Flutter 中的 Event Channel 和 Method Channel 如何理解

在Flutter中，Event Channel和Method Channel是用于原生平台（如iOS和Android）与Flutter应用之间进行通信的两种重要机制。它们各自有不同的用途和实现方式。

Method Channel

Method Channel主要用于从Flutter端调用原生平台的方法，或者从原生平台调用Flutter端的方法。它支持同步和异步调用。以下是一个简单的示例，展示了如何在Flutter和原生平台之间通过Method Channel进行通信。

Flutter端代码

首先，在Flutter中定义一个MethodChannel：

import 'package:flutter/services.dart';

class MyAppChannel {
  static const MethodChannel _channel = MethodChannel('com.example.myapp/channel');

  static Future<String?> getPlatformVersion() async {
    final String? version = await _channel.invokeMethod('getPlatformVersion');
    return version;
  }
}

iOS端代码

在iOS中，你需要在你的AppDelegate或SceneDelegate中设置并处理这个Channel：

import UIKit
import Flutter

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    GeneratedPluginRegistrant.register(with: self)
    
    let controller : FlutterViewController = window?.rootViewController as! FlutterViewController
    let channel = FlutterMethodChannel(name: "com.example.myapp/channel", binaryMessenger: controller)
    channel.setMethodCallHandler({
      (call: FlutterMethodCall, result: @escaping FlutterResult) in
      if call.method == "getPlatformVersion" {
        let version = UIDevice.current.systemVersion
        result(version)
      } else {
        result(FlutterMethodNotImplemented)
      }
    })
    
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}

Android端代码

在Android中，你需要在你的MainActivity中设置并处理这个Channel：

import io.flutter.embedding.android.FlutterActivity;
import io.flutter.embedding.engine.FlutterEngine;
import io.flutter.plugin.common.MethodChannel;
import android.os.Build;
import android.os.Bundle;

public class MainActivity extends FlutterActivity {
    private static final String CHANNEL = "com.example.myapp/channel";

    @Override
    public void configureFlutterEngine(FlutterEngine flutterEngine) {
        super.configureFlutterEngine(flutterEngine);
        new MethodChannel(flutterEngine.getDartExecutor().getBinaryMessenger(), CHANNEL)
            .setMethodCallHandler(
                (call, result) -> {
                    if (call.method.equals("getPlatformVersion")) {
                        String version = Build.VERSION.RELEASE;
                        result.success(version);
                    } else {
                        result.notImplemented();
                    }
                }
            );
    }
}

Event Channel

Event Channel主要用于从原生平台向Flutter端发送连续的事件流。以下是一个简单的示例，展示了如何使用Event Channel。

Flutter端代码

在Flutter中定义一个EventChannel：

import 'package:flutter/services.dart';

class MyEventChannel {
  static const EventChannel _channel = EventChannel('com.example.myapp/event_channel');

  static Stream<dynamic> subscribe() {
    return _channel.receiveBroadcastStream();
  }
}

iOS端代码

在iOS中，你需要设置并发送事件：

// 在上面的AppDelegate中继续添加
let eventChannel = FlutterEventChannel(name: "com.example.myapp/event_channel", binaryMessenger: controller)
eventChannel.setStreamHandler({
  (eventSink, errorSink) in
  // 模拟发送事件
  DispatchQueue.global(qos: .background).async {
    for i in 0..<10 {
      sleep(1)
      eventSink?(i)
    }
  }
})

Android端代码

在Android中，你需要设置并发送事件：

// 在上面的MainActivity中继续添加
new EventChannel(flutterEngine.getDartExecutor().getBinaryMessenger(), "com.example.myapp/event_channel")
    .setStreamHandler(
        new EventChannel.StreamHandler() {
            @Override
            public void onListen(Object arguments, EventChannel.EventSink events) {
                new Thread(() -> {
                    for (int i = 0; i < 10; i++) {
                        try {
                            Thread.sleep(1000);
                            events.success(i);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }).start();
            }

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

通过上述代码示例，你可以更好地理解Flutter中的Event Channel和Method Channel。Method Channel主要用于单次或偶发的请求，而Event Channel则用于持续的事件流。