produce<Type>诉Channel<Type>()

Question

试着理解频道。我想要引导安卓BluetoothLeScanner。为什么这样做：

fun startScan(filters: List<ScanFilter>, settings: ScanSettings = defaultSettings): ReceiveChannel<ScanResult?> {
    val channel = Channel<ScanResult>()
    scanCallback = object : ScanCallback() {
        override fun onScanResult(callbackType: Int, result: ScanResult) {
            channel.offer(result)
        }
    }
    scanner.startScan(filters, settings, scanCallback)

    return channel
}

但不是这个：

fun startScan(scope: CoroutineScope, filters: List<ScanFilter>, settings: ScanSettings = defaultSettings): ReceiveChannel<ScanResult?> = scope.produce {
    scanCallback = object : ScanCallback() {
        override fun onScanResult(callbackType: Int, result: ScanResult) {
            offer(result)
        }
    }
    scanner.startScan(filters, settings, scanCallback)
}

它告诉我Channel was closed什么时候想第一次调用offer。

EDIT1:，根据docs：The channel is closed when the coroutine completes.，这是有意义的。我知道我们可以将suspendCoroutine和resume一起用于一次callback-replacement。然而，这是一个侦听器/流-情况。我不想让协同线完成

Answer 1

使用produce，您可以将范围引入您的频道。这意味着，生成在通道上流的项的代码可以被取消。

这也意味着您的通道的生存期从produce的lambda开始，到此lambda结束时结束。

在您的示例中，produce调用的lambda几乎立即结束，这意味着您的通道几乎立即关闭。

将代码更改为如下内容：

fun CoroutineScope.startScan(filters: List<ScanFilter>, settings: ScanSettings = defaultSettings): ReceiveChannel<ScanResult?> = produce {
    scanCallback = object : ScanCallback() {
        override fun onScanResult(callbackType: Int, result: ScanResult) {
            offer(result)
        }
    }
    scanner.startScan(filters, settings, scanCallback)

    // now suspend this lambda forever (until its scope is canceled)
    suspendCancellableCoroutine<Nothing> { cont ->
        cont.invokeOnCancellation {
            scanner.stopScan(...)
        }
    }
}

...
val channel = scope.startScan(filter)
...
...
scope.cancel() // cancels the channel and stops the scanner.

我添加了行suspendCancellableCoroutine<Nothing> { ... }，使其挂起“永远”。

更新:使用produce并以结构化方式处理错误(允许结构化并发)：

fun CoroutineScope.startScan(filters: List<ScanFilter>, settings: ScanSettings = defaultSettings): ReceiveChannel<ScanResult?> = produce {
    // Suspend this lambda forever (until its scope is canceled)
    suspendCancellableCoroutine<Nothing> { cont ->
        val scanCallback = object : ScanCallback() {
            override fun onScanResult(callbackType: Int, result: ScanResult) {
                offer(result)
            }
            override fun onScanFailed(errorCode: Int) {
                cont.resumeWithException(MyScanException(errorCode))
            }
        }
        scanner.startScan(filters, settings, scanCallback)

        cont.invokeOnCancellation {
            scanner.stopScan(...)
        }
    }
}