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Jetpack Compose の snapshotFlow の動作を調べる
snapshotFlow とは
snapshotFlow を利用すると State<T> オブジェクトをコールド Flow に変換できる。
/**
* Create a [Flow] from observable [Snapshot] state. (e.g. state holders returned by
* [mutableStateOf][androidx.compose.runtime.mutableStateOf].)
*
* [snapshotFlow] creates a [Flow] that runs [block] when collected and emits the result,
* recording any snapshot state that was accessed. While collection continues, if a new [Snapshot]
* is applied that changes state accessed by [block], the flow will run [block] again,
* re-recording the snapshot state that was accessed.
* If the result of [block] is not [equal to][Any.equals] the previous result, the flow will emit
* that new result. (This behavior is similar to that of
* [Flow.distinctUntilChanged][kotlinx.coroutines.flow.distinctUntilChanged].) Collection will
* continue indefinitely unless it is explicitly cancelled or limited by the use of other [Flow]
* operators.
*
* @sample androidx.compose.runtime.samples.snapshotFlowSample
*
* [block] is run in a **read-only** [Snapshot] and may not modify snapshot data. If [block]
* attempts to modify snapshot data, flow collection will fail with [IllegalStateException].
*
* [block] may run more than once for equal sets of inputs or only once after many rapid
* snapshot changes; it should be idempotent and free of side effects.
*
* When working with [Snapshot] state it is useful to keep the distinction between **events** and
* **state** in mind. [snapshotFlow] models snapshot changes as events, but events **cannot** be
* effectively modeled as observable state. Observable state is a lossy compression of the events
* that produced that state.
*
* An observable **event** happens at a point in time and is discarded. All registered observers
* at the time the event occurred are notified. All individual events in a stream are assumed
* to be relevant and may build on one another; repeated equal events have meaning and therefore
* a registered observer must observe all events without skipping.
*
* Observable **state** raises change events when the state changes from one value to a new,
* unequal value. State change events are **conflated;** only the most recent state matters.
* Observers of state changes must therefore be **idempotent;** given the same state value the
* observer should produce the same result. It is valid for a state observer to both skip
* intermediate states as well as run multiple times for the same state and the result should
* be the same.
*/
fun <T> snapshotFlow(
block: () -> T
): Flow<T> {}
特徴
- snapshotFlow の block に State を渡して値を取得するようにしておくと、State が変化したときに block が実行される。
- block で取得した値は snapshoFlow から生成される Flow を通して 収集できるようになっている。
サンプル
以下のように LazyColumn の State に格納される firstVisibleItemIndex を収集する Flow を作成してみる。firstVisibleItemIndex は LazyColumn に一番上に表示されている項目の Index 値になります。
@Composable
fun SnapshotFlowSample() {
// LazyColumn の State を rememberLazyListState で保持しておく
val listState = rememberLazyListState()
// LazyColumn の State を Flow に変換する
LaunchedEffect(listState) {
snapshotFlow {
Log.v("TEST", "Enter")
listState.firstVisibleItemIndex
}.collect {
Log.v("TEST", "Collect FirstVisibleItemIndex $it")
}
}
// listState オブジェクトを利用するように state に渡す
LazyColumn(
state = listState,
verticalArrangement = Arrangement.spacedBy(10.dp),
modifier = Modifier.padding(8.dp)
) {
repeat(10) {
item {
Card(
modifier = Modifier
.fillMaxSize()
.height(100.dp)
) {
Text(text = it.toString())
}
}
}
}
}
動作結果
- LazyColumn をスクロールすると firstVisibleItemIndex が変化する
- firstVisibleItemIndex が変化すると snapshotFlow の block 処理が動作する
- block 処理が動作したあと Flow を通して firstVisibleItemIndex が通知される
2022-02-26 14:28:29.162 24001-24001/jp.kaleidot725.sample V/TEST: Enter
2022-02-26 14:28:29.162 24001-24001/jp.kaleidot725.sample V/TEST: Collect FirstVisibleItemIndex 0
2022-02-26 14:28:30.633 24001-24001/jp.kaleidot725.sample V/TEST: Enter
2022-02-26 14:28:30.633 24001-24001/jp.kaleidot725.sample V/TEST: Collect FirstVisibleItemIndex 1
2022-02-26 14:28:30.949 24001-24001/jp.kaleidot725.sample V/TEST: Enter
2022-02-26 14:28:30.949 24001-24001/jp.kaleidot725.sample V/TEST: Collect FirstVisibleItemIndex 2
2022-02-26 14:28:31.282 24001-24001/jp.kaleidot725.sample V/TEST: Enter
2022-02-26 14:28:31.282 24001-24001/jp.kaleidot725.sample V/TEST: Collect FirstVisibleItemIndex 3
2022-02-26 14:28:32.599 24001-24001/jp.kaleidot725.sample V/TEST: Enter
2022-02-26 14:28:32.599 24001-24001/jp.kaleidot725.sample V/TEST: Collect FirstVisibleItemIndex 2
2022-02-26 14:28:32.849 24001-24001/jp.kaleidot725.sample V/TEST: Enter
2022-02-26 14:28:32.849 24001-24001/jp.kaleidot725.sample V/TEST: Collect FirstVisibleItemIndex 1
2022-02-26 14:28:33.015 24001-24001/jp.kaleidot725.sample V/TEST: Enter
2022-02-26 14:28:33.015 24001-24001/jp.kaleidot725.sample V/TEST: Collect FirstVisibleItemIndex 0
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