协程的async使用

async与launch一样都是开启一个协程,但是async会返回一个Deferred对象,该Deferred也是一个job

 

async函数类似于 launch函数.它启动了一个单独的协程,这是一个轻量级的线程并与其它所有的协程一起并发的工作.不同之处在于 launch 返回一个 Job 并且不附带任何结果值,而 async 返回一个 Deferred —— 一个轻量级的非阻塞 future,这代表了一个将会在稍后提供结果的 promise.你可以使用 .await() 在一个延期的值上得到它的最终结果, 但是 Deferred 也是一个 Job

 

看一下async的使用:

GlobalScope.launch {
    var deffer1 = async(Dispatchers.IO) {
        async1()
    }.await()
    var deffer2 = async(Dispatchers.IO) {
        async2()
    }.await()
    val result = deffer1 + deffer2
    println("the sum is: $result")
}

private suspend fun async1(): Int {
    delay(1500)
    println("async1")
    return 1
}
private suspend fun async2(): Int {
    delay(300)
    println("async2")
    return 2
}

最终输出:

I/System.out: async1 
I/System.out: async2
I/System.out: the sum is: 3

async1先执行,说明其是顺序进行的,因为await本身就是一个挂起行数,这时候它的用法与withContext没有什么差别。

 

再看一个例子:

GlobalScope.launch {
    var deffer1 = async(Dispatchers.IO) {
        async1()
    }
    var deffer2 = async(Dispatchers.IO) {
        async2()
    }
    Log.d("GlobalScope","before sum")
    var r1 = deffer1.await()
    var r2 = deffer2.await()
    val result = r1 + r2
    Log.d("GlobalScope","the sum is -> $result")
}

private suspend fun async1(): Int {
    delay(1500)
    Log.d("GlobalScope","async1")
    return 1
}
private suspend fun async2(): Int {
    delay(300)
    Log.d("GlobalScope","async2")
    return 2
}

打印log:

D/GlobalScope: before sum
D/GlobalScope: async2
D/GlobalScope: async1
D/GlobalScope: the sum is -> 3

可以看到async2先执行,因为挂起函数在后面,await是挂起函数。

 

简化写法:

GlobalScope.launch {
    var deffer1 = async(Dispatchers.IO) {
        async1()
    }
    var deffer2 = async(Dispatchers.IO) {
        async2()
    }
    Log.d("GlobalScope","before sum")
    val result = deffer1.await() + deffer2.await()
    Log.d("GlobalScope","the sum is -> $result")
}

打印log:

D/GlobalScope: before sum
D/GlobalScope: async2
D/GlobalScope: async1
D/GlobalScope: the sum is -> 3

 

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