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
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
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
4. @zachklipp What for? What do they do? How do they work? What can we do with them?
5. @zachklipp
6. @zachklipp
7. @zachklipp
8. Snapshots are Git for your variables @zachklipp
9. Composition is the release manager @zachklipp
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
11. @zachklipp ACID “In-memory”
12. @zachklipp ACID tomicity On-disk “a”, Red “b”, Red “b”, Blue “b”, Blue
13. @zachklipp A CID onsistency On-disk “a” “b” “c” “c” “c” tomicity
14. @zachklipp AC ID solation On-disk “a”, Red “a”, Blue “b”, Red “b”, Blue onsistency urability
15. @zachklipp ACI D urability On-disk solation
16. @zachklipp ACI D urability On-disk In-memory solation
17. @zachklipp ACID “In-memory” tomicity onsistency solation urability
18. @zachklipp Database Snapshots ACID ACID transactional snapshots commit/rollback apply/dispose views derived state
19. @zachklipp How do they work?
20. @zachklipp
21. @zachklipp
22. No magic @zachklipp
23. Compiler plugin? @zachklipp from A Hitchhiker’s Guide to Jetpack Compose
24. No compiler plugin @zachklipp
25. @zachklipp (but shipped with it) Snapshots ≠ Compose
26. @zachklipp (but shipped with it) Snapshots ≠ Compose snapshots the rest of Compose ThreadLocal integers linked lists
27. @zachklipp snapshots ThreadLocal integers linked lists
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
29. @zachklipp mutableStateOf mutableStateOf Range “a” “b” “foo” “bar” snapA snapB snapC
30. @zachklipp Snapshots State Objects mutableStateOf mutableStateOf Range “a” “b” “foo” “bar” snapA snapB snapC
31. @zachklipp Snapshots State Objects records mutableStateOf mutableStateOf Range “a” “b” “foo” “bar” snapA snapB snapC
32. @zachklipp reads: 96 snapshots records 128 167 181 64 75 111 209 52 53 87 many : state “foo”
33. @zachklipp writes: : “bar” 210 96 128 167 181 64 75 111 209 52 53 87 state “foo”
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”
35. @zachklipp Snapshots API State Objects takeNestedSnapshot takeNestedMutableSnapshot withMutableSnapshot registerApplyObserver val state = mutableStateOf(“”) 
 mutableStateListOf mutableStateMapOf …whatever!
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
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
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
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
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
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
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
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
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
45. @zachklipp Git: commit SHAs
46. @zachklipp Snapshot IDs
47. @zachklipp Walkthrough: create child, apply val message = mutableStateOf("") val snapshot = Snapshot.takeMutableSnapshot() snapshot.enter { message.value = "hello" } snapshot.apply() message
48. @zachklipp Walkthrough: create child, apply message val message = mutableStateOf("") global snapshot
49. @zachklipp Walkthrough: create child, apply message val message = mutableStateOf("") global snapshot
50. @zachklipp Walkthrough: create child, apply message val message = mutableStateOf("") “” global snapshot
51. @zachklipp Walkthrough: create child, apply message val snapshot = Snapshot.takeMutableSnapshot() “” global snapshot snapshot
52. @zachklipp Walkthrough: create child, apply message val snapshot = Snapshot.takeMutableSnapshot() “” global snapshot snapshot
53. @zachklipp Walkthrough: create child, apply message val snapshot = Snapshot.takeMutableSnapshot() “” global snapshot snapshot
54. @zachklipp Walkthrough: create child, apply message val snapshot = Snapshot.takeMutableSnapshot() “” global snapshot snapshot
55. @zachklipp Walkthrough: create child, apply message val snapshot = Snapshot.takeMutableSnapshot() “” global snapshot snapshot
56. @zachklipp Walkthrough: create child, apply message message.value = "hello" “” global snapshot snapshot
57. @zachklipp Walkthrough: create child, apply message message.value = "hello" “hello” “” global snapshot snapshot
