生产<Type> vs Channel<Type>()

Question

试图了解渠道。我想对 android BluetoothLeScanner 进行通道化。为什么会这样：

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 第一次想呼叫offer。

EDIT1： 根据文档：The channel is closed when the coroutine completes. 这是有道理的。我知道我们可以使用suspendCoroutine 和resume 一次性使用callback-replacement。然而，这是一个监听器/流的情况。我不希望协程完成

Answer 1

使用produce，您可以为您的频道引入范围。这意味着，可以取消生成通过通道流式传输的项目的代码。

这也意味着您的 Channel 的生命周期从 produce 的 lambda 的开头开始，并在此 lambda 结束时结束。

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

把你的代码改成这样：

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 suspendCancellableCoroutine<Nothing> { ... } 行使其“永远”暂停。

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(...)
        }
    }
}