(ちょっと早いですが、時間が取れなかった場合、これをアドベントカレンダーのネタにするかもです)
このパターンでこれだとうまく動かない、こういう方法があるなど、何か指摘などあればぜひ教えて下さい。
どうやら、これからはStateFlowやSharedFlowの時代のようです。
**新しいコードで意図的にLiveDataを選択する理由はありません。**ということが書かれていました。
Twitterなどで話題になっていました。
https://twitter.com/codinginflow/status/1326628159077412866
(Kotlin LangのSlackより)
最初にまとめ
以下にLifecycleCoroutineScope.launchWhenStarted()を組み合わせて使います。
LiveData.postValueの挙動に近い
MutableStateFlow<String>("initial value")
最新のデータをViewに反映したいだけならこれです。
LiveData.setValueでの挙動に近い
MutableSharedFlow(replay = 1)
ちゃんと一つ一つのデータを流す必要があり、Viewに反映したいなら MutableSharedFlow(replay = 1)
Event通知
Flowで素直にやる方法が今の所ないみたいです。
以下の2つのやり方が一応あります。
-
Eventクラスでラップして、このEventが消費されたかを保持する方法 - 通知後にEmptyのオブジェクトを入れてを入れてなんとかする方法
Channel(capacity = UNLIMITED).receiveAsFlow()
他にもこんな方法でいけるとか、間違っているみたいなのがあれば教えていただきたいです。
以下考察
どんな動きが理想か？
LiveData
- observe時に今STARTED以降の状態であれば値を流し、そうでなければ次にSTARTED以降の状態になったときに値を流します。これは確実にViewに値を反映させつつ、FragmentなどonStart以降でしか動かすことができない(Exceptionが投げられる)AndroidのAPIがあってもうまく動かすためです。
liveData.observe(viewLifecycleOwer) { value ->
...
}
- LiveData#postValue(value)では複数回投げられると最新の1つの値で動きます。もし完全な表示のためのデータを持っているのであれば最新のデータを反映するだけでいいので、この仕様でもうまく動くはずです。
postValueのJavaDoc
If you called this method multiple times before a main thread executed a posted task, only the last value would be dispatched.
つまり以下ではobserverに"b"だけが通知されます。
liveData.postValue("a")
liveData.postValue("b")
-
LiveData#setValue(value)ではpostValue()のような制約なしに以下で"a"、"b"両方をobserverで実行することができます。しかし、"a"を流す必要ない場合もあるので、良し悪しがあります。
liveData.setValue("a")
liveData.setValue("b")
- 値が一緒でも流すという性質があります。これに関してはイベント通知として使っていると必要になるパターンがあるかもしれません。
Event
例えばSnackbarを表示したい、ダイアログを表示したいといったものです。
これに対応するAndroidのコンポーネントはないので、いろんなパターンで実装されてきたため、正確にこうというのはないですが、以下の仕様が良いででしょう。
- observe時には通知されない。例えば画面回転時にダイアログやSnackbarがまた出てしまうとつらいです。
- onStart以降で動かす。
- observeしていないときのイベントもobserve時に流れる。画面回転時にイベントが失われないようにするため。
- 複数回postしても、LiveData.postValueのように消えずに、ちゃんと全部が流れる。
- 同じ値を2回入れたときでも2回流れる。
FlowをSTARTED以降の状態で動かすには
LifecycleCoroutineScope.launchWhenStarted()を使うとCoroutinesの処理をpause、resumeしてくれます。
LifecycleCoroutineScope.launchWhenStarted()、なかなかおもしろい。。 pic.twitter.com/43zYUqD5ZD
— takahirom (@new_runnable) May 16, 2019
そのため、onStart以降でしか動かないようにする方法としてKotlin LangのSlackでIan Lakeさんが以下の方法を紹介していました。 これを使えば、onStart以降だけ動かすようにできそうです。
/**
* Similar to [kotlinx.coroutines.flow.launchIn] but using
* [androidx.lifecycle.LifecycleCoroutineScope.launchWhenStarted].
