Explain the method of Flutter calling Android Native in detail

Flutter calls the method of Android Native, which is implemented by MethodChannel:

On the Android side:

Create an Class to implement FlutterPlugin and MethodCallHandler interfaces Override onAttachedToEngine (), onDetachedFromEngine (), onMethodCall () In onAttachedToEngine (), MethodChannel is created according to the custom CHANNEL_NAME, and in onDetachedFromEngine, MethodChannel is released In onMethodCall, the user-defined METHOD_NAME is used to respond to the communication between invokeMethod and Native in Flutter. The code is as follows

class MethodChannelPlugin : FlutterPlugin, MethodChannel.MethodCallHandler {

  private var methodChannel: MethodChannel? = null
  private var mNoteForFlutterListener: NoteForFlutterListener? = null

  companion object {
    private const val CHANNEL_NAME = "method_channel"
    private const val METHOD_NAME = "saveNote"
    val instance: MethodChannelPlugin by lazy(mode = LazyThreadSafetyMode.SYNCHRONIZED) {
      MethodChannelPlugin()
    }
  }

  override fun onAttachedToEngine(binding: FlutterPlugin.FlutterPluginBinding) {
    methodChannel = MethodChannel(binding.binaryMessenger, CHANNEL_NAME)
    methodChannel?.setMethodCallHandler(this)
  }

  override fun onDetachedFromEngine(binding: FlutterPlugin.FlutterPluginBinding) {
    methodChannel?.setMethodCallHandler(null)
    methodChannel = null
  }

  override fun onMethodCall(call: MethodCall, result: MethodChannel.Result) {
    if (call.method == METHOD_NAME) {
      val content = call.argument<String>("content")
      mNoteForFlutterListener?.sendData(content!!)
      result.success("success")
    } else {
      result.notImplemented()
    }
  }

  fun setListener(noteForFlutterListener: NoteForFlutterListener) {
    mNoteForFlutterListener = noteForFlutterListener
  }

On the Flutter side:

Create MethodChannel from METHOD_NAME defined in Native Through MethodChannel.invokeMethod (METHOD_NAME, params), the parameter METHOD_NAME is METHOD_NAME defined in Native, and params is a passed parameter, you can communicate with Native. In onMethodCall method of Native, it is determined by call.method = = METHOD_NAME whether Flutter calls METHOD_NAME method defined in Native. The code is as follows

class NoteMainFulState extends State<NoteMainFul> {
 //flutter  And  native  Communication 
 static const _methodMessageChannel = MethodChannel("method_channel");
 TextField textField;
 TextEditingController textEditingController;

 @override
 Widget build(BuildContext context) {
  return Scaffold(
   body: Container(
    margin: EdgeInsets.fromLTRB(20.0, 60.0, 20.0, 20.0),
    child: Column(
     children: [
      Container(
       child: Column(
        children: [
         textField = TextField(
          // Remove the underline 
          decoration: InputDecoration(border: InputBorder.none),
          enabled: true,
          controller: textEditingController = NoteTextEditingController(),
          keyboardType: TextInputType.multiline,
          textInputAction: TextInputAction.newline,
          maxLines: null,
         ),
         RichText(
          text: TextSpan(),
         ),
        ],
       ),
      ),
      Container(
       color: Colors.deepPurple,
       height: 50,
       child: Row(
        mainAxisAlignment: MainAxisAlignment.spaceEvenly,
        children: [
         GestureDetector(
          child: Icon(
           Icons.forward,
          ),
          onTap: clickOut,
         )
        ],
       ),
      )
     ],
    ),
   ),
  );
 }

 void clickOut() async {
  // Call native Method to return to the home page 
  var content = textEditingController.text;
  Map<String, dynamic> map = {"content": content};
  var result = await _methodMessageChannel.invokeMethod("saveNote", map);
  print("result $result");
 }

The above is the process of Flutter calling Android Native method. Have time to analyze their implementation principle again
To be continued....
Attached, my current version of Flutter is:
Flutter 1.22.3 • channel stable • https://github.com/flutter/flutter
Framework • revision 8874f21e79 (3 months ago) • 2020-10-29 14:14:35 -0700
Engine • revision a1440ca392
Tools • Dart 2.10.3