Flutter 即學即用系列博客——09 EventChannel 實現原生與 Flutter 通訊（一）

時間 2019-12-07

標籤 flutter 即用系列博客 eventchannel 實現原生通訊简体版

原文原文鏈接

前言

緊接着上一篇，這一篇咱們講一下原生怎麼給 Flutter 發信號，即原生-> Fluttergit

仍是經過 Flutter 官網的 Example 來說解。github

案例

接着上一次，這一次咱們讓原生主動將電池的充電狀態發送給 Flutter 並在界面顯示。less

步驟以下。async

1. Flutter 界面修改

咱們在原先基礎上增長一列用於顯示文本。ide

String _chargingStatus = 'Battery status: unknown.';
Text(_chargingStatus),

2. Flutter 定義 EventChannel

咱們在 _BatteryWidgetState 裏面加入下面變量：ui

static const EventChannel eventChannel = EventChannel('samples.flutter.io/charging');

samples.flutter.io/charging 能夠本身指定，通常保證惟一，因此 samples 實際使用能夠替換爲包名。主要是要跟原生對應便可。this

3. Flutter 在 initState 實現 EventChannel 監聽並實現對應回調方法

@override
  void initState() {
    super.initState();
    eventChannel.receiveBroadcastStream().listen(_onEvent, onError: _onError);
  }

  void _onEvent(Object event) {
    setState(() {
      _chargingStatus =
      "Battery status: ${event == 'charging' ? '' : 'dis'}charging.";
    });
  }

  void _onError(Object error) {
    setState(() {
        PlatformException exception = error;
        _chargingStatus = exception?.message ?? 'Battery status: unknown.';
    });
  }

能夠看到若是原生髮送 charging 顯示 charging，不然顯示 discharging。3d

固然錯誤顯示的是原生髮送過來的錯誤信息。code

注意這裏若是要獲取到錯誤信息，須要經過orm

PlatformException exception = error;

這個轉換語句才能夠。

4. 原生定義 EventChannel

private static final String CHARGING_CHANNEL = "samples.flutter.io/charging";

注意須要跟 Flutter 的一一對應。

5. 原生建立 EventChannel 並經過 StreamHandler 的 EventSink 發送內容給 Flutter

new EventChannel((FlutterView) flutterView, CHARGING_CHANNEL).setStreamHandler(
        new EventChannel.StreamHandler() {

            @Override
            public void onListen(Object arguments, EventChannel.EventSink events) {
            }

            @Override
            public void onCancel(Object arguments) {
            }
        }
);

具體到這裏爲：

new EventChannel((FlutterView)flutterView, CHARGING_CHANNEL).setStreamHandler(
        new EventChannel.StreamHandler() {
            private BroadcastReceiver chargingStateChangeReceiver;
            @Override
            public void onListen(Object arguments, EventChannel.EventSink events) {
                chargingStateChangeReceiver = createChargingStateChangeReceiver(events);
                registerReceiver(
                        chargingStateChangeReceiver, new IntentFilter(Intent.ACTION_BATTERY_CHANGED));
            }

            @Override
            public void onCancel(Object arguments) {
                unregisterReceiver(chargingStateChangeReceiver);
                chargingStateChangeReceiver = null;
            }
        }
);

private BroadcastReceiver createChargingStateChangeReceiver(final EventChannel.EventSink events) {
    return new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            int status = intent.getIntExtra(BatteryManager.EXTRA_STATUS, -1);

            if (status == BatteryManager.BATTERY_STATUS_UNKNOWN) {
                events.error("UNAVAILABLE", "Charging status unavailable", null);
            } else {
                boolean isCharging = status == BatteryManager.BATTERY_STATUS_CHARGING ||
                        status == BatteryManager.BATTERY_STATUS_FULL;
                events.success(isCharging ? "charging" : "discharging");
            }
        }
    };
}

這裏的 events.success 和 events.error 分別會調用 Flutter 的對應方法。

其中 error 的參數對應 Flutter 的 PlatformException 的參數。

PlatformException({
  @required this.code,
  this.message,
  this.details,
}) : assert(code != null);

這裏經過廣播的方式將電量狀態變化發送給 Flutter。

效果以下：

擴展

其實咱們點擊 Flutter 的 EventChannel，會看到源碼裏面的 receiveBroadcastStream 方法是對 MethodChannel 作了封裝。

Stream<dynamic> receiveBroadcastStream([dynamic arguments]) {
    final MethodChannel methodChannel = MethodChannel(name, codec);
    StreamController<dynamic> controller;
    controller = StreamController<dynamic>.broadcast(onListen: () async {
      BinaryMessages.setMessageHandler(name, (ByteData reply) async {
        if (reply == null) {
          controller.close();
        } else {
          try {
            controller.add(codec.decodeEnvelope(reply));
          } on PlatformException catch (e) {
            controller.addError(e);
          }
        }
        return null;
      });
      try {
        await methodChannel.invokeMethod('listen', arguments);
      } catch (exception, stack) {
        FlutterError.reportError(FlutterErrorDetails(
          exception: exception,
          stack: stack,
          library: 'services library',
          context: 'while activating platform stream on channel $name',
        ));
      }
    }, onCancel: () async {
      BinaryMessages.setMessageHandler(name, null);
      try {
        await methodChannel.invokeMethod('cancel', arguments);
      } catch (exception, stack) {
        FlutterError.reportError(FlutterErrorDetails(
          exception: exception,
          stack: stack,
          library: 'services library',
          context: 'while de-activating platform stream on channel $name',
        ));
      }
    });
    return controller.stream;
  }

因此其實原生-> Flutter 的通訊也是能夠用 MethodChannel 直接實現。

那怎麼實現呢？