八. Flow 其他的操作符
8.1 Transform operators
transform
在使用 transform 操作符时，可以任意多次调用 emit ，这是 transform 跟 map 最大的区别：
fun main() = runBlocking {
(1..5).asFlow()
.transform {
emit(it * 2)
delay(100)
emit(it * 4)
}
.collect { println(it) }
}
transform 也可以使用 emit 发射任意值：
fun main() = runBlocking {
(1..5).asFlow()
.transform {
emit(it * 2)
delay(100)
emit("emit $it")
}
.collect { println(it) }
}
8.2 Size-limiting operators
take
take 操作符只取前几个 emit 发射的值。
fun main() = runBlocking {
(1..5).asFlow()
.take(2)
.collect { println(it) }
}
8.3 Terminal flow operators
在 Kotlin Coroutines Flow 系列(一) Flow 基本使用 一文最后，我整理了 Flow 相关的 Terminal 操作符。本文介绍 reduce 和 fold 两个操作符。
reduce
类似于 Kotlin 集合中的 reduce 函数，能够对集合进行计算操作。
例如，对平方数列求和：
fun main() = runBlocking {
val sum = (1..5).asFlow()
.map { it * it }
.reduce { a, b -> a + b }
println(sum)
}
例如，计算阶乘：
fun main() = runBlocking {
val sum = (1..5).asFlow().reduce { a, b -> a * b }
println(sum)
}
fold
也类似于 Kotlin 集合中的 fold 函数，fold 也需要设置初始值。
fun main() = runBlocking {
val sum = (1..5).asFlow()
.map { it * it }
.fold(0) { a, b -> a + b }
println(sum)
}
在上述代码中，初始值为0就类似于使用 reduce 函数实现对平方数列求和。
而对于计算阶乘：
fun main() = runBlocking {
val sum = (1..5).asFlow().fold(1) { a, b -> a * b }
println(sum)
}
初始值为1就类似于使用 reduce 函数实现计算阶乘。
8.4 Composing flows operators
zip
zip 是可以将2个 flow 进行合并的操作符。
fun main() = runBlocking {
val flowA = (1..5).asFlow()
val flowB = flowOf("one", "two", "three","four","five")
flowA.zip(flowB) { a, b -> "$a and $b" }
.collect { println(it) }
}
执行结果：
1 and one
2 and two
3 and three
4 and four
5 and five
zip 操作符会把 flowA 中的一个 item 和 flowB 中对应的一个 item 进行合并。即使 flowB 中的每一个 item 都使用了 delay() 函数，在合并过程中也会等待 delay() 执行完后再进行合并。
fun main() = runBlocking {
val flowA = (1..5).asFlow()
val flowB = flowOf("one", "two", "three", "four", "five").onEach { delay(100) }
val time = measureTimeMillis {
flowA.zip(flowB) { a, b -> "$a and $b" }
.collect { println(it) }
}
println("Cost $time ms")
}
执行结果：
1 and one
2 and two
3 and three
4 and four
5 and five
Cost 561 ms
如果 flowA 中 item 个数大于 flowB 中 item 个数：
fun main() = runBlocking {
val flowA = (1..6).asFlow()
val flowB = flowOf("one", "two", "three","four","five")
flowA.zip(flowB) { a, b -> "$a and $b" }
.collect { println(it) }
}
执行合并后新的 flow 的 item 个数 = 较小的 flow 的 item 个数。
执行结果：
1 and one
2 and two
3 and three
4 and four
5 and five
combine
combine 虽然也是合并，但是跟 zip 不太一样。
使用 combine 合并时，每次从 flowA 发出新的 item ，会将其与 flowB 的最新的 item 合并。
fun main() = runBlocking {
val flowA = (1..5).asFlow().onEach { delay(100) }
val flowB = flowOf("one", "two", "three","four","five").onEach { delay(200) }
flowA.combine(flowB) { a, b -> "$a and $b" }
.collect { println(it) }
}
执行结果：
1 and one
2 and one
3 and one
3 and two
4 and two
5 and two
5 and three
5 and four
5 and five
flattenMerge
其实，flattenMerge 不会组合多个 flow ，而是将它们作为单个流执行。
fun main() = runBlocking {
val flowA = (1..5).asFlow()
val flowB = flowOf("one", "two", "three","four","five")
flowOf(flowA,flowB)
.flattenConcat()
.collect{ println(it) }
}
执行结果：
1
2
3
4
5
one
two
three
four
five
为了能更清楚地看到 flowA、flowB 作为单个流的执行，对他们稍作改动。
fun main() = runBlocking {