58. @zachklipp Walkthrough: create child, apply message snapshot.apply() “” “hello” global snapshot snapshot
59. @zachklipp Walkthrough: create child, apply message snapshot.apply() “” “hello” global snapshot snapshot
60. @zachklipp Walkthrough: create child, apply message snapshot.apply() “” “hello” global snapshot
61. @zachklipp Walkthrough: create child, apply message snapshot.apply() “” “hello” global snapshot
62. @zachklipp Walkthrough: create child, apply message snapshot.apply() “” “hello” global snapshot
63. @zachklipp Walkthrough: create child, apply message snapshot.apply() “” “hello” global snapshot
64. @zachklipp Walkthrough: create child, apply message snapshot.apply() “” “hello” global snapshot
65. @zachklipp Walkthrough: create child, apply message snapshot.apply() “hello” “” global snapshot
66. @zachklipp Walkthrough: create child, apply message “hello” “” merged! global snapshot
67. @zachklipp Snapshot rules: Create child Apply Advance
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
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()
70. @zachklipp Walkthrough: create 2 children, apply val message = mutableStateOf("") val color = mutableStateOf(Red) message color global snapshot
71. @zachklipp Walkthrough: create 2 children, apply val message = mutableStateOf("") val color = mutableStateOf(Red) message “” color Red global snapshot
72. @zachklipp Walkthrough: create 2 children, apply val snapshotA = Snapshot.takeMutableSnapshot() message “” color Red snapshotA global snapshot
73. @zachklipp Walkthrough: create 2 children, apply val snapshotA = Snapshot.takeMutableSnapshot() message “” color Red snapshotA global snapshot
74. @zachklipp Walkthrough: create 2 children, apply val snapshotA = Snapshot.takeMutableSnapshot() message “” color Red snapshotA global snapshot
75. @zachklipp Walkthrough: create 2 children, apply val snapshotA = Snapshot.takeMutableSnapshot() message “” color Red snapshotA global snapshot
76. @zachklipp Walkthrough: create 2 children, apply val snapshotA = Snapshot.takeMutableSnapshot() message “” color Red snapshotA global snapshot
77. @zachklipp Walkthrough: create 2 children, apply message “” color Red snapshotA global snapshot val snapshotB = Snapshot.takeMutableSnapshot() snapshotB
78. @zachklipp Walkthrough: create 2 children, apply message “” color Red snapshotA global snapshot val snapshotB = Snapshot.takeMutableSnapshot() snapshotB
79. @zachklipp Walkthrough: create 2 children, apply message “” color Red snapshotA global snapshot val snapshotB = Snapshot.takeMutableSnapshot() snapshotB
80. @zachklipp Walkthrough: create 2 children, apply message “” color Red snapshotA global snapshot val snapshotB = Snapshot.takeMutableSnapshot() snapshotB
81. @zachklipp Walkthrough: create 2 children, apply message “” color Red snapshotA global snapshot val snapshotB = Snapshot.takeMutableSnapshot() snapshotB
82. @zachklipp Walkthrough: create 2 children, apply message “” color Red snapshotA global snapshot val snapshotB = Snapshot.takeMutableSnapshot() snapshotB
83. @zachklipp Walkthrough: create 2 children, apply message “” color Red snapshotA global snapshot snapshotB snapshotA.enter { message.value = "hello" }
84. @zachklipp Walkthrough: create 2 children, apply message “” color Red snapshotA global snapshot snapshotB snapshotA.enter { message.value = "hello" }
85. @zachklipp Walkthrough: create 2 children, apply message “” color Red snapshotA global snapshot snapshotB “hello” snapshotA.enter { message.value = "hello" }
86. @zachklipp Walkthrough: create 2 children, apply message “” color Red snapshotA global snapshot snapshotB “hello” snapshotB.enter { color.value = Blue }
87. @zachklipp Walkthrough: create 2 children, apply message “” color Red snapshotA global snapshot snapshotB “hello” snapshotB.enter { color.value = Blue }
88. @zachklipp Walkthrough: create 2 children, apply message “” color Red snapshotA global snapshot snapshotB “hello” Blue snapshotB.enter { color.value = Blue }
89. @zachklipp Walkthrough: create 2 children, apply message “” color Red snapshotA global snapshot snapshotB “hello” Blue snapshotA.apply()
