如何使用 Flutter MethodChannel 从原生 swift 代码调用 dart 代码中的方法？

Question

我已经查看了很多关于这个主题的类似问题，但没有一个解决方案对我有用。我正在 Flutter 中开发一个应用程序，但想从 AppDelegate.swift 调用我的 main.dart 文件中的特定方法在原生 iOS 项目中。

为了删除所有其他变量，我已将问题提取到一个新的 dart 项目中。我正在尝试使用methodChannel.invokeMethod() 从AppDelegate.swift 调用setChannelText()，但没有成功。

有人知道我哪里出错了吗？我知道我没有对methodChannel.invokeMethod() 中的“名称”参数采取行动，但那是因为我只希望调用调用该方法...

这是我的 main.dart 文件：

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  MethodChannel channel =
      new MethodChannel("com.example.channeltest/changetext");
  String centerText;
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        backgroundColor: Colors.purple,
        body: Center(
          child: Text(
            centerText,
            style: TextStyle(
              color: Colors.white,
              fontWeight: FontWeight.bold,
              fontSize: 30.0,
            ),
          ),
        ),
      ),
    );
  }

  @override
  void initState() {
    super.initState();
    this.channel.setMethodCallHandler((call) async => await setChannelText());
    this.centerText = "Hello World!";
  }

  Future setChannelText() async {
    Future.delayed(Duration(milliseconds: 200));
    setState(() => this.centerText = "Another Text.");
  }
}

这是我的 AppDelegate.swift 文件：

import UIKit
import Flutter

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
    var methodChannel: FlutterMethodChannel!
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions:                 

[UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    
    let rootViewController : FlutterViewController = window?.rootViewController as! FlutterViewController
    methodChannel = FlutterMethodChannel(name: "com.example.channeltest/changetext", binaryMessenger: rootViewController as! FlutterBinaryMessenger)
    
    //This call would obviously be somewhere else in a real world example, but I'm just
    //testing if I can invoke the method in my dart code at all..
    methodChannel.invokeMethod("some_method_name", arguments: nil)
    
    GeneratedPluginRegistrant.register(with: self)
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}

最后，我试图在启动后立即更改文本，但事实并非如此。

Screenshot of app running on iOS simulator

提前感谢您的帮助！

Answer 1

问题

问题是您的平台端（在本例中为 iOS）正在调用 Flutter 端的方法在 Flutter 准备好之前。无法从平台端进行检查，因此您的 Flutter 应用必须告诉您的平台端。您在 Android 上也会遇到同样的问题。

解决方案

要克服这个问题，您必须告诉平台方应用程序已准备就绪（通过发送平台方法）并将其保存在布尔值中，或实例化一个类并调用一个方法。然后平台端就可以开始发送消息了。

您真的应该阅读日志，它应该会警告您以下内容：“没有任何东西在听这个，或者 Flutter 引擎未连接”。

import 'dart:async';

import 'package:flutter/src/services/platform_channel.dart';

class StringService {
  final methodChannel =
      const MethodChannel("com.example.app_name.method_channel.strings");

  final StreamController<String> _stringStreamController =
      StreamController<String>();

  Stream<String> get stringStream => _stringStreamController.stream;

  StringService() {
    // Set method call handler before telling platform side we are ready to receive.
    methodChannel.setMethodCallHandler((call) async {
      print('Just received ${call.method} from platform');
      if (call.method == "new_string") {
        _stringStreamController.add(call.arguments as String);
      } else {
        print("Method not implemented: ${call.method}");
      }
    });
    // Tell platform side we are ready!
    methodChannel.invokeMethod("isReady");
  }
}

您可以在reverse_platform_methods，尤其是AppDelegate.swift 看到一个工作项目。我没有为 Android 实现它，但您可以在 MainActivity.kt 中以类似的方式实现它。

问题

大多数应用不希望代码首先从平台端调用。 您的用例是什么？根据您的回答，我可能会提供更好的建议。我实现了这个来处理推送到设备的推送通知，所以“事件”肯定是从平台端触发的。

此外，如果您遇到错误和警告，您应该显示它们，例如No implementation found for method $method on channel $name'.

