord(value)
     
     
     override fun assign(value: StateRecord) {
     
     
     this.value = (value as StateStateRecord).value
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

152. @zachklipp
     internal class SnapshotMutableStateImpl(
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     private var next: StateStateRecord = StateStateRecord(value)
     
     
     override var value: T
     
     
     get() = TODO("???")
     
     
     set(value) { TODO("???") }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = next
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     TODO("???")
     
     
     }
     
     
     private class StateStateRecord(myValue: T) : StateRecord() {
     
     
     var value: T = myValue
     
     
     override fun create(): StateRecord = StateStateRecord(value)
     
     
     override fun assign(value: StateRecord) {
     
     
     this.value = (value as StateStateRecord).value
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

153. @zachklipp
     internal class SnapshotMutableStateImpl(
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     private var next: StateStateRecord = StateStateRecord(value)
     
     
     override var value: T
     
     
     get() = TODO("???")
     
     
     set(value) { TODO("???") }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = next
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     next = value as StateStateRecord
     
     
     }
     
     
     private class StateStateRecord(myValue: T) : StateRecord() {
     
     
     var value: T = myValue
     
     
     override fun create(): StateRecord = StateStateRecord(value)
     
     
     override fun assign(value: StateRecord) {
     
     
     this.value = (value as StateStateRecord).value
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

154. @zachklipp
     internal class SnapshotMutableStateImpl(
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     private var next: StateStateRecord = StateStateRecord(value)
     
     
     override var value: T
     
     
     get() = next.value
     
     
     set(value) { TODO("???") }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = next
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     next = value as StateStateRecord
     
     
     }
     
     
     private class StateStateRecord(myValue: T) : StateRecord() {
     
     
     var value: T = myValue
     
     
     override fun create(): StateRecord = StateStateRecord(value)
     
     
     override fun assign(value: StateRecord) {
     
     
     this.value = (value as StateStateRecord).value
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

155. @zachklipp
     internal class SnapshotMutableStateImpl(
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     private var next: StateStateRecord = StateStateRecord(value)
     
     
     override var value: T
     
     
     get() = next.readable(this).value
     
     
     set(value) { TODO("???") }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = next
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     next = value as StateStateRecord
     
     
     }
     
     
     private class StateStateRecord(myValue: T) : StateRecord() {
     
     
     var value: T = myValue
     
     
     override fun create(): StateRecord = StateStateRecord(value)
     
     
     override fun assign(value: StateRecord) {
     
     
     this.value = (value as StateStateRecord).value
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

156. @zachklipp
     internal class SnapshotMutableStateImpl(
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     private var next: StateStateRecord = StateStateRecord(value)
     
     
     override var value: T
     
     
     get() = next.readable(this).value
     
     
     set(value) {
     
     
     next.writable(this) {
     
     
     this.value = value
     
     
     }
     
     
     }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = next
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     next = value as StateStateRecord
     
     
     }
     
     
     private class StateStateRecord(myValue: T) : StateRecord() {
     
     
     var value: T = myValue
     
     
     override fun create(): StateRecord = StateStateRecord(value)
     
     
     override fun assign(value: StateRecord) {
     
     
     this.value = (value as StateStateRecord).value
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

157. @zachklipp
     internal class SnapshotMutableStateImpl(
     
     
     val firstStateRecord: StateRecord
     
     
     fun prependStateRecord(value: StateRecord)
     
     
     fun mergeRecords(
     
     
     previous: StateRecord,
     
     
     current: StateRecord,
     
     
     applied: StateRecord
     
     
     ): StateRecord? = null
     
     
     }
     

     View Slide

158. @zachklipp
     internal class SnapshotMutableStateImpl(
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     override fun mergeRecords(
     
     
     previous: StateRecord,
     
     
     current: StateRecord,
     
     
     applied: StateRecord
     
     
     ): StateRecord? {
     
     
     }
     
     
     }
     

     View Slide

159. @zachklipp
     internal class SnapshotMutableStateImpl(
     
     
     value: T,
     
     
     override val policy: SnapshotMutationPolicy
     
     
     ) : StateObject, SnapshotMutableState {
     
     
     override fun mergeRecords(
     
     
     previous: StateRecord,
     
     
     current: StateRecord,
     
     
     applied: StateRecord
     
     
     ): StateRecord? {
     
     
     val previousRecord = previous as StateStateRecord
     
     
     val currentRecord = current as StateStateRecord
     
     
     val appliedRecord = applied as StateStateRecord
     
     
     return if (policy.equivalent(currentRecord.value, appliedRecord.value))
     
     
     current
     
     
     else {
     
     
     val merged = policy.merge(
     
     
     previousRecord.value,
     
     
     currentRecord.value,
     
     
     appliedRecord.value
     
     
     )
     
     
     if (merged != null) {
     
     
     appliedRecord.create().also {
     
     
     (it as StateStateRecord).value = merged
     
     
     }
     
     
     } else {
     
     
     null
     
     
     }
     
     
     }
     
     
     }
     
     
     }
     

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160. @zachklipp
     What can we do with them?
     