*/
fun <T> Flow<T>.launchWhenStartedIn(
lifecycleOwner: LifecycleOwner
) = lifecycleOwner.lifecycleScope.launchWhenStarted {
collect()
}
viewModel.yourFlow.onEach { items ->
// Do something with the items
}.launchWhenStartedIn(viewLifecycleOwner)
検証コード
基本的に複数の種類のFlowに対してそれぞれforEach{}で同じプログラムで、チェックしているだけです。
ここから動作見られます。
https://pl.kotl.in/9lm5AT-8N
import kotlinx.coroutines.*
import kotlinx.coroutines.channels.BufferOverflow
import kotlinx.coroutines.channels.Channel
import kotlinx.coroutines.channels.Channel.Factory.UNLIMITED
import kotlinx.coroutines.flow.*
class App {
/**
* public fun <T> MutableSharedFlow(
* replay: Int = 0,
* extraBufferCapacity: Int = 0,
* onBufferOverflow: BufferOverflow = BufferOverflow.SUSPEND
)
*/
val sharedFlow1 = MutableSharedFlow<String>()
val sharedFlow2 = MutableSharedFlow<String>(replay = 1)
val sharedFlow3 = MutableSharedFlow<String>(extraBufferCapacity = 1)
val sharedFlow4 = MutableSharedFlow<String>(
extraBufferCapacity = 1,
onBufferOverflow = BufferOverflow.DROP_LATEST
)
val sharedFlow5 = MutableSharedFlow<String>(
replay = 1,
extraBufferCapacity = 1,
onBufferOverflow = BufferOverflow.DROP_OLDEST
)
val sharedFlow6 = MutableSharedFlow<String>(extraBufferCapacity = 1, onBufferOverflow = BufferOverflow.DROP_OLDEST)
// The resulting flow can be collected just once and throws
// * [IllegalStateException] when trying to collect it more than once.
// val channelConsumeAsFlow = Channel<String>(capacity = UNLIMITED)
// val flowChannelConsumeAsFlow = channelConsumeAsFlow.consumeAsFlow()
val channelReceiveAsFlow = Channel<String>(capacity = UNLIMITED)
val flowChannelReceiveAsFlow = channelReceiveAsFlow.receiveAsFlow()
data class Event<T>(var isConsumed: Boolean, val value: T)
val mutableSharedFlowForWrapEvent = MutableSharedFlow<Event<String>>(replay = 1)
val flowWrapWithEvent = mutableSharedFlowForWrapEvent
val sharedFlowSetEmptyWhenCollect = MutableSharedFlow<String>(replay = 1)
val stateFlow = MutableStateFlow("initial value")
}
open class FlowCheck<T>(
open val name: String,
open val collect: suspend ((T) -> Unit) -> Unit,
open val emit: suspend (String) -> Unit
)
class DefaultFlowCheck<T>(
name: String,
flow: Flow<T>
) : FlowCheck<T>(
name = name,
collect = { collectFun ->
flow.collect { value -> collectFun(value) }
},
emit = { value ->
flow as MutableSharedFlow<String>
flow.emit(value)
}
)
suspend fun main(args: Array<String>) {
val app = App()
val flows = listOf<FlowCheck<*>>(
DefaultFlowCheck(
"MutableSharedFlow(replay = 0, extraBufferCapacity = 0, onBufferOverflow = BufferOverflow.SUSPEND) default MutableSharedFlow",
app.sharedFlow1
),
DefaultFlowCheck(
"MutableSharedFlow(replay = 1, extraBufferCapacity = 0, onBufferOverflow = BufferOverflow.SUSPEND)",
app.sharedFlow2
),
DefaultFlowCheck(
"MutableSharedFlow(replay = 0, extraBufferCapacity = 1, onBufferOverflow = BufferOverflow.SUSPEND)",
app.sharedFlow3
),
DefaultFlowCheck(
"MutableSharedFlow(replay = 0, extraBufferCapacity = 1, onBufferOverflow = BufferOverflow.DROP_OLDEST)",
app.sharedFlow6
),
DefaultFlowCheck(
"MutableSharedFlow(replay = 0, extraBufferCapacity = 1, onBufferOverflow = BufferOverflow.DROP_LATEST)",
app.sharedFlow4
),
DefaultFlowCheck(
"MutableSharedFlow(replay = 1, extraBufferCapacity = 1, onBufferOverflow = BufferOverflow.DROP_OLDEST)",
app.sharedFlow5
),
FlowCheck(
"Event",
collect = { function: (App.Event<String>) -> Unit ->
app.flowWrapWithEvent.collect { event ->
