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# Android View

建议使用 Android Studio 进行开发,在 Android Studio 左侧 project tab下选中 android 目录下任意一个文件,右上角会出现 Open for Editing in Android Studio

点击即可打开,打开后 project tab 并不是一个 Android 项目,而是项目中所有 Android 项目,包含第三方:

app 目录是当前项目的 android 目录,其他则是第三方的 android 目录。

App 项目的 java/包名 目录下创建嵌入 Flutter 中的 Android View,此 View 继承 PlatformView

class MyFlutterView(context: Context) : PlatformView {
    override fun getView(): View {
        TODO("Not yet implemented")
    }

    override fun dispose() {
        TODO("Not yet implemented")
    }
}
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  • getView :返回要嵌入 Flutter 层次结构的Android View
  • dispose:释放此View时调用,此方法调用后 View 不可用,此方法需要清除所有对象引用,否则会造成内存泄漏。

返回一个简单的 TextView

class MyFlutterView(context: Context, messenger: BinaryMessenger, viewId: Int, args: Map<String, Any>?) : PlatformView {

    val textView: TextView = TextView(context)

    init {
        textView.text = "我是Android View"
    }

    override fun getView(): View {

        return textView
    }

    override fun dispose() {
        TODO("Not yet implemented")
    }
}
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  • messenger:用于消息传递,后面介绍 Flutter 与 原生通信时用到此参数。
  • viewId:View 生成时会分配一个唯一 ID。
  • args:Flutter 传递的初始化参数。

# 注册PlatformView

创建PlatformViewFactory:

class MyFlutterViewFactory(val messenger: BinaryMessenger) : PlatformViewFactory(StandardMessageCodec.INSTANCE) {

    override fun create(context: Context, viewId: Int, args: Any?): PlatformView {
        val flutterView = MyFlutterView(context, messenger, viewId, args as Map<String, Any>?)
        return flutterView
    }

}
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创建 MyPlugin

class MyPlugin : FlutterPlugin {

    override fun onAttachedToEngine(binding: FlutterPlugin.FlutterPluginBinding) {
        val messenger: BinaryMessenger = binding.binaryMessenger
        binding
                .platformViewRegistry
                .registerViewFactory(
                        "plugins.flutter.io/custom_platform_view", MyFlutterViewFactory(messenger))
    }

    companion object {
        
        fun registerWith(registrar: PluginRegistry.Registrar) {
            registrar
                    .platformViewRegistry()
                    .registerViewFactory(
                            "plugins.flutter.io/custom_platform_view",
                            MyFlutterViewFactory(registrar.messenger()))
        }
    }

    override fun onDetachedFromEngine(binding: FlutterPlugin.FlutterPluginBinding) {

    }
}
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记住 plugins.flutter.io/custom_platform_view ,这个字符串在 Flutter 中需要与其保持一致。

App 中 MainActivity 中注册:

class MainActivity : FlutterActivity() {
    override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
        super.configureFlutterEngine(flutterEngine)
        flutterEngine.plugins.add(MyPlugin())
    }
}
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如果是 Flutter Plugin,没有MainActivity,则在对应的 Plugin onAttachedToEngine 和 registerWith 方法修改如下:

public class CustomPlatformViewPlugin : FlutterPlugin,MethodCallHandler {
    /// The MethodChannel that will the communication between Flutter and native Android
    ///
    /// This local reference serves to register the plugin with the Flutter Engine and unregister it
    /// when the Flutter Engine is detached from the Activity
    private lateinit var channel: MethodChannel

    override fun onAttachedToEngine( flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
        channel = MethodChannel(flutterPluginBinding.getFlutterEngine().getDartExecutor(), "custom_platform_view")
        channel.setMethodCallHandler(this)

        val messenger: BinaryMessenger = flutterPluginBinding.binaryMessenger
        flutterPluginBinding
                .platformViewRegistry
                .registerViewFactory(
                        "plugins.flutter.io/custom_platform_view", MyFlutterViewFactory(messenger))

