在 Android 中使用可完成的期货时，调用的 Flutter 插件方法将无法完成

Question

我正在构建一个 Android 独有的 Flutter 插件，该插件将严重依赖其 Android 端的异步任务。为此，我正在尝试构建一个简单的示例，该示例使用 Java 中的 CompletableFutures 以模拟延迟测试这种通信。我用

创建了这个项目

flutter create --org org.example --template=plugin --platforms=android -a java zcspos

我的示例代码只是延迟回显：

ZcsposPlugin.java

    private CompletableFuture<Void> waitAsync(int milliseconds) {
        return CompletableFuture.runAsync(() -> {
            try {
                Thread.sleep(milliseconds);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
    }

    @Override
    public void onMethodCall(@NonNull MethodCall call, @NonNull final Result result) {
        switch (call.method) {
            case "echo": {
                final String value = call.argument("value");
                this.waitAsync(1000).thenRunAsync(() -> { // wait 1 second
                    Log.d(this.TAG, "Echoing '" + value + "'");
                    result.success(value);
                });
                break;
            }
            default:
                result.notImplemented();
        }
    }

zcspos.dart

  static Future<String?> echo(String value) async {
    return await _channel.invokeMethod<String>('echo', <String, dynamic>{'value': value});
  }

ma​​in.dart

  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Plugin example app'),
        ),
        body: Container(
          margin: EdgeInsets.all(8),
          child: Center(
            child: Column(
              children: [
                RawMaterialButton(
                    padding: EdgeInsets.all(8),
                    fillColor: Colors.blue,
                    highlightColor: Colors.blueAccent,
                    child: Text(
                      "ECHO TEST",
                      style: TextStyle(
                        fontWeight: FontWeight.bold,
                        color: Colors.white,
                      ),
                    ),
                    onPressed: () async {
                      try {
                        print(await Zcspos.echo("Echo 1"));
                        print(await Zcspos.echo("Echo 2"));
                        print(await Zcspos.echo("Echo 3"));
                      } catch (e) {
                        print(e.toString());
                      }
                    }),
              ],
            ),
          ),
        ),
      ),
    );
  }

输出只有：

Echoing 'Echo 1'

但应该是：

Echoing 'Echo 1'
Echo 1
Echoing 'Echo 2'
Echo 2
Echoing 'Echo 3'
Echo 3

我调试了 Android 代码，它正确地调用了result.success：

我该如何解决这个问题？

Answer 1

在编写问题时得到了解决方案，因此它可能对将来的某人有用。

结果应该在主线程上实现，它不会在不同的线程上工作。一种可能的解决方案是make sure to have access to the Activity，然后您可以简单地将result.success 包装成runOnUiThread()：

Zcspos.java

public class ZcsposPlugin implements FlutterPlugin, MethodCallHandler, ActivityAware {
    private Activity activity;
    private MethodChannel channel;

    @Override
    public void onAttachedToActivity(ActivityPluginBinding binding) {
        this.activity = binding.getActivity();
    }

    @Override
    public void onReattachedToActivityForConfigChanges(ActivityPluginBinding binding) {
        this.activity = binding.getActivity();
    }

    @Override
    public void onAttachedToEngine(@NonNull FlutterPluginBinding flutterPluginBinding) {
        channel = new MethodChannel(flutterPluginBinding.getBinaryMessenger(), "zcspos");
        channel.setMethodCallHandler(this);
    }

    private CompletableFuture<Void> waitAsync(int milliseconds) {
        return CompletableFuture.runAsync(() -> {
            try {
                Thread.sleep(milliseconds);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
    }

    @Override
    public void onMethodCall(@NonNull MethodCall call, @NonNull final Result result) {
        switch (call.method) {
            case "echo": {
                final String value = call.argument("value");
                this.waitAsync(1000).thenRunAsync(() -> {
                    this.activity.runOnUiThread(() -> { // <- HERE WE GO
                        Log.d(this.TAG, "Echoing '" + value + "'");
                        result.success(value);
                    });
                });
                break;
            }
            default:
                result.notImplemented();
        }
    }

    @Override
    public void onDetachedFromEngine(@NonNull FlutterPluginBinding binding) {
        channel.setMethodCallHandler(null);
    }

    @Override
    public void onDetachedFromActivityForConfigChanges() {
        this.activity = null;
    }

    @Override
    public void onDetachedFromActivity() {
        this.activity = null;
    }

不确定这是否是最好的解决方案，但它就像魅力一样。