90. @zachklipp Walkthrough: create 2 children, apply message “” color Red snapshotA global snapshot snapshotB “hello” Blue snapshotA.apply()
91. @zachklipp Walkthrough: create 2 children, apply message “” color Red global snapshot snapshotB “hello” Blue snapshotA.apply()
92. @zachklipp Walkthrough: create 2 children, apply message “” color Red global snapshot snapshotB “hello” Blue snapshotA.apply()
93. @zachklipp Walkthrough: create 2 children, apply message “” color Red global snapshot snapshotB “hello” Blue snapshotA.apply()
94. @zachklipp Walkthrough: create 2 children, apply message “” color Red global snapshot snapshotA.apply() snapshotB “hello” Blue
95. @zachklipp Walkthrough: create 2 children, apply message “” color Red global snapshot snapshotA.apply() snapshotB “hello” Blue
96. @zachklipp Walkthrough: create 2 children, apply message “” color Red global snapshot snapshotB.apply() snapshotB “hello” Blue
97. @zachklipp Walkthrough: create 2 children, apply message “” color Red global snapshot snapshotB.apply() snapshotB “hello” Blue
98. @zachklipp Walkthrough: create 2 children, apply message “” color Red global snapshot snapshotB.apply() “hello” Blue
99. @zachklipp Walkthrough: create 2 children, apply message “” color Red global snapshot snapshotB.apply() “hello” Blue
100. @zachklipp Walkthrough: create 2 children, apply message “” color Red global snapshot snapshotB.apply() “hello” Blue
101. @zachklipp Walkthrough: create 2 children, apply message “” color Red global snapshot snapshotB.apply() “hello” Blue
102. @zachklipp Walkthrough: create 2 children, apply message “” color Red global snapshot snapshotB.apply() “hello” Blue
103. @zachklipp Walkthrough: create 2 children, apply message “” color Red global snapshot “hello” Blue snapshotB.apply()
104. @zachklipp Walkthrough: create 2 children, apply message “” color Red global snapshot “hello” Blue merged!
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()
106. @zachklipp val message = mutableStateOf("") val color = mutableStateOf(Red) message color global snapshot Walkthrough: created nested child, apply
107. @zachklipp val message = mutableStateOf("") val color = mutableStateOf(Red) message “” color Red global snapshot Walkthrough: created nested child, apply
108. @zachklipp val snapshotA = Snapshot.takeMutableSnapshot() message “” color Red snapshotA global snapshot Walkthrough: created nested child, apply
109. @zachklipp val snapshotA = Snapshot.takeMutableSnapshot() message “” color Red snapshotA global snapshot Walkthrough: created nested child, apply
110. @zachklipp val snapshotA = Snapshot.takeMutableSnapshot() message “” color Red snapshotA global snapshot Walkthrough: created nested child, apply
111. @zachklipp val snapshotA = Snapshot.takeMutableSnapshot() message “” color Red snapshotA global snapshot Walkthrough: created nested child, apply
112. @zachklipp val snapshotA = Snapshot.takeMutableSnapshot() message “” color Red snapshotA global snapshot Walkthrough: created nested child, apply
113. @zachklipp snapshotA.enter { message.value = "hello" message “” color Red snapshotA global snapshot Walkthrough: created nested child, apply
114. @zachklipp snapshotA.enter { message.value = "hello" message “” color Red snapshotA global snapshot Walkthrough: created nested child, apply
115. @zachklipp snapshotA.enter { message.value = "hello" message “” color Red snapshotA global snapshot “hello” Walkthrough: created nested child, apply
116. @zachklipp snapshotA.enter { val snapshotB = Snapshot.takeMutableSnapshot() message “” color Red snapshotA global snapshot snapshotB “hello” Walkthrough: created nested child, apply
117. @zachklipp snapshotA.enter { val snapshotB = Snapshot.takeMutableSnapshot() message “” color Red snapshotA global snapshot snapshotB “hello” Walkthrough: created nested child, apply
118. @zachklipp snapshotA.enter { val snapshotB = Snapshot.takeMutableSnapshot() message “” color Red snapshotA global snapshot snapshotB “hello” Walkthrough: created nested child, apply
119. @zachklipp snapshotA.enter { val snapshotB = Snapshot.takeMutableSnapshot() message “” color Red snapshotA global snapshot snapshotB “hello” Walkthrough: created nested child, apply