val flowA = (1..5).asFlow().onEach { delay(100) }
val flowB = flowOf("one", "two", "three","four","five").onEach { delay(200) }
flowOf(flowA,flowB)
.flattenMerge(2)
.collect{ println(it) }
}
执行结果：
1
one
2
3
two
4
5
three
four
five
8.5 Flattening flows operators
flatMapConcat、flatMapMerge 类似于 RxJava 的 concatMap、flatMap 操作符。
flatMapConcat
flatMapConcat 由 map、flattenConcat 操作符实现。
@FlowPreview
public fun <T, R> Flow<T>.flatMapConcat(transform: suspend (value: T) -> Flow<R>): Flow<R> =
map(transform).flattenConcat()
在调用 flatMapConcat 后，collect 函数在收集新值之前会等待 flatMapConcat 内部的 flow 完成。
fun currTime() = System.currentTimeMillis()
var start: Long = 0
fun main() = runBlocking {
(1..5).asFlow()
.onStart { start = currTime() }
.onEach { delay(100) }
.flatMapConcat {
flow {
emit("$it: First")
delay(500)
emit("$it: Second")
}
}
.collect {
println("$it at ${System.currentTimeMillis() - start} ms from start")
}
}
执行结果：
1: First at 114 ms from start
1: Second at 619 ms from start
2: First at 719 ms from start
2: Second at 1224 ms from start
3: First at 1330 ms from start
3: Second at 1830 ms from start
4: First at 1932 ms from start
4: Second at 2433 ms from start
5: First at 2538 ms from start
5: Second at 3041 ms from start
flatMapMerge
flatMapMerge 由 map、flattenMerge 操作符实现。
@FlowPreview
public fun <T, R> Flow<T>.flatMapMerge(
concurrency: Int = DEFAULT_CONCURRENCY,
transform: suspend (value: T) -> Flow<R>
): Flow<R> = map(transform).flattenMerge(concurrency)
flatMapMerge 是顺序调用内部代码块，并且并行地执行 collect 函数。
fun currTime() = System.currentTimeMillis()
var start: Long = 0
fun main() = runBlocking {
(1..5).asFlow()
.onStart { start = currTime() }
.onEach { delay(100) }
.flatMapMerge {
flow {
emit("$it: First")
delay(500)
emit("$it: Second")
}
}
.collect {
println("$it at ${System.currentTimeMillis() - start} ms from start")
}
}
执行结果：
1: First at 116 ms from start
2: First at 216 ms from start
3: First at 319 ms from start
4: First at 422 ms from start
5: First at 525 ms from start
1: Second at 618 ms from start
2: Second at 719 ms from start
3: Second at 822 ms from start
4: Second at 924 ms from start
5: Second at 1030 ms from start
flatMapMerge 操作符有一个参数 concurrency ，它默认使用DEFAULT_CONCURRENCY，如果想更直观地了解 flatMapMerge 的并行，可以对这个参数进行修改。例如改成2，就会发现不一样的执行结果。
flatMapLatest
当发射了新值之后，上个 flow 就会被取消。
fun currTime() = System.currentTimeMillis()
var start: Long = 0
fun main() = runBlocking {
(1..5).asFlow()
.onStart { start = currTime() }
.onEach { delay(100) }
.flatMapLatest {
flow {
emit("$it: First")
delay(500)
emit("$it: Second")
}
}
.collect {
println("$it at ${System.currentTimeMillis() - start} ms from start")
}
}
执行结果：
1: First at 114 ms from start
2: First at 220 ms from start
3: First at 321 ms from start
4: First at 422 ms from start
5: First at 524 ms from start
5: Second at 1024 ms from start
九. Flow VS Reactive Streams
天生的多平台支持
由于 Kotlin 语言自身对多平台的支持，使得 Flow 也可以在多平台上使用。
互操作性
Flow 仍然属于响应式范畴。开发者通过 kotlinx-coroutines-reactive 模块中 Flow.asPublisher() 和 Publisher.asFlow() ，可以方便地将 Flow 跟 Reactive Streams 进行互操作。
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