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What does
Recomposition
mean to your app?
Aida Issayeva
Senior Software Engineer
Android GDE
@aida_isay
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TODAY’S
AGENDA
02
What is Recomposition?
03
Smart Recomposition
04
Tools to detect & debug
05
Practical tips
01
Deep into the foundation
06
Jetpack Compose Paradigm
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Jetpack Compose paradigm
UI widgets are functions
They are stateless
Different arguments, passed to
the functions, trigger updates
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SlotTable
Android OS
Composer
Applier
UI Node Tree
State Snapshot System
Change List
Applier
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Deep into the
foundation
02
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Compose Phases
1
Composition
Calls composable functions and
creates a description of the UI
2
Layout
Measures the layout elements
and their children and places
them on the screen
3
Drawing
Draws the UI elements to the
screen
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Compose Layers
UI
Compiler
Runtime
1
Composition
2
Layout
3
Drawing
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Compose Layers
UI
Compiler
Runtime
1
Composition
2
Layout
3
Drawing
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From Android OS:
● Context
● Configuration
● Lifecycle
● Keyboard state
● SavedStateInstance
● etc
CompositionLocals
Entrance to Jetpack Compose
class MainActivity : ComponentActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
...
setContent {
Card()
}
}
}
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Compose Layers
UI
Compiler
Runtime
1
Composition
2
Layout
3
Drawing
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UI
Compiler
Runtime
1
Composition
2
Layout
3
Drawing
Composition:
1. SlotTable
2. Composer
3. Changes List
Applier:
UI materializer
Recomposer:
1. State Snapshot System
2. Compositions - list
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UI
Compiler
Runtime
1
Composition
2
Layout
3
Drawing
Composition:
1. SlotTable
2. Composer
3. Changes List
Applier:
UI materializer
Recomposer:
1. State Snapshot System
2. Compositions - list
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scheduler for performing
recomposition and applying updates
to one or more Compositions.
Recomposer
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UI
Compiler
Runtime
1
Composition
2
Layout
3
Drawing
Composition:
1. SlotTable
2. Composer
3. Changes List
Applier:
UI materializer
Recomposer:
1. State Snapshot System
2. Compositions - list
WindowRecomposer:
LifecycleAware recomposer
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Android Lifecycle & Compose
viewTreeLifecycleOwner .lifecycle.addObserver(
object : LifecycleEventObserver {
override fun onStateChanged (lifecycleOwner: LifecycleOwner , event:
Lifecycle .Event) {
val self = this
when (event) {
Lifecycle.Event.ON_CREATE ->
runRecomposeScope .launch(start = CoroutineStart .UNDISPATCHED) {
try {
recomposer.runRecomposeAndApplyChanges()
} finally {
lifecycleOwner .lifecycle.removeObserver( self)
}
}
Lifecycle .Event.ON_START -> pausableClock ?.resume()
Lifecycle .Event.ON_STOP -> pausableClock ?.pause()
Lifecycle.Event.ON_DESTROY -> {
recomposer.cancel()
}
Lifecycle .Event.ON_PAUSE -> {
// Nothing
}
Lifecycle .Event.ON_RESUME -> {
// Nothing
}
Lifecycle .Event.ON_ANY -> {
// Nothing
}
}
}
}
)
Android OS
State Snapshot
System
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State(“Android”)
State Snapshot System
@Composable
fun StateSnapshotSystem() {
val name = remember {
mutableStateOf(Android)
}
val isImportant = remember {
mutableStateOf(false)
}
Column {
Checkbox(checked = isImportant.value,
onCheckedChange = {
isImportant.value = !isImportant.value
})
Text(text = name.value,
modifier = Modifier.clickable {
name.value = swapName(name.value)
}
)
}
}
State(false)
State(“Android”) State(true)
State(false)
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UI
Compiler
Runtime
1
Composition
2
Layout
3
Drawing
Composition:
1. SlotTable
2. Composer
3. Changes List
Applier:
UI materializer
Recomposer:
1. State Snapshot System
2. Compositions - list
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Composition
process of executing composable
functions
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Composition
Composer
SlotTable
Change
List
Manager Update
State
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Composer & SlotTable
@Composable
fun Card() {
Column {
Text(
text = "Compose",
fontSize = 24.sp,
fontWeight = FontWeight.Bold
)
Button(onClick = {
}) {
Text(text = "Click me")
}
}
}
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Composer & SlotTable
@Composable
fun Card() {
Column {
Text(
text = "Compose",
fontSize = 24.sp,
fontWeight = FontWeight.Bold
)
Button(onClick = {
}) {
Text(text = "Click me")
}
}
}
…..