     View Slide

161. @zachklipp
     Custom state objects
     class Range {
     
     
     var start: Int = 0
     
     
     var end: Int = 0
     
     
     }
     

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162. @zachklipp
     Custom state objects
     class Range {
     
     
     private var next = Record()
     
     
     var start: Int = 0
     
     
     var end: Int = 0
     
     
     private class Record : StateRecord() {
     
     
     var start: Int = 0
     
     
     var end: Int = 0
     
     
     override fun create(): StateRecord = Record()
     
     
     override fun assign(value: StateRecord) {
     
     
     value as Record
     
     
     start = value.start
     
     
     end = value.end
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

163. @zachklipp
     Custom state objects
     class Range {
     
     
     private var next = Record()
     
     
     var start: Int
     
     
     get() = next.readable(this).start
     
     
     set(value) {
     
     
     next.writable(this) { start = value }
     
     
     }
     
     
     var end: Int
     
     
     get() = next.readable(this).end
     
     
     set(value) {
     
     
     next.writable(this) { end = value }
     
     
     }
     
     
     private class Record : StateRecord() {
     
     
     var start: Int = 0
     
     
     var end: Int = 0
     
     
     override fun create(): StateRecord = Record()
     
     
     override fun assign(value: StateRecord) {
     
     
     value as Record
     
     
     start = value.start
     
     
     end = value.end
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

164. @zachklipp
     Custom state objects
     class Range : StateObject {
     
     
     private var next = Record()
     
     
     var start: Int
     
     
     get() = next.readable(this).start
     
     
     set(value) {
     
     
     next.writable(this) { start = value }
     
     
     }
     
     
     var end: Int
     
     
     get() = next.readable(this).end
     
     
     set(value) {
     
     
     next.writable(this) { end = value }
     
     
     }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = next
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     next = value as Record
     
     
     }
     
     
     private class Record : StateRecord() {
     
     
     var start: Int = 0
     
     
     var end: Int = 0
     
     
     override fun create(): StateRecord = Record()
     
     
     override fun assign(value: StateRecord) {
     
     
     value as Record
     
     
     start = value.start
     
     
     end = value.end
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

165. @zachklipp
     Custom state objects
     class Range : StateObject {
     
     
     private var next = Record()
     
     
     var start: Int
     
     
     get() = next.readable(this).start
     
     
     set(value) {
     
     
     next.writable(this) { start = value }
     
     
     }
     
     
     var end: Int
     
     
     get() = next.readable(this).end
     
     
     set(value) {
     
     
     next.writable(this) { end = value }
     
     
     }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = next
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     next = value as Record
     
     
     }
     
     
     private class Record : StateRecord() {
     
     
     var start: Int = 0
     
     
     var end: Int = 0
     
     
     override fun create(): StateRecord = Record()
     
     
     override fun assign(value: StateRecord) {
     
     
     value as Record
     
     
     start = value.start
     
     
     end = value.end
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

166. @zachklipp
     Custom state objects
     class Range : StateObject {
     
     
     private var next = Record()
     
     
     var start: Int
     
     
     get() = next.readable(this).start
     
     
     set(value) {
     
     
     require(value <= next.readable(this).end)
     
     
     next.writable(this) { start = value }
     
     
     }
     
     
     var end: Int
     
     
     get() = next.readable(this).end
     
     
     set(value) {
     
     
     require(value >= next.readable(this).start)
     
     
     next.writable(this) { end = value }
     
     
     }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = next
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     next = value as Record
     