if (!event.isConsumed) {
event.isConsumed = true
function(event)
}
}
},
emit = {
app.mutableSharedFlowForWrapEvent.emit(App.Event(false, it))
}
),
// DefaultFlowCheck(
// "channel.consumeAsFlow()",
// app.flowChannelConsumeAsFlow,
// { s: String -> app.channelConsumeAsFlow.send(s) }
// ),
FlowCheck<String>(
"channel.receiveAsFlow()",
collect = { collectFun ->
app.flowChannelReceiveAsFlow.collect { collectFun(it) }
},
emit = { s: String -> app.channelReceiveAsFlow.send(s) }
),
FlowCheck<String>(
"SharedFlowSetEmptyWhenCollect",
collect = { collectFun ->
app.sharedFlowSetEmptyWhenCollect.collect {
if (it.isNotEmpty()) {
collectFun(it)
app.sharedFlowSetEmptyWhenCollect.tryEmit("")
}
}
},
emit = { s: String -> app.sharedFlowSetEmptyWhenCollect.emit(s) }
),
DefaultFlowCheck(
"MutableStateFlow<String>(\"initial value\")",
app.stateFlow
),
)
flows.forEach { flowCheck ->
println("----")
println("start ${flowCheck.name}")
println("#1 collect and emit")
val job = GlobalScope.launch {
flowCheck.collect {
println("- #1 received: $it")
}
}
delay(30)
flowCheck.emit("hello 1")
delay(30)
job.cancel()
delay(30)
println("#2 emit and collect")
flowCheck.emit("hello 2")
delay(30)
val job2 = GlobalScope.launch {
flowCheck.collect {
println("- #2 received: $it")
}
}
delay(30)
job2.cancel()
delay(30)
println("#3 collect and emit × 3")
delay(30)
val job3 = GlobalScope.launch {
flowCheck.collect {
println("- #3 received: $it")
}
}
delay(30)
flowCheck.emit("hello 3-1")
flowCheck.emit("hello 3-2")
flowCheck.emit("hello 3-3")
delay(30)
job3.cancel()
println("#4 just collect")
delay(30)
val job4 = GlobalScope.launch {
flowCheck.collect {
println("- #4 received: $it")
}
}
delay(30)
job4.cancel()
delay(30)
println("#5 collect and same emit × 2")
val job5 = GlobalScope.launch {
flowCheck.collect {
println("- #5 received: $it")
}
}
delay(30)
flowCheck.emit("hello 5")
delay(30)
flowCheck.emit("hello 5")
delay(30)
job5.cancel()
delay(30)
println("#6 same emit × 2 and collect")
flowCheck.emit("hello 6")
delay(30)
flowCheck.emit("hello 6")
delay(30)
val job6 = GlobalScope.launch {
flowCheck.collect {
println("- #6 received: $it")
}
}
delay(30)
job6.cancel()
delay(30)
}
delay(30)
}
----
start MutableSharedFlow(replay = 0, extraBufferCapacity = 0, onBufferOverflow = BufferOverflow.SUSPEND) default MutableSharedFlow
#1 collect and emit
- #1 received: hello 1
#2 emit and collect
#3 collect and emit × 3
- #3 received: hello 3-1
- #3 received: hello 3-2
- #3 received: hello 3-3
#4 just collect
#5 collect and same emit × 2
- #5 received: hello 5
- #5 received: hello 5
#6 same emit × 2 and collect
----
start MutableSharedFlow(replay = 1, extraBufferCapacity = 0, onBufferOverflow = BufferOverflow.SUSPEND)
#1 collect and emit
- #1 received: hello 1
#2 emit and collect
- #2 received: hello 2
#3 collect and emit × 3
- #3 received: hello 2
- #3 received: hello 3-1
- #3 received: hello 3-2
- #3 received: hello 3-3
#4 just collect
- #4 received: hello 3-3
#5 collect and same emit × 2
- #5 received: hello 3-3
- #5 received: hello 5
- #5 received: hello 5
#6 same emit × 2 and collect
- #6 received: hello 6
----
start MutableSharedFlow(replay = 0, extraBufferCapacity = 1, onBufferOverflow = BufferOverflow.SUSPEND)
#1 collect and emit
- #1 received: hello 1
#2 emit and collect
#3 collect and emit × 3
- #3 received: hello 3-1
- #3 received: hello 3-2
- #3 received: hello 3-3
#4 just collect
