将广播接收器包装到流中（协程）答案

【问题标题】：Wrap Broadcast receiver into Flow (coroutine)将广播接收器包装到流中（协程）
【发布时间】：2021-04-13 09:04:36
【问题描述】：

我有一个用于 wifi 扫描结果的广播接收器作为数据源，我想以协程方式制作它。我在这里找到了暂停功能的答案： https://stackoverflow.com/a/53520496/5938671

suspend fun getCurrentScanResult(): List<ScanResult> =
    suspendCancellableCoroutine { cont ->
        //define broadcast reciever
        val wifiScanReceiver = object : BroadcastReceiver() {
            override fun onReceive(c: Context, intent: Intent) {
                if (intent.action?.equals(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION) == true) {
                    context.unregisterReceiver(this)
                    cont.resume(wifiManager.scanResults)
                }
            }
        }
        //setup cancellation action on the continuation
        cont.invokeOnCancellation {
            context.unregisterReceiver(wifiScanReceiver)
        }
        //register broadcast reciever
        context.registerReceiver(wifiScanReceiver, IntentFilter(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION))
        //kick off scanning to eventually receive the broadcast
        wifiManager.startScan()
    }

这对于信号发射来说很好，但如果我想在扫描过程中获得结果，那么我会崩溃，因为cont.resume() 只能被调用一次。然后我决定尝试Flow。这是我的代码：

suspend fun getCurrentScanResult(): Flow<List<ScanResult>> =
    flow{
        val wifiScanReceiver = object : BroadcastReceiver() {
            override fun onReceive(c: Context, intent: Intent) {
                if (intent.action?.equals(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION) == true) {
                    //context.unregisterReceiver(this)
                    emit(wifiManager.scanResults)
                }
            }
        }
        //setup cancellation action on the continuation
        //register broadcast reciever
        context.registerReceiver(wifiScanReceiver, IntentFilter(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION))
        //kick off scanning to eventually receive the broadcast
        wifiManager.startScan()
    }

但是现在Android Stuidio 说Suspension functions can be called only within coroutine body for function emit(wifiManager.scanResults) 有没有办法在这里使用Flow？

【问题讨论】：

标签： android kotlin broadcastreceiver kotlin-coroutines android-wifi

【解决方案1】：

请查看专为此用例设计的callback flow。像这样的东西可以完成这项工作：

callbackFlow {
  val receiver = object : BroadcastReceiver() {
    override fun onReceive(context: Context?, intent: Intent) {
      if (intent.action == WifiManager.SCAN_RESULTS_AVAILABLE_ACTION) {
        sendBlocking(wifiManager.scanResults) // or non-blocking offer()
      }
    }
  } 
  context.registerReceiver(receiver, intentFilter)

  awaitClose {
      context.unregisterReceiver(receiver)
  }
}

您可能还想与例如共享此流程。 shareIn 运营商，以避免为每个流订阅者注册一个新的接收者。

【讨论】：

它的实验性api