     
     }
     
     
     private class Record : StateRecord() {
     
     
     var start: Int = 0
     
     
     var end: Int = 0
     
     
     override fun create(): StateRecord = Record()
     
     
     override fun assign(value: StateRecord) {
     
     
     value as Record
     
     
     start = value.start
     
     
     end = value.end
     
     
     }
     
     
     }
     
     
     }
     

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167. @zachklipp
     Custom state objects
     class Range : StateObject {
     
     
     private var next = Record()
     
     
     var start: Int
     
     
     get() = next.readable(this).start
     
     
     set(value) {
     
     
     next.withCurrent { require(value <= it.end) }
     
     
     next.writable(this) { start = value }
     
     
     }
     
     
     var end: Int
     
     
     get() = next.readable(this).end
     
     
     set(value) {
     
     
     next.withCurrent { require(value >= it.start) }
     
     
     next.writable(this) { end = value }
     
     
     }
     
     
     override val firstStateRecord: StateRecord
     
     
     get() = next
     
     
     override fun prependStateRecord(value: StateRecord) {
     
     
     next = value as Record
     
     
     }
     
     
     private class Record : StateRecord() {
     
     
     var start: Int = 0
     
     
     var end: Int = 0
     
     
     override fun create(): StateRecord = Record()
     
     
     override fun assign(value: StateRecord) {
     
     
     value as Record
     
     
     start = value.start
     
     
     end = value.end
     
     
     }
     
     
     }
     
     
     }
     

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168. @zachklipp
     Custom state objects
     class Range : StateObject {
     
     
     }
     

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169. @zachklipp
     Custom state objects
     class Range : StateObject {
     
     
     override fun mergeRecords(
     
     
     previous: StateRecord,
     
     
     current: StateRecord,
     
     
     applied: StateRecord
     
     
     ): StateRecord? {
     
     
     }
     
     
     }
     

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170. @zachklipp
     Custom state objects
     class Range : StateObject {
     
     
     override fun mergeRecords(
     
     
     previous: StateRecord,
     
     
     current: StateRecord,
     
     
     applied: StateRecord
     
     
     ): StateRecord? {
     
     
     val previousRecord = previous as Record
     
     
     val currentRecord = current as Record
     
     
     val appliedRecord = applied as Record
     
     
     if (currentRecord.start == appliedRecord.start &&
     
     
     currentRecord.end == appliedRecord.end
     
     
     ) {
     
     
     return current
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

171. @zachklipp
     Custom state objects
     class Range : StateObject {
     
     
     override fun mergeRecords(
     
     
     previous: StateRecord,
     
     
     current: StateRecord,
     
     
     applied: StateRecord
     
     
     ): StateRecord? {
     
     
     val previousRecord = previous as Record
     
     
     val currentRecord = current as Record
     
     
     val appliedRecord = applied as Record
     
     
     if (currentRecord.start == appliedRecord.start &&
     
     
     currentRecord.end == appliedRecord.end
     
     
     ) {
     
     
     return current
     
     
     }
     
     
     val currentChangedStart = previousRecord.start != currentRecord.start
     
     
     val currentChangedEnd = previousRecord.end != currentRecord.end
     
     
     val appliedChangedStart = previousRecord.start != appliedRecord.start
     
     
     val appliedChangedEnd = previousRecord.end != appliedRecord.end
     
     
     if ((currentChangedStart && appliedChangedStart) ||
     
     
     (currentChangedEnd && appliedChangedEnd)
     
     
     ) {
     
     
     return null
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

172. @zachklipp
     Custom state objects
     class Range : StateObject {
     
     
     override fun mergeRecords(
     
     
     previous: StateRecord,
     
     
     current: StateRecord,
     
     
     applied: StateRecord
     
     
     ): StateRecord? {
     
     
     val previousRecord = previous as Record
     
     
     val currentRecord = current as Record
     
     
     val appliedRecord = applied as Record
     
     
     if (currentRecord.start == appliedRecord.start &&
     
     
     currentRecord.end == appliedRecord.end
     
     
     ) {
     
     
     return current
     
     
     }
     
     
     val currentChangedStart = previousRecord.start != currentRecord.start
     
     
     val currentChangedEnd = previousRecord.end != currentRecord.end
     
     
     val appliedChangedStart = previousRecord.start != appliedRecord.start
     
     
     val appliedChangedEnd = previousRecord.end != appliedRecord.end
     
     
     if ((currentChangedStart && appliedChangedStart) ||
     
     
     (currentChangedEnd && appliedChangedEnd)
     
     
     ) {
     
     
     return null
     
     
     }
     
     
     return Record().apply {
     
     
     start = if (currentChangedStart) currentRecord.start else appliedRecord.start
     
     
     end = if (currentChangedEnd) currentRecord.end else appliedRecord.end
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

173. @zachklipp
     State diffing
     github.com/zach-klippenstein/compose-autotransition
     