120. @zachklipp snapshotA.enter { snapshotB.enter { color.value = Blue } message “” color Red snapshotA global snapshot snapshotB “hello” Walkthrough: created nested child, apply
121. @zachklipp snapshotA.enter { snapshotB.enter { color.value = Blue } message “” color Red snapshotA global snapshot snapshotB “hello” Walkthrough: created nested child, apply
122. @zachklipp snapshotA.enter { snapshotB.enter { color.value = Blue } message “” color Red snapshotA global snapshot snapshotB “hello” Blue Walkthrough: created nested child, apply
123. @zachklipp snapshotA.enter { snapshotB.apply() } message “” color Red snapshotA global snapshot snapshotB “hello” Blue Walkthrough: created nested child, apply
124. @zachklipp snapshotA.enter { snapshotB.apply() } message “” color Red snapshotA global snapshot snapshotB “hello” Blue Walkthrough: created nested child, apply
125. @zachklipp snapshotA.enter { snapshotB.apply() } message “” color Red snapshotA global snapshot “hello” Blue Walkthrough: created nested child, apply
126. @zachklipp snapshotA.enter { snapshotB.apply() } message “” color Red snapshotA global snapshot “hello” Blue Walkthrough: created nested child, apply
127. @zachklipp snapshotA.enter { snapshotB.apply() } message “” color Red snapshotA global snapshot “hello” Blue Walkthrough: created nested child, apply
128. @zachklipp snapshotA.enter { snapshotB.apply() } message “” color Red snapshotA global snapshot “hello” Blue Walkthrough: created nested child, apply
129. @zachklipp snapshotA.enter { snapshotB.apply() } message “” color Red snapshotA global snapshot “hello” Blue Walkthrough: created nested child, apply
130. @zachklipp message “” color Red snapshotA global snapshot “hello” Blue snapshotA.apply() Walkthrough: created nested child, apply
131. @zachklipp snapshotA.apply() message “” color Red snapshotA global snapshot “hello” Blue Walkthrough: created nested child, apply
132. @zachklipp snapshotA.apply() message “” color Red global snapshot “hello” Blue Walkthrough: created nested child, apply
133. @zachklipp snapshotA.apply() message “” color Red global snapshot “hello” Blue Walkthrough: created nested child, apply
134. @zachklipp snapshotA.apply() message “” color Red global snapshot “hello” Blue Walkthrough: created nested child, apply
135. @zachklipp snapshotA.apply() message “” color Red global snapshot “hello” Blue Walkthrough: created nested child, apply
136. @zachklipp snapshotA.apply() message “” color Red global snapshot “hello” Blue Walkthrough: created nested child, apply
137. @zachklipp snapshotA.apply() message “” color Red global snapshot “hello” Blue Walkthrough: created nested child, apply
138. @zachklipp snapshotA.apply() message “” color Red global snapshot “hello” Blue Walkthrough: created nested child, apply
139. @zachklipp message “” color Red global snapshot “hello” Blue merged! Walkthrough: created nested child, apply
140. @zachklipp MutableState message “” “hello” interface MutableState<T> : State<T> {
141. @zachklipp MutableState message “” “hello” interface MutableState<T> : State<T> { override var value: T operator fun component1(): T operator fun component2(): (T) -> Unit }
142. @zachklipp SnapshotMutableState MutableState message “” “hello” interface SnapshotMutableState<T> : MutableState<T> { val policy: SnapshotMutationPolicy<T> } interface MutableState<T> : State<T> {
143. @zachklipp SnapshotMutableStateImpl SnapshotMutableState message “” “hello” internal class SnapshotMutableStateImpl<T>( value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { override var value: T get() = TODO("???") set(value) { TODO("???") } } interface SnapshotMutableState<T> : MutableState<T> {
144. @zachklipp SnapshotMutableStateImpl StateObject message “” “hello” internal class SnapshotMutableStateImpl<T>( val firstStateRecord: StateRecord fun prependStateRecord(value: StateRecord) fun mergeRecords( previous: StateRecord, current: StateRecord, applied: StateRecord ): StateRecord? = null } interface SnapshotMutableState<T> : MutableState<T> {