    }

    // This static function is optional and equivalent to onAttachedToEngine. It supports the old
    // pre-Flutter-1.12 Android projects. You are encouraged to continue supporting
    // plugin registration via this function while apps migrate to use the new Android APIs
    // post-flutter-1.12 via https://flutter.dev/go/android-project-migration.
    //
    // It is encouraged to share logic between onAttachedToEngine and registerWith to keep
    // them functionally equivalent. Only one of onAttachedToEngine or registerWith will be called
    // depending on the user's project. onAttachedToEngine or registerWith must both be defined
    // in the same class.
    companion object {
        
        fun registerWith(registrar: Registrar) {
            val channel = MethodChannel(registrar.messenger(), "custom_platform_view")
            channel.setMethodCallHandler(CustomPlatformViewPlugin())

            registrar
                    .platformViewRegistry()
                    .registerViewFactory(
                            "plugins.flutter.io/custom_platform_view",
                            MyFlutterViewFactory(registrar.messenger()))
        }
    }

    override fun onMethodCall( call: MethodCall,  result: Result) {
        if (call.method == "getPlatformVersion") {
            result.success("Android ${android.os.Build.VERSION.RELEASE}")
        } else {
            result.notImplemented()
        }
    }

    override fun onDetachedFromEngine( binding: FlutterPlugin.FlutterPluginBinding) {
        channel.setMethodCallHandler(null)
    }
}
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# 嵌入Flutter

在 Flutter 中调用

class PlatformViewDemo extends StatelessWidget {
  
  Widget build(BuildContext context) {
    Widget platformView(){
      if(defaultTargetPlatform == TargetPlatform.android){
        return AndroidView(
          viewType: 'plugins.flutter.io/custom_platform_view',
        );
      }
    }
    return Scaffold(
      appBar: AppBar(),
      body: Center(
        child: platformView(),
      ),
    );
  }
}
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上面嵌入的是 Android View,因此通过 defaultTargetPlatform == TargetPlatform.android 判断当前平台加载,在 Android 上运行效果:

# 设置初始化参数

Flutter 端修改如下:

AndroidView(
          viewType: 'plugins.flutter.io/custom_platform_view',
          creationParams: {'text': 'Flutter传给AndroidTextView的参数'},
          creationParamsCodec: StandardMessageCodec(),
        )
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  • creationParams :传递的参数,插件可以将此参数传递给 AndroidView 的构造函数。
  • creationParamsCodec :将 creationParams 编码后再发送给平台侧,它应该与传递给构造函数的编解码器匹配。值的范围:
    • StandardMessageCodec
    • JSONMessageCodec
    • StringCodec
    • BinaryCodec

修改 MyFlutterView :

class MyFlutterView(context: Context, messenger: BinaryMessenger, viewId: Int, args: Map<String, Any>?) : PlatformView {

    val textView: TextView = TextView(context)

    init {
        args?.also {
            textView.text = it["text"] as String
        }
    }

    override fun getView(): View {

        return textView
    }

    override fun dispose() {
        TODO("Not yet implemented")
    }
}
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最终效果:

# Flutter 向 Android View 发送消息

修改 Flutter 端,创建 MethodChannel 用于通信:

class PlatformViewDemo extends StatefulWidget {
  
  _PlatformViewDemoState createState() => _PlatformViewDemoState();
}

class _PlatformViewDemoState extends State<PlatformViewDemo> {
  static const platform =
      const MethodChannel('com.flutter.guide.MyFlutterView');

  
  Widget build(BuildContext context) {
    Widget platformView() {
      if (defaultTargetPlatform == TargetPlatform.android) {
        return AndroidView(
          viewType: 'plugins.flutter.io/custom_platform_view',
          creationParams: {'text': 'Flutter传给AndroidTextView的参数'},
          creationParamsCodec: StandardMessageCodec(),
        );
      }
    }

    return Scaffold(
      appBar: AppBar(),
      body: Column(children: [
        RaisedButton(
          child: Text('传递参数给原生View'),
          onPressed: () {
            platform.invokeMethod('setText', {'name': 'laomeng', 'age': 18});
          },
        ),
        Expanded(child: platformView()),
      ]),
    );
  }
}
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在 原生View 中也创建一个 MethodChannel 用于通信:

class MyFlutterView(context: Context, messenger: BinaryMessenger, viewId: Int, args: Map<String, Any>?) : PlatformView, MethodChannel.MethodCallHandler {

    val textView: TextView = TextView(context)
    private var methodChannel: MethodChannel

    init {
        args?.also {
            textView.text = it["text"] as String
        }
        methodChannel = MethodChannel(messenger, "com.flutter.guide.MyFlutterView")
        methodChannel.setMethodCallHandler(this)
    }

    override fun getView(): View {

        return textView
    }

    override fun dispose() {
        methodChannel.setMethodCallHandler(null)
    }

    override fun onMethodCall(call: MethodCall, result: MethodChannel.Result) {
        if (call.method == "setText") {
            val name = call.argument("name") as String?
            val age = call.argument("age") as Int?