     View Slide

174. @zachklipp
     State diffing
     github.com/zach-klippenstein/compose-autotransition
     

     View Slide

175. @zachklipp
     State diffing
     suspend fun withAnimation(changes: () -> Unit) {
     
     
     val statesToAnimate = mutableSetOf()
     
     
     val targetValues = mutableMapOf()
     
     
     val snapshot = Snapshot.takeMutableSnapshot(
     
     
     writeObserver = { stateObject -> statesToAnimate += stateObject }
     
     
     )
     
     
     snapshot.enter {
     
     
     changes()
     
     
     statesToAnimate.forEach { stateObject -> targetValues[stateObject] = stateObject.value }
     
     
     }
     
     
     snapshot.dispose()
     
     
     coroutineScope {
     
     
     statesToAnimate.forEach { stateObject ->
     
     
     launch {
     
     
     animate(stateObject.value, targetValues[stateObject])
     
     
     }
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

176. @zachklipp
     State diffing
     suspend fun withAnimation(changes: () -> Unit) {
     
     
     val statesToAnimate = mutableSetOf()
     
     
     val targetValues = mutableMapOf()
     
     
     val snapshot = Snapshot.takeMutableSnapshot(
     
     
     writeObserver = { stateObject -> statesToAnimate += stateObject }
     
     
     )
     
     
     snapshot.enter {
     
     
     changes()
     
     
     statesToAnimate.forEach { stateObject -> targetValues[stateObject] = stateObject.value }
     
     
     }
     
     
     snapshot.dispose()
     
     
     coroutineScope {
     
     
     statesToAnimate.forEach { stateObject ->
     
     
     launch {
     
     
     animate(stateObject.value, targetValues[stateObject])
     
     
     }
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

177. @zachklipp
     State diffing
     suspend fun withAnimation(changes: () -> Unit) {
     
     
     val statesToAnimate = mutableSetOf()
     
     
     val targetValues = mutableMapOf()
     
     
     val snapshot = Snapshot.takeMutableSnapshot(
     
     
     writeObserver = { stateObject -> statesToAnimate += stateObject }
     
     
     )
     
     
     snapshot.enter {
     
     
     changes()
     
     
     statesToAnimate.forEach { stateObject -> targetValues[stateObject] = stateObject.value }
     
     
     }
     
     
     snapshot.dispose()
     
     
     coroutineScope {
     
     
     statesToAnimate.forEach { stateObject ->
     
     
     launch {
     
     
     animate(stateObject.value, targetValues[stateObject])
     
     
     }
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

178. @zachklipp
     State diffing
     suspend fun withAnimation(changes: () -> Unit) {
     
     
     val statesToAnimate = mutableSetOf()
     
     
     val targetValues = mutableMapOf()
     
     
     val snapshot = Snapshot.takeMutableSnapshot(
     
     
     writeObserver = { stateObject -> statesToAnimate += stateObject }
     
     
     )
     
     
     snapshot.enter {
     
     
     changes()
     
     
     statesToAnimate.forEach { stateObject -> targetValues[stateObject] = stateObject.value }
     
     
     }
     
     
     snapshot.dispose()
     
     
     coroutineScope {
     
     
     statesToAnimate.forEach { stateObject ->
     
     
     launch {
     
     
     animate(stateObject.value, targetValues[stateObject])
     
     
     }
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

179. @zachklipp
     State diffing
     suspend fun withAnimation(changes: () -> Unit) {
     
     
     val statesToAnimate = mutableSetOf()
     
     
     val targetValues = mutableMapOf()
     
     
     val snapshot = Snapshot.takeMutableSnapshot(
     
     
     writeObserver = { stateObject -> statesToAnimate += stateObject }
     
     
     )
     
     
     snapshot.enter {
     
     
     changes()
     
     
     statesToAnimate.forEach { stateObject -> targetValues[stateObject] = stateObject.value }
     
     
     }
     
     
     snapshot.dispose()
     
     
     coroutineScope {
     
     
     statesToAnimate.forEach { stateObject ->
     
     
     launch {
     
     
     animate(stateObject.value, targetValues[stateObject])
     
     
     }
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

180. @zachklipp
     State diffing
     github.com/zach-klippenstein/compose-autotransition
     

     View Slide

181. @zachklipp
     State diffing
     github.com/zach-klippenstein/compose-autotransition
     

     View Slide

182. @zachklipp
     Persistence
     “In-memory”
     tomicity
     onsistency
     solation
     urability
     ACID
     

