Coroutines Flow 入門 / Coroutines Flow Introduction

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Coroutines Flow 入門 Kenji Abe

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Coroutines Flow とは •雑に言うと、RxJavaみたいなやつ •Cold Stream ‣ 受信しはじめてから動き始める ‣ Hot Stream は Channel

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Coroutines Flow とは • ↓ の資料が分かりやすいので、そっちを見て ‣ https://speakerdeck.com/sys1yagi/5fen- tewakarukotlin-coroutines-ﬂow •この資料では色々な使い方を紹介します

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基本的なやつ fun main() = runBlocking { val flow = flow { repeat(3) { emit(it) } } flow.collect { println(it) } }

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基本的なやつ fun main() = runBlocking { val flow = flow { repeat(3) { emit(it) } } flow.collect { println(it) } } Flow builder

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基本的なやつ fun main() = runBlocking { val flow = flow { repeat(3) { emit(it) } } flow.collect { println(it) } } 値を送出

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基本的なやつ fun main() = runBlocking { val flow = flow { repeat(3) { emit(it) } } flow.collect { println(it) } } 値を受信

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Flow builders •Flow { } •ﬂowOf(...) ‣ ﬂowOf(1, 2, 3) •(() -> T).asFlow() ‣ ({ 1 }).asFlow()

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Intermediate operators •map •ﬁlter •take •zip

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Flow operators fun main() = runBlocking { val flow = flowOf(1, 2, 3, 4, 5) flow.map { it * 2 }.collect { println(it) } } 値を2倍

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Flow operators fun main() = runBlocking { val flow = flowOf(1, 2, 3, 4, 5) flow.filter { it % 2 == 0 }.collect { println(it) } } 偶数のみ

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Terminal operators •collect •single •reduce •toList

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Terminal operators fun main() = runBlocking { val flow = flowOf(1, 2, 3, 4, 5) val value = flow .filter { it == 5 } .single() println(value) // 5 } 複数の値の場合は例外になる

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Terminal operators fun main() = runBlocking { val flow = flowOf(1, 2, 3, 4, 5) val value = flow .reduce { accumulator, value -> accumulator + value } // 1 + 2 + 3 + 4 + 5 println(value) // 15 }

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Terminal operators fun main() = runBlocking { val flow = flowOf(1, 2, 3, 4, 5) val value = flow .toList() println(value) // [1, 2, 3, 4, 5] }

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Change context •ﬂowOn ‣ 上流のContextを変更する ‣ Flowにおいて唯一Contextを切り替える方法 •emitするときにContextが異なると例外

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Change context fun main() = runBlocking { val flow = flowOf(1, 2, 3, 4, 5) flow .map { println("map: ${Thread.currentThread().name}") it + 1 } .flowOn(Dispatchers.IO) .collect { println("collect: ${Thread.currentThread().name}") } } worker main

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Change context fun main() = runBlocking { val flow = flowOf(1, 2, 3, 4, 5) flow .map { } .flowOn(Dispatchers.IO) .filter { } .flowOn(Dispatchers.Default) .map { } .flowOn(Dispatchers.IO) .collect { } }

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Change context fun main() = runBlocking { val flow = flowOf(1, 2, 3, 4, 5) flow .map { } .flowOn(Dispatchers.IO) .flowOn(Dispatchers.Default) .collect { } } 最初のやつが優先

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Change context fun main() = runBlocking { val flow = flow { emit(1) launch { emit(2) } } flow.collect { } } emit時にContextが違うので例外になる

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Exception •catchを使って捕捉できる •上流のストリームの例外のみ捕捉 •onCompletionでﬁnallyみたいなこともできる

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Exception fun main() = runBlocking { flow { } .map { } .catch { } .map { } .catch { } .collect { } } ① ② ③ ③の例外はここで捕捉 ①と②の例外はここで捕捉それ以降は実行されない

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Exception fun main() = runBlocking { flow { emit(1) }.catch { emit(-1) }.collect { println(it) } } 例外発生時に別の値を送出

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Exception fun main() = runBlocking { flow { } .map { } .onCompletion { } .map { } .onCompletion { } .collect { } } ﬁnallyみたいなやつ

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おわり