Answer 2

嗯，问题在于初始化过程。在 dart/flutter 部分准备好处理它之前，您尝试从 swift 代码中调用您的方法。

您必须执行以下步骤才能获得结果：

1. 重要。在你的AppDelegate for ios 中使用applicationDidBecomeActive 方法

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
    
 var methodChannel: FlutterMethodChannel? = nil

  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {

    print("Setup methodChannel from Swift")
    let rootViewController : FlutterViewController = window?.rootViewController as! FlutterViewController
    methodChannel = FlutterMethodChannel(name: "com.example.channeltest/changetext", binaryMessenger: rootViewController as! FlutterBinaryMessenger)

    GeneratedPluginRegistrant.register(with: self)
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
    
    //THIS METHOD
    override func applicationDidBecomeActive(_ application: UIApplication) {
        methodChannel?.invokeMethod("some_method_name", arguments: "ios string")
    }
}

对于安卓onStart()方法：

class MainActivity : FlutterActivity() {
    var channel: MethodChannel? = null

    override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
        super.configureFlutterEngine(flutterEngine)

        channel = MethodChannel(
            flutterEngine.dartExecutor.binaryMessenger,
            "com.example.channeltest/changetext"
        )

    }

    override fun onStart() {
        super.onStart()
        channel?.invokeMethod("some_method_name", "android str")
    }
}

1. 使用MethodChannel 创建您自己的课程（如上一个答案）

class TestChannel {
  static MethodChannel channel =
  const MethodChannel("com.example.channeltest/changetext");

  final StreamController<String> _controller =
  StreamController<String>();

  Stream<String> get stringStream => _controller.stream;

  TestChannel() {
    channel.setMethodCallHandler((call) async {
      if (call.method == "some_method_name") {
        _controller.add(call.arguments as String);
      } else {
        print("Method not implemented: ${call.method}");
      }
    });
  }
}

1. 重要。创建它的全局实例

final _changeTextChannel = TestChannel(); //<--- like this

void main() {
  runApp(MyApp());
}

1. 在 UI 中处理

class TestPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
          child: StreamBuilder<String>(
        stream: _changeTextChannel.stringStream,
        builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
          if (snapshot.hasError) {
            return Text("Error");
          }

          if (!snapshot.hasData) {
            return Text("Loading");
          }

          return Text(snapshot.data ?? "NO_DATA");
        },
      )),
    );
  }
}

Answer 3

Flutter 端代码：

import 'dart:async';

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

class _MyHomePageState extends State<MyHomePage> {
  static const platform = MethodChannel('samples.flutter.dev/battery');

  // Get battery level.
  String _batteryLevel = 'Unknown battery level.';

  Future<void> _getBatteryLevel() async {
    String batteryLevel;
    try {
      final int result = await platform.invokeMethod('getBatteryLevel');
      batteryLevel = 'Battery level at $result % .';
    } on PlatformException catch (e) {
      batteryLevel = "Failed to get battery level: '${e.message}'.";
    }

    setState(() {
      _batteryLevel = batteryLevel;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Material(
      child: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            ElevatedButton(
              child: Text('Get Battery Level'),
              onPressed: _getBatteryLevel,
            ),
            Text(_batteryLevel),
          ],
        ),
      ),
    );
  }
}

此处为 Swift 代码：

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    let controller : FlutterViewController = window?.rootViewController as! FlutterViewController
    let batteryChannel = FlutterMethodChannel(name: "samples.flutter.dev/battery",
                                              binaryMessenger: controller.binaryMessenger)
    batteryChannel.setMethodCallHandler({
      [weak self] (call: FlutterMethodCall, result: FlutterResult) -> Void in
      // Note: this method is invoked on the UI thread.
      guard call.method == "getBatteryLevel" else {
        result(FlutterMethodNotImplemented)
        return
      }
      self?.receiveBatteryLevel(result: result)
    })

    GeneratedPluginRegistrant.register(with: self)
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}

或者参考这个链接：

Platform Channels