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183. @zachklipp
     Persistence
     “In-memory”
     tomicity
     onsistency
     solation
     urability
     ACID
     

     View Slide

184. @zachklipp
     Persistence
     fun observeUserAsState(name: String): State {
     
     
     val cached = cachedStates.getOrPut(name) {
     
     
     val flow = users.findByName(name)
     
     
     val state = mutableStateOf(null)
     
     
     val job = scope.launch {
     
     
     flow.collect {
     
     
     state.value = it
     
     
     }
     
     
     }
     
     
     return@getOrPut UserState(state, job)
     
     
     }
     
     
     return cached.state
     
     
     }
     Read
     

     View Slide

185. @zachklipp
     Persistence
     Read
     fun observeUserAsState(name: String): State {
     
     
     val cached = cachedStates.getOrPut(name) {
     
     
     val flow = users.findByName(name)
     
     
     val state = mutableStateOf(null)
     
     
     val job = scope.launch {
     
     
     flow.collect {
     
     
     state.value = it
     
     
     }
     
     
     }
     
     
     return@getOrPut UserState(state, job)
     
     
     }
     
     
     return cached.state
     
     
     }
     

     View Slide

186. @zachklipp
     Persistence
     Read
     fun observeUserAsState(name: String): State {
     
     
     val cached = cachedStates.getOrPut(name) {
     
     
     val flow = users.findByName(name)
     
     
     val state = mutableStateOf(null)
     
     
     val job = scope.launch {
     
     
     flow.collect {
     
     
     state.value = it
     
     
     }
     
     
     }
     
     
     return@getOrPut UserState(state, job)
     
     
     }
     
     
     return cached.state
     
     
     }
     

     View Slide

187. @zachklipp
     Persistence
     Read
     fun observeUserAsState(name: String): State {
     
     
     val cached = cachedStates.getOrPut(name) {
     
     
     val flow = users.findByName(name)
     
     
     val state = mutableStateOf(null)
     
     
     val job = scope.launch {
     
     
     flow.collect {
     
     
     state.value = it
     
     
     }
     
     
     }
     
     
     return@getOrPut UserState(state, job)
     
     
     }
     
     
     return cached.state
     
     
     }
     

     View Slide

188. @zachklipp
     Persistence
     Read
     fun observeUserAsState(name: String): State {
     
     
     val cached = cachedStates.getOrPut(name) {
     
     
     val flow = users.findByName(name)
     
     
     val state = mutableStateOf(null)
     
     
     val job = scope.launch {
     
     
     flow.collect {
     
     
     state.value = it
     
     
     }
     
     
     }
     
     
     return@getOrPut UserState(state, job)
     
     
     }
     
     
     return cached.state
     
     
     }
     

     View Slide

189. @zachklipp
     Persistence
     Update
     Snapshot.registerApplyObserver { changedStateObjects, snapshot ->
     
     
     val changedDbStates = changedStateObjects.mapNotNull {
     
     
     if (it in cachedStates) it as MutableState else null
     
     
     }
     
     
     
     
     if (changedDbStates.isNotEmpty()) {
     
     
     scope.launch(Dispatchers.IO) {
     
     
     val readSnapshot = snapshot.takeNestedSnapshot()
     
     
     try {
     
     
     readSnapshot.enter {
     
     
     changedDbStates.forEach { (user, _) ->
     
     
     user?.let { users.updateUser(user) }
     
     
     }
     
     
     }
     
     
     } finally {
     
     
     readSnapshot.dispose()
     
     
     }
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

190. @zachklipp
     Persistence
     Update
     Snapshot.registerApplyObserver { changedStateObjects, snapshot ->
     
     
     val changedDbStates = changedStateObjects.mapNotNull {
     
     
     if (it in cachedStates) it as MutableState else null
     
     
     }
     
     
     
     
     if (changedDbStates.isNotEmpty()) {
     
     
     scope.launch(Dispatchers.IO) {
     
     
     val readSnapshot = snapshot.takeNestedSnapshot()
     
     
     try {
     
     
     readSnapshot.enter {
     
     
     changedDbStates.forEach { (user, _) ->
     
     
     user?.let { users.updateUser(user) }
     
     
     }
     
     
     }
     
     
     } finally {
     
     
     readSnapshot.dispose()
     