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Composer & SlotTable
@Composable
fun Card() {
Column {
Text(
text = "Compose",
fontSize = 24.sp,
fontWeight = FontWeight.Bold
)
Button(onClick = {
}) {
Text(text = "Click me")
}
}
}
Group(123)
…..
C
A
R
D
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Composer & SlotTable
@Composable
fun Card() {
Column {
Text(
text = "Compose",
fontSize = 24.sp,
fontWeight = FontWeight.Bold
)
Button(onClick = {
}) {
Text(text = "Click me")
}
}
}
Group(123)
…..
Group(456)
C
A
R
D
C
O
L
U
M
N
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Composer & SlotTable
@Composable
fun Card() {
Column {
Text(
text = "Compose",
fontSize = 24.sp,
fontWeight = FontWeight.Bold
)
Button(onClick = {
}) {
Text(text = "Click me")
}
}
}
Group(123)
…..
Group(456)
“Compose”
C
A
R
D
C
O
L
U
M
N
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Composer & SlotTable
@Composable
fun Card() {
Column {
Text(
text = "Compose",
fontSize = 24.sp,
fontWeight = FontWeight.Bold
)
Button(onClick = {
}) {
Text(text = "Click me")
}
}
}
Group(123)
…..
Group(456)
“Compose”
Group(789)
C
A
R
D
C
O
L
U
M
N
B
U
T
T
O
N
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Composer & SlotTable
@Composable
fun Card() {
Column {
Text(
text = "Compose",
fontSize = 24.sp,
fontWeight = FontWeight.Bold
)
Button(onClick = {
}) {
Text(text = "Click me")
}
}
}
Group(123)
…..
Group(456)
“Compose”
Group(789)
C
A
R
D
C
O
L
U
M
N
B
U
T
T
O
N
{ ... }
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Composer & SlotTable
@Composable
fun Card() {
Column {
Text(
text = "Compose",
fontSize = 24.sp,
fontWeight = FontWeight.Bold
)
Button(onClick = {
}) {
Text(text = "Click me")
}
}
}
Group(123)
…..
Group(456)
“Compose”
Group(789)
“Click me”
C
A
R
D
C
O
L
U
M
N
B
U
T
T
O
N
{ ... }
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UI
Compiler
Runtime
1
Composition
2
Layout
3
Drawing
Composition:
1. SlotTable
2. Composer
3. Changes List
Applier:
UI materializer
Recomposer:
1. State Snapshot System
2. Compositions - list
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Applier
interface for materializing UI tree
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UI
Compiler
Runtime
1
Composition
2
Layout
3
Drawing
Composition:
1. SlotTable
2. Composer
3. Changes List
Applier:
UI materializer
Recomposer:
1. State Snapshot System
2. Compositions - list
UiApplier:
Applier implementation
WindowRecomposer:
LifecycleAware recomposer
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Layout phase steps
Column
LayoutNode
Button
LayoutNode
Text
LayoutNode
Text
LayoutNode
Root Node
1. Add child nodes bottom-up to the
root node
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Layout phase steps
Column
LayoutNode
Button
LayoutNode
Text
LayoutNode
Text
LayoutNode
Root Node
1. Add child nodes bottom-up to the
root node
2. Measure the child nodes
C
O
N
S
T
R
A
I
N
T
S
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Layout phase steps
Column
LayoutNode
Button
LayoutNode
Text
LayoutNode
Text
LayoutNode
Root Node
1. Add child nodes bottom-up to the
root node
2. Measure the child nodes
3. Define the size of a parent node
C
O
N
S
T
R
A
I
N
T
S
M
E
A
S
U
R
E
D
S
I
Z
E
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Layout phase steps
Column
LayoutNode
Button
LayoutNode
Text
LayoutNode
Text
LayoutNode
Root Node
1. Add child nodes bottom-up to the
root node
2. Measure the child nodes
3. Define the size of a parent node
4. Place the child nodes on screen
C
O
N
S
T
R
A
I
N
T
S
M
E
A
S
U
R
E
D
S
I
Z
E
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UI
Compiler
Runtime
1
Composition
2
Layout
3
Drawing
Composition:
1. SlotTable
2. Composer
3. Changes List
Applier:
UI materializer
Recomposer:
1. State Snapshot System
2. Compositions - list
UiApplier:
Applier implementation
WindowRecomposer:
LifecycleAware recomposer
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Final result
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What is
recomposition?
