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

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

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@zachklipp

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@zachklipp

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@zachklipp

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Snapshots are Git for your variables @zachklipp

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Composition is the release manager @zachklipp

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@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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@zachklipp ACID “In-memory”

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

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

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

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@zachklipp ACI D urability On-disk solation

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

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

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

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@zachklipp How do they work?

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@zachklipp

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@zachklipp

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No magic @zachklipp

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

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No compiler plugin @zachklipp

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@zachklipp (but shipped with it) Snapshots ≠ Compose

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

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@zachklipp snapshots ThreadLocal integers linked lists

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

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

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

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

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

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@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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@zachklipp Snapshots API State Objects takeNestedSnapshot takeNestedMutableSnapshot withMutableSnapshot registerApplyObserver val state = mutableStateOf(“”) 
 mutableStateListOf mutableStateMapOf …whatever!

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@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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@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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@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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@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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@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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@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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@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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@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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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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@zachklipp Git: commit SHAs

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@zachklipp Snapshot IDs

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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@zachklipp Snapshot rules: Create child Apply Advance

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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@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 {

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@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 {

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

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@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 :

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@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("???") } }

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@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("???") } } }

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@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("???") } } }

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@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 } } }

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@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 } } }

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@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 } } }

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@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 } } }

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@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 } } }

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@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 } } }

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@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 } } }

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

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

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@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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@zachklipp What can we do with them?

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

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@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 } } }

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@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 } } }

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@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 } } }

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@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 } } }

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

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

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@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 } } }

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@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 } } }

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@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 } } }

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

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

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@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]) } } } }

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@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]) } } } }

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@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]) } } } }

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@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]) } } } }

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@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]) } } } }

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

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

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

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

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@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

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@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 }

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@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 }

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@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 }

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@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 }

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@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() } } } }

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@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() } } } }

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@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() } } } }

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@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() } } } }

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@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() } } } }

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@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() } } } }

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@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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@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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Snapshots are Git for your variables @zachklipp

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

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