Kotlin 协程 异步 异步流

runBlocking

因为launch只在CoroutineScope中声明

runBlocking 的名字意味着运行它的线程(在本例中是主线程)在调用期间被阻塞，直到runBlocking中所有的协程{…}完成他们的执行。您经常会在应用程序的顶层看到这样使用runBlocking，而在真正的代码中却很少这样使用，因为线程是昂贵的资源，阻塞它们的效率很低，而且通常是不可取的。

 runBlocking {
    
        launch {
    
            delay(1000)
            println("word!")
        }
        println("Helllo")
    }

Helllo
word!

提取launch{…}转换成一个单独的函数

fun main(args: Array<String>) {
    
    runBlocking {
    
       launch {
     doWorld() }
        println("Helllo")
    }
}
suspend fun doWorld(){
    
    delay(1000)
    println("word!")
}

Helllo
word!

任何挂起函数中使用coroutineScope

fun main(args: Array<String>) {
    
    runBlocking {
    
        doWorld()
    }
}

suspend fun doWorld() = coroutineScope {
    
    launch {
    
        delay(1000L)
        println("World!")
    }
    println("Hello")
}

Hello
World!

coroutineScope构建器可以执行多个并发操作

fun main(args: Array<String>) {
    
    runBlocking {
    
        doWorld()
        println("Done")
    }
}

suspend fun doWorld() = coroutineScope {
    
    launch {
    
        delay(2000L)
        println("World 2")
    }
    launch {
    
        delay(1000L)
        println("World 1")
    }
    println("Hello")
}

Hello
World 1
World 2
Don

等待子协程完成，然后打印“Done”字符串

 runBlocking {
    
        val job = launch {
     // launch a new coroutine and keep a reference to its Job
            delay(1000L)
            println("World!")
        }
        println("Hello")
        job.join() // wait until child coroutine completes
        println("Done")
    }

Hello
World!
Done

组合挂起函数，等拿到每个条件之后，在得出结论

fun main() = runBlocking<Unit> {
    
//sampleStart
    val time = measureTimeMillis {
    
        println("The answer is ${concurrentSum()}")
    }
    println("Completed in $time ms")
//sampleEnd    
}

suspend fun concurrentSum(): Int = coroutineScope {
    
    val one = async {
     doSomethingUsefulOne() }
    val two = async {
     doSomethingUsefulTwo() }
    one.await() + two.await()
}

suspend fun doSomethingUsefulOne(): Int {
    
    delay(1000L) // 假设我们在这里做了些有用的事
    return 13
}

suspend fun doSomethingUsefulTwo(): Int {
    
    delay(1000L) // 假设我们在这里也做了些有用的事
    return 29
}

The answer is 42
Completed in 1017 ms

异步流Flow倒计时

@RequiresApi(Build.VERSION_CODES.N)
fun main(args: Array<String>) {
    
    runBlocking {
    
        launch {
    
            val ret = countdown(60_000, 2_000) {
     remianTime ->
                println("剩余时间$remianTime")
            }.onStart {
    
                println("countdown start")
            }.onCompletion {
    
                println("countdown end")
            }.reduce {
     acc, value ->
                println("acc$acc value $value ")
            }
            println("coutdown acc ret = $ret")
        }
    }
}

fun <T> countdown(
    duration: Long,
    interval: Long,
    onCountdown: suspend (Long) -> T
): Flow<T> = flow {
    
    (duration - interval downTo 0 step interval).forEach {
     emit(it) }
}.onEach {
     delay(interval) }
    .onStart {
     emit(duration) }
    .map {
     onCountdown(it) }
    .flowOn(Dispatchers.Default)

countdown start
剩余时间60000
剩余时间58000
acckotlin.Unit value kotlin.Unit 
剩余时间56000
acckotlin.Unit value kotlin.Unit 
剩余时间54000
........
acckotlin.Unit value kotlin.Unit 
剩余时间2000
acckotlin.Unit value kotlin.Unit 
剩余时间0
acckotlin.Unit value kotlin.Unit 
countdown end
coutdown acc ret = kotlin.Unit

参考文章 ：https://book.kotlincn.net/text/flow.html
Kotlin 异步 | Flow 应用场景及原理 https://juejin.cn/post/6989032238079803429