145. @zachklipp message “” “hello” interface StateObject { value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { override var value: T get() = TODO("???") set(value) { TODO("???") } } SnapshotMutableStateImpl StateObject :
146. @zachklipp message “” “hello” interface StateObject { value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { override var value: T get() = TODO("???") set(value) { TODO("???") } override val firstStateRecord: StateRecord get() = TODO("???") override fun prependStateRecord(value: StateRecord) { TODO("???") } } SnapshotMutableStateImpl StateObject :
147. @zachklipp internal class SnapshotMutableStateImpl<T>( value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { override var value: T get() = TODO("???") set(value) { TODO("???") } override val firstStateRecord: StateRecord get() = TODO("???") override fun prependStateRecord(value: StateRecord) { TODO("???") } }
148. @zachklipp internal class SnapshotMutableStateImpl<T>( value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { override var value: T get() = TODO("???") set(value) { TODO("???") } override val firstStateRecord: StateRecord get() = TODO("???") override fun prependStateRecord(value: StateRecord) { TODO("???") } private class StateStateRecord<T>(myValue: T) : StateRecord() { var value: T = myValue override fun create(): StateRecord = TODO("???") override fun assign(value: StateRecord) { TODO("???") } } }
149. @zachklipp internal class SnapshotMutableStateImpl<T>( value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { override var value: T get() = TODO("???") set(value) { TODO("???") } override val firstStateRecord: StateRecord get() = TODO("???") override fun prependStateRecord(value: StateRecord) { TODO("???") } private class StateStateRecord<T>(myValue: T) : StateRecord() { var value: T = myValue override fun create(): StateRecord = StateStateRecord(value) override fun assign(value: StateRecord) { TODO("???") } } }
150. @zachklipp internal class SnapshotMutableStateImpl<T>( value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { override var value: T get() = TODO("???") set(value) { TODO("???") } override val firstStateRecord: StateRecord get() = TODO("???") override fun prependStateRecord(value: StateRecord) { TODO("???") } private class StateStateRecord<T>(myValue: T) : StateRecord() { var value: T = myValue override fun create(): StateRecord = StateStateRecord(value) override fun assign(value: StateRecord) { this.value = (value as StateStateRecord<T>).value } } }
151. @zachklipp internal class SnapshotMutableStateImpl<T>( value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { private var next: StateStateRecord<T> = 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<T>(myValue: T) : StateRecord() { var value: T = myValue override fun create(): StateRecord = StateStateRecord(value) override fun assign(value: StateRecord) { this.value = (value as StateStateRecord<T>).value } } }
152. @zachklipp internal class SnapshotMutableStateImpl<T>( value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { private var next: StateStateRecord<T> = 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<T>(myValue: T) : StateRecord() { var value: T = myValue override fun create(): StateRecord = StateStateRecord(value) override fun assign(value: StateRecord) { this.value = (value as StateStateRecord<T>).value } } }
153. @zachklipp internal class SnapshotMutableStateImpl<T>( value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { private var next: StateStateRecord<T> = 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<T> } private class StateStateRecord<T>(myValue: T) : StateRecord() { var value: T = myValue override fun create(): StateRecord = StateStateRecord(value) override fun assign(value: StateRecord) { this.value = (value as StateStateRecord<T>).value } } }
154. @zachklipp internal class SnapshotMutableStateImpl<T>( value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { private var next: StateStateRecord<T> = 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<T> } private class StateStateRecord<T>(myValue: T) : StateRecord() { var value: T = myValue override fun create(): StateRecord = StateStateRecord(value) override fun assign(value: StateRecord) { this.value = (value as StateStateRecord<T>).value } } }