     
     }
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

191. @zachklipp
     Persistence
     Update
     Snapshot.registerApplyObserver { changedStateObjects, snapshot ->
     
     
     val changedDbStates = changedStateObjects.mapNotNull {
     
     
     if (it in cachedStates) it as MutableState else null
     
     
     }
     
     
     
     
     if (changedDbStates.isNotEmpty()) {
     
     
     scope.launch(Dispatchers.IO) {
     
     
     val readSnapshot = snapshot.takeNestedSnapshot()
     
     
     try {
     
     
     readSnapshot.enter {
     
     
     changedDbStates.forEach { (user, _) ->
     
     
     user?.let { users.updateUser(user) }
     
     
     }
     
     
     }
     
     
     } finally {
     
     
     readSnapshot.dispose()
     
     
     }
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

192. @zachklipp
     Persistence
     Update
     Snapshot.registerApplyObserver { changedStateObjects, snapshot ->
     
     
     val changedDbStates = changedStateObjects.mapNotNull {
     
     
     if (it in cachedStates) it as MutableState else null
     
     
     }
     
     
     
     
     if (changedDbStates.isNotEmpty()) {
     
     
     scope.launch(Dispatchers.IO) {
     
     
     val readSnapshot = snapshot.takeNestedSnapshot()
     
     
     try {
     
     
     readSnapshot.enter {
     
     
     changedDbStates.forEach { (user, _) ->
     
     
     user?.let { users.updateUser(user) }
     
     
     }
     
     
     }
     
     
     } finally {
     
     
     readSnapshot.dispose()
     
     
     }
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

193. @zachklipp
     Persistence
     Update
     Snapshot.registerApplyObserver { changedStateObjects, snapshot ->
     
     
     val changedDbStates = changedStateObjects.mapNotNull {
     
     
     if (it in cachedStates) it as MutableState else null
     
     
     }
     
     
     
     
     if (changedDbStates.isNotEmpty()) {
     
     
     scope.launch(Dispatchers.IO) {
     
     
     val readSnapshot = snapshot.takeNestedSnapshot()
     
     
     try {
     
     
     readSnapshot.enter {
     
     
     changedDbStates.forEach { (user, _) ->
     
     
     user?.let { users.updateUser(user) }
     
     
     }
     
     
     }
     
     
     } finally {
     
     
     readSnapshot.dispose()
     
     
     }
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

194. @zachklipp
     Persistence
     Update
     Snapshot.registerApplyObserver { changedStateObjects, snapshot ->
     
     
     val changedDbStates = changedStateObjects.mapNotNull {
     
     
     if (it in cachedStates) it as MutableState else null
     
     
     }
     
     
     
     
     if (changedDbStates.isNotEmpty()) {
     
     
     scope.launch(Dispatchers.IO) {
     
     
     val readSnapshot = snapshot.takeNestedSnapshot()
     
     
     try {
     
     
     readSnapshot.enter {
     
     
     changedDbStates.forEach { (user, _) ->
     
     
     user?.let { users.updateUser(user) }
     
     
     }
     
     
     }
     
     
     } finally {
     
     
     readSnapshot.dispose()
     
     
     }
     
     
     }
     
     
     }
     
     
     }
     

     View Slide

195. @zachklipp
     Persistence
     Usage
     @Composable
     
     
     fun UserNameEditor(database: SnapshotDatabase) {
     
     
     val userState = remember(database) {
     
     
     database.observeUserAsState("Rhaenyra")
     
     
     }
     
     
     userState.value?.let { user ->
     
     
     BasicTextField(
     
     
     value = user.name,
     
     
     onValueChange = {
     
     
     userState.value = user.copy(name = it)
     
     
     }
     
     
     )
     
     
     }
     
     
     }
     

     View Slide

196. @zachklipp
     Persistence
     Usage
     @Composable
     
     
     fun UserNameEditor(database: SnapshotDatabase) {
     
     
     val userState = remember(database) {
     
     
     database.observeUserAsState("Rhaenyra")
     
     
     }
     
     
     userState.value?.let { user ->
     
     
     BasicTextField(
     
     
     value = user.name,
     
     
     onValueChange = {
     
     
     userState.value = user.copy(name = it)
     
     
     }
     
     
     )
     
     
     }
     
     
     }
     

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197. Snapshots are Git for your variables
     @zachklipp
     

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198. Further reading:
     More info about implementing custom StateObjects:
     Slides:
     Whitepaper on multiversion concurrency control:
     @zachklipp
     

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199. Questions?
     twitter.com/ @zachklipp
     More info about implementing custom StateObjects:
     Slides:
     Whitepaper on multiversion concurrency control:
     

     View Slide