Flutter iOS之间通信

Flutter iOS之间通信

1.Flutter架构

flutter架构.png

Flutter框架包括：Framework和Engine，他们运行在各自的Platform上。

Framework是Dart语言开发的，包括Material Design风格的Widgets和Cupertino(iOS-style)风格的Widgets，以及文本、图片、按钮等基础Widgets；还包括渲染、动画、绘制、手势等基础能力。

Engine是C++实现的，包括Skia（二维图形库）；Dart VM（Dart Runtime）；Text（文本渲染）等。

实际上，Flutter的上层能力都是Engine提供的。Flutter正是通过Engine将各个Platform的差异化抹平。而我们今天要讲的Flutter 与 Native之间的通信，正是通过Engine提供的Platform Channel实现的。

2. Platform Channel

2.1 Flutter App调用Native APIs：

flutter调用Native API.png

通过上图，我们看到Flutter App是通过Platform Channel调用的Native APIs。

2.2Platform Channel 架构图：

Platform Channel 架构图.png

Platform Channel：

• Flutter App (Client)，通过MethodChannel类向Platform发送调用消息；
• iOS Platform (Host)，通过FlutterMethodChannel类接收调用消息。

PS：消息编解码器，是JSON格式的二进制序列化，所以调用方法的参数类型必须是可JSON序列化的
PS：方法调用，也可以反向发送调用消息。

3.Flutter部分

3.1MethodChannel：Flutter App调用Native APIs

/**
   * （1）MethodChannel：Flutter App调用Native APIs
   */
  static const MethodChannel _methodChannel = const MethodChannel('samples.flutter.io/battery');
  
  // 
  Future getBatteryLevel() async {
    String batteryLevel;
    try {
      final int result = await _methodChannel.invokeMethod('getBatteryLevel',{'paramName':'paramVale'});
      batteryLevel = 'Battery level: $result%.';
    } catch(e) {
      batteryLevel = 'Failed to get battery level.';
    }
    return batteryLevel;
  }

首先，我们实例_methodChannel(Channel名称必须唯一)，然后调用invokeMethod()方法。invokeMethod()有2个参数：
（1）方法名，不能为空；
（2）调用方法的参数，该参数必须可JSON序列化，可以为空。

3.2EventChannel：Native调用Flutter App

/**
   * （2）EventChannel：Native调用Flutter App
   */
  static const EventChannel _eventChannel = const EventChannel('samples.flutter.io/charging');

  void listenNativeEvent() {
    _eventChannel.receiveBroadcastStream().listen(_onEvent, onError:_onError);
  }

  void _onEvent(dynamic event) {
    print("Battery status: ${event == 'charging' ? '' : 'dis'}charging.");
  }

  void _onError(Object error) {
    print('Battery status: unknown.');
  }

4.iOS端实现

/**
 * Channel名称：必须与Flutter App的Channel名称一致
 */
#define METHOD_CHANNEL "samples.flutter.io/battery";
#define EVENT_CHANNEL "samples.flutter.io/charging";

@implementation AppDelegate

- (BOOL)application:(UIApplication*)application didFinishLaunchingWithOptions:(NSDictionary*)launchOptions {
    /**
     * FlutterViewController
     */
    FlutterViewController* controller = (FlutterViewController*)self.window.rootViewController;

    /**
     * FlutterMethodChannel & Handler
     */
    FlutterMethodChannel* batteryChannel = [FlutterMethodChannel methodChannelWithName:METHOD_CHANNEL binaryMessenger:controller];
    [batteryChannel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
        if ([@"getBatteryLevel" isEqualToString:call.method]) {
            int batteryLevel = [self getBatteryLevel];
            result(@(batteryLevel));
        } else {
            result(FlutterMethodNotImplemented);
        }
    }];
    
    /**
     * FlutterEventChannel & Handler
     */
    FlutterEventChannel* chargingChannel = [FlutterEventChannel eventChannelWithName:EVENT_CHANNEL binaryMessenger:controller];
    [chargingChannel setStreamHandler:self];
    
    return [super application:application didFinishLaunchingWithOptions:launchOptions];
}

@end

（1）Channel名称：必须与Flutter App的Channel名称一致；
（2）FlutterMethodChannel和FlutterEventChannel初始化的时候都需要传递binaryMessenger，即FlutterViewController；
（3）设置FlutterMethodChannel的Handler；
（4）设置FlutterEventChannel的Handler；

3.4.2 FlutterStreamHandler：

@interface AppDelegate () 

@property (nonatomic, copy)   FlutterEventSink     eventSink;

@end

- (FlutterError*)onListenWithArguments:(id)arguments eventSink:(FlutterEventSink)eventSink {
    self.eventSink = eventSink;

    // 监听电池状态
    [[NSNotificationCenter defaultCenter] addObserver:self
                                           selector:@selector(onBatteryStateDidChange:)
                                               name:UIDeviceBatteryStateDidChangeNotification
                                             object:nil];
    return nil;
}

- (FlutterError*)onCancelWithArguments:(id)arguments {
    [[NSNotificationCenter defaultCenter] removeObserver:self];
    self.eventSink = nil;
    return nil;
}

- (void)onBatteryStateDidChange:(NSNotification*)notification {
    if (self.eventSink == nil) return;
    UIDeviceBatteryState state = [[UIDevice currentDevice] batteryState];
    switch (state) {
            case UIDeviceBatteryStateFull:
            case UIDeviceBatteryStateCharging:
            self.eventSink(@"charging");
            break;
            case UIDeviceBatteryStateUnplugged:
            self.eventSink(@"discharging");
            break;
        default:
            self.eventSink([FlutterError errorWithCode:@"UNAVAILABLE"
                                           message:@"Charging status unavailable"
                                           details:nil]);
            break;
    }
}

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