155. @zachklipp internal class SnapshotMutableStateImpl<T>( value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { private var next: StateStateRecord<T> = 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<T> } private class StateStateRecord<T>(myValue: T) : StateRecord() { var value: T = myValue override fun create(): StateRecord = StateStateRecord(value) override fun assign(value: StateRecord) { this.value = (value as StateStateRecord<T>).value } } }
156. @zachklipp internal class SnapshotMutableStateImpl<T>( value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { private var next: StateStateRecord<T> = 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<T> } private class StateStateRecord<T>(myValue: T) : StateRecord() { var value: T = myValue override fun create(): StateRecord = StateStateRecord(value) override fun assign(value: StateRecord) { this.value = (value as StateStateRecord<T>).value } } }
157. @zachklipp internal class SnapshotMutableStateImpl<T>( val firstStateRecord: StateRecord fun prependStateRecord(value: StateRecord) fun mergeRecords( previous: StateRecord, current: StateRecord, applied: StateRecord ): StateRecord? = null }
158. @zachklipp internal class SnapshotMutableStateImpl<T>( value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { override fun mergeRecords( previous: StateRecord, current: StateRecord, applied: StateRecord ): StateRecord? { } }
159. @zachklipp internal class SnapshotMutableStateImpl<T>( value: T, override val policy: SnapshotMutationPolicy<T> ) : StateObject, SnapshotMutableState<T> { override fun mergeRecords( previous: StateRecord, current: StateRecord, applied: StateRecord ): StateRecord? { val previousRecord = previous as StateStateRecord<T> val currentRecord = current as StateStateRecord<T> val appliedRecord = applied as StateStateRecord<T> 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<T>).value = merged } } else { null } } } }
160. @zachklipp What can we do with them?
161. @zachklipp Custom state objects class Range { var start: Int = 0 var end: Int = 0 }
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 } } }
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 } } }
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 } } }
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 } } }
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 } } }
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 } } }
168. @zachklipp Custom state objects class Range : StateObject { }
169. @zachklipp Custom state objects class Range : StateObject { override fun mergeRecords( previous: StateRecord, current: StateRecord, applied: StateRecord ): StateRecord? { } }
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 } } }
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 } } }
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 } } }
173. @zachklipp State diffing github.com/zach-klippenstein/compose-autotransition
174. @zachklipp State diffing github.com/zach-klippenstein/compose-autotransition
175. @zachklipp State diffing suspend fun withAnimation(changes: () -> Unit) { val statesToAnimate = mutableSetOf<Any>() val targetValues = mutableMapOf<Any, Any>() 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]) } } } }
176. @zachklipp State diffing suspend fun withAnimation(changes: () -> Unit) { val statesToAnimate = mutableSetOf<Any>() val targetValues = mutableMapOf<Any, Any>() 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]) } } } }
177. @zachklipp State diffing suspend fun withAnimation(changes: () -> Unit) { val statesToAnimate = mutableSetOf<Any>() val targetValues = mutableMapOf<Any, Any>() 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]) } } } }
178. @zachklipp State diffing suspend fun withAnimation(changes: () -> Unit) { val statesToAnimate = mutableSetOf<Any>() val targetValues = mutableMapOf<Any, Any>() 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]) } } } }