            textView.text = "hello,$name,年龄:$age"
        } else {
            result.notImplemented()
        }
    }
}
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# Flutter 向 Android View 获取消息

与上面发送信息不同的是,Flutter 向原生请求数据,原生返回数据到 Flutter 端,修改 MyFlutterView onMethodCall

override fun onMethodCall(call: MethodCall, result: MethodChannel.Result) {
    if (call.method == "setText") {
        val name = call.argument("name") as String?
        val age = call.argument("age") as Int?
        textView.text = "hello,$name,年龄:$age"
    } else if (call.method == "getData") {
        val name = call.argument("name") as String?
        val age = call.argument("age") as Int?

        var map = mapOf("name" to "hello,$name",
                "age" to "$age"
        )
        result.success(map)
    } else {
        result.notImplemented()
    }
}
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result.success(map) 是返回的数据。

Flutter 端接收数据:

var _data = '获取数据';

RaisedButton(
  child: Text('$_data'),
  onPressed: () async {
    var result = await platform
        .invokeMethod('getData', {'name': 'laomeng', 'age': 18});
    setState(() {
      _data = '${result['name']},${result['age']}';
    });
  },
),
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# 解决多个原生View通信冲突问题

当然页面有3个原生View,

class PlatformViewDemo extends StatefulWidget {
  
  _PlatformViewDemoState createState() => _PlatformViewDemoState();
}

class _PlatformViewDemoState extends State<PlatformViewDemo> {
  static const platform =
      const MethodChannel('com.flutter.guide.MyFlutterView');

  var _data = '获取数据';

  
  Widget build(BuildContext context) {
    Widget platformView() {
      if (defaultTargetPlatform == TargetPlatform.android) {
        return AndroidView(
          viewType: 'plugins.flutter.io/custom_platform_view',
          creationParams: {'text': 'Flutter传给AndroidTextView的参数'},
          creationParamsCodec: StandardMessageCodec(),
        );
      }
    }

    return Scaffold(
      appBar: AppBar(),
      body: Column(children: [
        Row(
          children: [
            RaisedButton(
              child: Text('传递参数给原生View'),
              onPressed: () {
                platform
                    .invokeMethod('setText', {'name': 'laomeng', 'age': 18});
              },
            ),
            RaisedButton(
              child: Text('$_data'),
              onPressed: () async {
                var result = await platform
                    .invokeMethod('getData', {'name': 'laomeng', 'age': 18});
                setState(() {
                  _data = '${result['name']},${result['age']}';
                });
              },
            ),
          ],
        ),
        Expanded(child: Container(color: Colors.red, child: platformView())),
        Expanded(child: Container(color: Colors.blue, child: platformView())),
        Expanded(child: Container(color: Colors.yellow, child: platformView())),
      ]),
    );
  }
}
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此时点击 传递参数给原生View 按钮哪个View会改变内容,实际上只有最后一个会改变。

如何改变指定View的内容?重点是 MethodChannel,只需修改上面3个通道的名称不相同即可:

  • 第一种方法:将一个唯一 id 通过初始化参数传递给原生 View,原生 View使用这个id 构建不同名称的 MethodChannel
  • 第二种方法(推荐):原生 View 生成时,系统会为其生成唯一id:viewId,使用 viewId 构建不同名称的 MethodChannel

原生 View 使用 viewId 构建不同名称的 MethodChannel

class MyFlutterView(context: Context, messenger: BinaryMessenger, viewId: Int, args: Map<String, Any>?) : PlatformView, MethodChannel.MethodCallHandler {

    val textView: TextView = TextView(context)
    private var methodChannel: MethodChannel

    init {
        args?.also {
            textView.text = it["text"] as String
        }
        methodChannel = MethodChannel(messenger, "com.flutter.guide.MyFlutterView_$viewId")
        methodChannel.setMethodCallHandler(this)
    }
  ...
}
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Flutter 端为每一个原生 View 创建不同的MethodChannel

var platforms = [];

AndroidView(
  viewType: 'plugins.flutter.io/custom_platform_view',
  onPlatformViewCreated: (viewId) {
    print('viewId:$viewId');
    platforms
        .add(MethodChannel('com.flutter.guide.MyFlutterView_$viewId'));
  },
  creationParams: {'text': 'Flutter传给AndroidTextView的参数'},
  creationParamsCodec: StandardMessageCodec(),
)
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给第一个发送消息:

platforms[0]
    .invokeMethod('setText', {'name': 'laomeng', 'age': 18});
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