03
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Recomposition
re-execution of composable functions
in response to state changes
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UI
Compiler
Runtime
1
Composition
2
Layout
3
Drawing
Composition:
1. SlotTable
2. Composer
3. Changes List
Applier:
UI materializer
Recomposer:
1. State Snapshot System
2. Compositions - list
UiApplier:
Applier implementation
WindowRecomposer:
LifecycleAware recomposer
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Recomposition in SlotTable:
example 1
@Composable
fun Card() {
val counter = remember {
mutableStateOf(0)
}
Column {
Text(text = "Compose")
Button(onClick = {
counter.value++
}) {
Text(text = "Click me $counter")
}
}
}
Group(456)
…..
Group(789)
“Compose”
Group(101)
“Click me 0”
C
A
R
D
C
O
L
U
M
N
B
U
T
T
O
N
{ ... }
Group(123)
State(0)
R
E
M
E
M
B
E
R
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Recomposition in SlotTable:
example 1
@Composable
fun Card() {
val counter = remember {
mutableStateOf(0)
}
Column {
Text(text = "Compose")
Button(onClick = {
counter.value++
}) {
Text(text = "Click me $counter")
}
}
}
Group(456)
…..
Group(789)
“Compose”
Group(101)
“Click me 0”
C
A
R
D
C
O
L
U
M
N
B
U
T
T
O
N
{ ... }
Group(123)
State(0)
R
E
M
E
M
B
E
R
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Recomposition in SlotTable:
example 1
@Composable
fun Card() {
val counter = remember {
mutableStateOf(0)
}
Column {
Text(text = "Compose")
Button(onClick = {
counter.value++
}) {
Text(text = "Click me $counter")
}
}
}
Group(456)
…..
Group(789)
“Compose”
Group(101)
“Click me 1”
C
A
R
D
C
O
L
U
M
N
B
U
T
T
O
N
{ ... }
Group(123)
State(1)
R
E
M
E
M
B
E
R
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Recomposition in SlotTable:
example 2
@Composable
fun Card() {
val counter = remember {
mutableStateOf(0)
}
Column {
Text(text = "Compose $counter")
Button(onClick = {
counter.value++
}) {
Text(text = "Click me")
}
}
}
Group(456)
…..
Group(789)
“Compose 0”
Group(101)
“Click me”
C
A
R
D
C
O
L
U
M
N
B
U
T
T
O
N
{ ... }
Group(123)
State(0)
R
E
M
E
M
B
E
R
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Recomposition in SlotTable:
example 2
@Composable
fun Card() {
val counter = remember {
mutableStateOf(0)
}
Column {
Text(text = "Compose $counter")
Button(onClick = {
counter.value++
}) {
Text(text = "Click me")
}
}
}
Group(456)
…..
Group(789)
“Compose 1”
Group(101)
“Click me”
C
A
R
D
C
O
L
U
M
N
B
U
T
T
O
N
{ ... }
Group(123)
State(1)
R
E
M
E
M
B
E
R
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Compose Phases
1
Composition
Calls composable functions and
creates a description of the UI
2
Layout
Measures the layout elements
and their children and places
them on the screen
3
Drawing
Draws the UI elements to the
screen
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Smart
recomposition
04
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Inputs are stable
Smart recomposition is an intelligent way to skip the
recomposition of composable functions when their
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UI
Compiler
Runtime
1
Composition
2
Layout
3
Drawing
Composition:
1. SlotTable
2. Composer
3. Changes List
Applier:
UI materializer
Recomposer:
1. State Snapshot System
2. Compositions - list
UiApplier:
Applier implementation
WindowRecomposer:
LifecycleAware recomposer
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Stable type contract rules
The result of
`equals()`
function is
the same
All public
properties
are stable
Public
property is
changed -
composition
is notified
Rule 1 Rule 3
Rule 2
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Stable property types
Primitive types:
Int, Boolean, Long, Char, Float, etc
String
Annotated with @Stable
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What about custom
data types?
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Inferring Class stability
1
Visit each
eligible class
2
Annotate with
@StabilityInferred
3
Help Compiler
to determine
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Exceptions
Enum Enum Entry
Interface Annotation
Companion Object
Inline Anonymous Object
Expect Element
Inner class
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Tools to
detect & debug
recomposition
05
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1. Compose Compiler Metrics
{
"skippableComposables" : 64,
"restartableComposables" :
76,
"readonlyComposables" : 0,
"totalComposables" : 76
}
Full analysis
✅
Release build
❗
Skippable fun
✅
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2. Recompose Highlighter
Visual appeal
✅
Nested functions
❗
Labor-consuming
❗
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3. Logging Modifier
Too much noise
❗
Easy to add
✅
Labor-consuming
❗
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№ of compositions
✅
№ of skips
✅
No manual add
✅
Canary build
❗
JC v1.2.0-alpha03+
❗
4. Recomposition counter in Android Studio EE
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Practical tips to
leverage smart
recomposition
06
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1. Break down composable
functions
/**
* 👍 : only associated column with changes gets
recomposed.