179. @zachklipp State diffing suspend fun withAnimation(changes: () -> Unit) { val statesToAnimate = mutableSetOf<Any>() val targetValues = mutableMapOf<Any, Any>() 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]) } } } }
180. @zachklipp State diffing github.com/zach-klippenstein/compose-autotransition
181. @zachklipp State diffing github.com/zach-klippenstein/compose-autotransition
182. @zachklipp Persistence “In-memory” tomicity onsistency solation urability ACID
183. @zachklipp Persistence “In-memory” tomicity onsistency solation urability ACID
184. @zachklipp Persistence fun observeUserAsState(name: String): State<User?> { val cached = cachedStates.getOrPut(name) { val flow = users.findByName(name) val state = mutableStateOf<User?>(null) val job = scope.launch { flow.collect { state.value = it } } return@getOrPut UserState(state, job) } return cached.state } Read
185. @zachklipp Persistence Read fun observeUserAsState(name: String): State<User?> { val cached = cachedStates.getOrPut(name) { val flow = users.findByName(name) val state = mutableStateOf<User?>(null) val job = scope.launch { flow.collect { state.value = it } } return@getOrPut UserState(state, job) } return cached.state }
186. @zachklipp Persistence Read fun observeUserAsState(name: String): State<User?> { val cached = cachedStates.getOrPut(name) { val flow = users.findByName(name) val state = mutableStateOf<User?>(null) val job = scope.launch { flow.collect { state.value = it } } return@getOrPut UserState(state, job) } return cached.state }
187. @zachklipp Persistence Read fun observeUserAsState(name: String): State<User?> { val cached = cachedStates.getOrPut(name) { val flow = users.findByName(name) val state = mutableStateOf<User?>(null) val job = scope.launch { flow.collect { state.value = it } } return@getOrPut UserState(state, job) } return cached.state }
188. @zachklipp Persistence Read fun observeUserAsState(name: String): State<User?> { val cached = cachedStates.getOrPut(name) { val flow = users.findByName(name) val state = mutableStateOf<User?>(null) val job = scope.launch { flow.collect { state.value = it } } return@getOrPut UserState(state, job) } return cached.state }
189. @zachklipp Persistence Update Snapshot.registerApplyObserver { changedStateObjects, snapshot -> val changedDbStates = changedStateObjects.mapNotNull { if (it in cachedStates) it as MutableState<User?> 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() } } } }
190. @zachklipp Persistence Update Snapshot.registerApplyObserver { changedStateObjects, snapshot -> val changedDbStates = changedStateObjects.mapNotNull { if (it in cachedStates) it as MutableState<User?> 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() } } } }
191. @zachklipp Persistence Update Snapshot.registerApplyObserver { changedStateObjects, snapshot -> val changedDbStates = changedStateObjects.mapNotNull { if (it in cachedStates) it as MutableState<User?> 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() } } } }
192. @zachklipp Persistence Update Snapshot.registerApplyObserver { changedStateObjects, snapshot -> val changedDbStates = changedStateObjects.mapNotNull { if (it in cachedStates) it as MutableState<User?> 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() } } } }
193. @zachklipp Persistence Update Snapshot.registerApplyObserver { changedStateObjects, snapshot -> val changedDbStates = changedStateObjects.mapNotNull { if (it in cachedStates) it as MutableState<User?> 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() } } } }
194. @zachklipp Persistence Update Snapshot.registerApplyObserver { changedStateObjects, snapshot -> val changedDbStates = changedStateObjects.mapNotNull { if (it in cachedStates) it as MutableState<User?> 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() } } } }
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) } ) } }
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) } ) } }
197. Snapshots are Git for your variables @zachklipp
198. Further reading: More info about implementing custom StateObjects: Slides: Whitepaper on multiversion concurrency control: @zachklipp
199. Questions? twitter.com/ @zachklipp More info about implementing custom StateObjects: Slides: Whitepaper on multiversion concurrency control: