android - 如何在主线程上使用 Kotlin 协程 await()

Question

我刚开始学习 Kotlin 协程，并试图通过在 UI 上显示结果来模拟一些长时间的 API 调用:

class MainActivity : AppCompatActivity() {
    fun log(msg: String) = println("[${Thread.currentThread().name}] $msg")

    override
    fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        this.setContentView(R.layout.activity_main)
        val resultTV = findViewById(R.id.text) as TextView

        val a = async(CommonPool) {
            delay(1_000L)
            6
        }

        val b = async(CommonPool) {
            delay(1_000L)
            7
        }

        launch(< NEED UI thread here >) {
            val aVal = a.await()
            val bVal = b.await()
            resultTV.setText((aVal * bVal).toString())
        }
    }
}

我不明白我怎么可能在 main 上下文中使用 launch 方法。

不幸的是，我找不到任何关于在 the official tutorial for coroutines 上为某些特定线程提供结果的信息。 .

Answer 1

编辑:

另见 an official example in Kotlin repo

您需要实现 Continuation对 Android UI 线程和 Coroutine context 进行回调的接口(interface)

例如(来自here)

private class AndroidContinuation<T>(val cont: Continuation<T>) : Continuation<T> by cont {
    override fun resume(value: T) {
        if (Looper.myLooper() == Looper.getMainLooper()) cont.resume(value)
        else Handler(Looper.getMainLooper()).post { cont.resume(value) }
    }
    override fun resumeWithException(exception: Throwable) {
        if (Looper.myLooper() == Looper.getMainLooper()) cont.resumeWithException(exception)
        else Handler(Looper.getMainLooper()).post { cont.resumeWithException(exception) }
    }
}

object Android : AbstractCoroutineContextElement(ContinuationInterceptor), ContinuationInterceptor {
    override fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T> =
        AndroidContinuation(continuation)
}

那就试试吧:

launch(Android) {
    val aVal = a.await()
    val bVal = b.await()
    resultTV.setText((aVal * bVal).toString()) 
}

更多信息:

https://medium.com/@macastiblancot/android-coroutines-getting-rid-of-runonuithread-and-callbacks-cleaner-thread-handling-and-more-234c0a9bd8eb#.r2buf5e6h