#5 collect and same emit × 2
- #5 received: hello 5
- #5 received: hello 5
#6 same emit × 2 and collect
----
start MutableSharedFlow(replay = 0, extraBufferCapacity = 1, onBufferOverflow = BufferOverflow.DROP_OLDEST)
#1 collect and emit
- #1 received: hello 1
#2 emit and collect
#3 collect and emit × 3
- #3 received: hello 3-3
#4 just collect
#5 collect and same emit × 2
- #5 received: hello 5
- #5 received: hello 5
#6 same emit × 2 and collect
----
start MutableSharedFlow(replay = 0, extraBufferCapacity = 1, onBufferOverflow = BufferOverflow.DROP_LATEST)
#1 collect and emit
- #1 received: hello 1
#2 emit and collect
#3 collect and emit × 3
- #3 received: hello 3-1
#4 just collect
#5 collect and same emit × 2
- #5 received: hello 5
- #5 received: hello 5
#6 same emit × 2 and collect
----
start MutableSharedFlow(replay = 1, extraBufferCapacity = 1, onBufferOverflow = BufferOverflow.DROP_OLDEST)
#1 collect and emit
- #1 received: hello 1
#2 emit and collect
- #2 received: hello 2
#3 collect and emit × 3
- #3 received: hello 2
- #3 received: hello 3-2
- #3 received: hello 3-3
#4 just collect
- #4 received: hello 3-3
#5 collect and same emit × 2
- #5 received: hello 3-3
- #5 received: hello 5
- #5 received: hello 5
#6 same emit × 2 and collect
- #6 received: hello 6
----
start Event
#1 collect and emit
- #1 received: Event(isConsumed=true, value=hello 1)
#2 emit and collect
- #2 received: Event(isConsumed=true, value=hello 2)
#3 collect and emit × 3
- #3 received: Event(isConsumed=true, value=hello 3-1)
- #3 received: Event(isConsumed=true, value=hello 3-2)
- #3 received: Event(isConsumed=true, value=hello 3-3)
#4 just collect
#5 collect and same emit × 2
- #5 received: Event(isConsumed=true, value=hello 5)
- #5 received: Event(isConsumed=true, value=hello 5)
#6 same emit × 2 and collect
- #6 received: Event(isConsumed=true, value=hello 6)
----
start channel.receiveAsFlow()
#1 collect and emit
- #1 received: hello 1
#2 emit and collect
- #2 received: hello 2
#3 collect and emit × 3
- #3 received: hello 3-1
- #3 received: hello 3-2
- #3 received: hello 3-3
#4 just collect
#5 collect and same emit × 2
- #5 received: hello 5
- #5 received: hello 5
#6 same emit × 2 and collect
- #6 received: hello 6
- #6 received: hello 6
----
start SharedFlowSetEmptyWhenCollect
#1 collect and emit
- #1 received: hello 1
#2 emit and collect
- #2 received: hello 2
#3 collect and emit × 3
- #3 received: hello 3-1
- #3 received: hello 3-2
- #3 received: hello 3-3
#4 just collect
#5 collect and same emit × 2
- #5 received: hello 5
- #5 received: hello 5
#6 same emit × 2 and collect
- #6 received: hello 6
----
start MutableStateFlow<String>("initial value")
#1 collect and emit
- #1 received: initial value
- #1 received: hello 1
#2 emit and collect
- #2 received: hello 2
#3 collect and emit × 3
- #3 received: hello 2
- #3 received: hello 3-3
#4 just collect
- #4 received: hello 3-3
#5 collect and same emit × 2
- #5 received: hello 3-3
- #5 received: hello 5
#6 same emit × 2 and collect
- #6 received: hello 6
結果
LiveData.postValueに近いもの(observe時に値が流れ、かつ、最新以外が失われるもの)
MutableSharedFlow(replay = 1, extraBufferCapacity = 1, onBufferOverflow = BufferOverflow.DROP_OLDEST)かMutableStateFlow<String>("initial value")ですが、この結果であれば、MutableStateFlowで良さそうに思えます。
MutableSharedFlow(replay = 1, extraBufferCapacity = 1, onBufferOverflow = BufferOverflow.DROP_OLDEST)ではreplay=1によって、observe時(collect時)に保持している値が流れ、BufferOverflow.DROP_OLDESTによって最新の値が使われることによって、LiveData.postValue()に近い動きが実現されています。