*/
@Composable
fun GoodGreetingsTip1() {
Column {
Greeting("Android", 1, Gray200)
Greeting("Compose", 2, Brown200)
}
}
@Composable
fun Greeting(name: String, columnNumber: Int,
background: Color) {
Column {
. . .
}
}
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2. Use the key composable
/**
* 👍 : the key is set, and when item in the middle is
removed, only LazyColumn gets recomposed
* and not its children
*/
@Composable
fun GoodGreetingsTip2(fruits: List) {
LazyColumn(
modifier = Modifier
.recompositionCounter("lazyColumn")
) {
items(
items = fruits,
key = { it.name }
) { fruit ->
Item(fruit)
}
}
}
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3. Read the state value at the
lowest composable function
/**
* 👍 : only inner column and its children get
recomposed, because the read happens
* in one of the children.
*/
@Composable
fun GoodGreetingsTip3(cardInfo: () -> CardInfo) {
Column {
UpperRow()
GoodRow(cardInfo = cardInfo)
}
}
@Composable
private fun GoodRow(cardInfo: () -> CardInfo) {
Row {
GoodInnerColumn(cardInfo = cardInfo)
}
}
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4. Use modifier lambdas
/**
* 👍: With the receiver lambda function
(Modifier.offset{}) in place, this composable
function doesn't get recomposed.
*/
@Composable
fun GoodGreetingsTip4(yAxis: () -> Int) {
Row(
modifier = Modifier
.offset {
IntOffset(y = yAxis(), x = 0)
},
verticalAlignment = Alignment.Bottom
) {
. . .
}
}
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5. Utilize `derivedStateOf()`
/**
* 👍 : Only BottomBar gets recomposed whenever
hideBottomBar's calculated state changes
*/
@Composable
fun GoodGreetingsTip5(list: List) {
val listState = rememberLazyListState()
val hideBottomBar = remember {
derivedStateOf { listState.isScrollInProgress }
}
Scaffold(bottomBar = {
AnimatedVisibility(visible = !hideBottomBar.value)
{
BottomAppBar(content = { BottomNavigationItems
()})
}
}, content = {
LazyColumn(state = listState) {
. . .
}
})
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6. Declare properties as
read-only
/**
* 👍: `name` property inside Fruit is declared as
read-only. When Checkbox is toggled, the
recomposition of Text UI widget is skipped
*/
data class Fruit(val name: String)
@Composable
fun GoodGreetingsTip6(fruit: Fruit) {
var isChecked by remember {
mutableStateOf(false)
}
Column {
Text(text = fruit.name)
Checkbox(checked = isChecked,
onCheckedChange = {
isChecked = !isChecked
})
}
}
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7. Leverage KotlinX immutable
collections
/**
* 👍: When it comes to reg collections, Compiler can't
determine their stability, because they're interfaces and
underlying implementation can be mutable, therefore it marks
them unstable.When using kotlinX immutable collections, it's
guaranteed they'll be immutable, meaning they will be
inferred by the Compiler.
*/
@Composable
fun GoodGreetingsTip7() {
fruits: ImmutableList = persistentListOf()
Column {
fruits.forEach { fruit ->
Text(text = fruit.name)
}
}
}
in alpha
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7. Tips worth repeating:
Logic out of
composables
Execution in
any order &
parallel
Side-effect
free
ViewModels Callbacks
Optimization
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Recap
UI widgets are
composable functions
3 Compose Phases
UI widgets go through are Composition,
Layout, Draw
Recomposition is an
expected process
3 Compose Layers
Jetpack Compose uses to create
composable functions: Compiler,
Runtime, UI
Practical tips
should be used to avoid common
recomposition pitfalls
Smart recomposition
should be leveraged as much as
possible to teach Compiler
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Resources
1. Jetpack Compose Internals
2. Jetpack Compose source code
3. Understanding Jetpack Compose — part 1 of 2
4. Understanding Jetpack Compose — part 2 of 2
5. Recomposition made easy
6. RecomposeHighlighter code snippet
7. Compose Compiler Metrics
8. A historical introduction to the Compose reactive state
model
9. What is “donut-hole” skipping in Jetpack Compose?
10. Compose performance
11. Composable Metrics
12. Thinking in Compose
13. Jetpack Compose Phases
14. Jetpack Compose Lifecycle
15. Jetpack Compose Stability Explained
16. Compose Tooling
Recomposition tips
code samples
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THANK YOU
www.cupsofcode.com
Code | Live | Grow
@aida_isay