MutableStateFlow<String>("initial value")はデフォルトでそのような動きになっているようで、理想的な動きになっています。
start MutableSharedFlow(replay = 1, extraBufferCapacity = 1, onBufferOverflow = BufferOverflow.DROP_OLDEST)
#1 collect and emit
- #1 received: hello 1
#2 emit and collect
- #2 received: hello 2
#3 collect and emit × 3
- #3 received: hello 2
- #3 received: hello 3-2
- #3 received: hello 3-3
#4 just collect
- #4 received: hello 3-3
#5 collect and same emit × 2
- #5 received: hello 3-3
- #5 received: hello 5
- #5 received: hello 5
#6 same emit × 2 and collect
- #6 received: hello 6
---
start MutableStateFlow<String>("initial value")
#1 collect and emit
- #1 received: initial value
- #1 received: hello 1
#2 emit and collect
- #2 received: hello 2
#3 collect and emit × 3
- #3 received: hello 2
- #3 received: hello 3-3
#4 just collect
- #4 received: hello 3-3
#5 collect and same emit × 2
- #5 received: hello 3-3
- #5 received: hello 5
#6 same emit × 2 and collect
- #6 received: hello 6
LiveData.setValueに近いもの(observe時に値が流れ、かつ、情報が失われないもの)
MutableSharedFlow(replay = 1)がそのような挙動になります。
start MutableSharedFlow(replay = 1, extraBufferCapacity = 0, onBufferOverflow = BufferOverflow.SUSPEND)
#1 collect and emit
- #1 received: hello 1
#2 emit and collect
- #2 received: hello 2
#3 collect and emit × 3
- #3 received: hello 2
- #3 received: hello 3-1
- #3 received: hello 3-2
- #3 received: hello 3-3
#4 just collect
- #4 received: hello 3-3
#5 collect and same emit × 2
- #5 received: hello 3-3
- #5 received: hello 5
- #5 received: hello 5
#6 same emit × 2 and collect
- #6 received: hello 6
イベント通知(observe時に値が流れず、observeしていないときのイベントも流れる)
これは以下が考えられます。Channel APIがちょっと複雑というか理解するべきことが増えるので、厄介だというのがあるので、Eventがいいかもしれません。
-
Eventクラスでラップして、このEventが消費されたかを保持する方法 - 通知後にEmptyのオブジェクトを入れてなんとかする方法
Channel(capacity = UNLIMITED).receiveAsFlow()
しかし、一つ問題があって、Channelを使うやり方でないとcollectされていない状態で複数イベントが通知された場合に1つしか動きません。
Channel(capacity = UNLIMITED)を使うとLinkedListChannelが使われ、linked-listで管理されているBufferがうまく動いてくれるようです。
start Event
#1 collect and emit
- #1 received: Event(isConsumed=true, value=hello 1)
#2 emit and collect
- #2 received: Event(isConsumed=true, value=hello 2)
#3 collect and emit × 3
- #3 received: Event(isConsumed=true, value=hello 3-1)
- #3 received: Event(isConsumed=true, value=hello 3-2)
- #3 received: Event(isConsumed=true, value=hello 3-3)
#4 just collect
#5 collect and same emit × 2
- #5 received: Event(isConsumed=true, value=hello 5)
- #5 received: Event(isConsumed=true, value=hello 5)
#6 same emit × 2 and collect
- #6 received: Event(isConsumed=true, value=hello 6)
----
start channel.receiveAsFlow()
#1 collect and emit
- #1 received: hello 1
#2 emit and collect
- #2 received: hello 2
#3 collect and emit × 3
- #3 received: hello 3-1
- #3 received: hello 3-2
- #3 received: hello 3-3
#4 just collect
#5 collect and same emit × 2
- #5 received: hello 5
- #5 received: hello 5
#6 same emit × 2 and collect
- #6 received: hello 6
- #6 received: hello 6
----
start SharedFlowSetEmptyWhenCollect
#1 collect and emit
- #1 received: hello 1
#2 emit and collect
- #2 received: hello 2
#3 collect and emit × 3
- #3 received: hello 3-1
- #3 received: hello 3-2
- #3 received: hello 3-3
#4 just collect
#5 collect and same emit × 2
- #5 received: hello 5
- #5 received: hello 5
#6 same emit × 2 and collect
- #6 received: hello 6