解here
实现ActivityAware，其中一个方法是

override fun onAttachedToActivity(binding: ActivityPluginBinding) {
    this.activity = binding.activity;
  }

注：

你可以通过实现ActivityAware接口来获取对Activity的引用但是如果你在onAttachToEngine()中使用setMethodCallHandler(...)方法，onAttachToActivity()将永远不会被调用，你也永远无法访问Activity
看看下面的例子

**什么不起作用：**在下面的示例中，onAttachToActivity()从未被调用

class AndroidLongTaskPlugin : FlutterPlugin, ActivityAware {
  private var activity: FlutterActivity? = null

  

  override fun onAttachedToEngine(@NonNull flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
    //activity is null here 
    //also onAttachToActivity will never be called because we are calling setMethodHandler here
    channel = MethodChannel(binaryMessenger, CHANNEL_NAME)
    channel.setMethodCallHandler { call, result ->
        //our code
    }

  }

  override fun onDetachedFromEngine(@NonNull binding: FlutterPlugin.FlutterPluginBinding) {
    channel?.setMethodCallHandler(null)
  }

  override fun onAttachedToActivity(binding: ActivityPluginBinding) {
    activity = binding.activity as FlutterActivity
  }

  //rest of the methods
}

以下是一个工作示例：

class MyPlugin : FlutterPlugin, ActivityAware {
  private var activity: FlutterActivity? = null
  private var binaryMessenger: BinaryMessenger? = null

  override fun onAttachedToEngine(@NonNull flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
    binaryMessenger = flutterPluginBinding.binaryMessenger
  }

  override fun onDetachedFromEngine(@NonNull binding: FlutterPlugin.FlutterPluginBinding) {
    Log.d("DART/NATIVE", "onDetachedFromEngine")
    channel?.setMethodCallHandler(null)
  }

  override fun onAttachedToActivity(binding: ActivityPluginBinding) {
    Log.d("DART/NATIVE", "onAttachedToActivity")
    activity = binding.activity as FlutterActivity
    //here we have access to activity
    //also make sure to setMethodCallHandler here
    channel.setMethodCallHandler { call, result ->
        //our code
    }
  }

  //rest of the methods
}

在最新版本Flutter 3.0.5+中，这是我的工作：
确保在类实现中添加ActivityAware

import io.flutter.embedding.engine.plugins.FlutterPlugin;
import io.flutter.plugin.common.MethodCall;
import io.flutter.plugin.common.MethodChannel;
import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
import io.flutter.plugin.common.MethodChannel.Result;
import io.flutter.embedding.engine.plugins.activity.ActivityAware;
import io.flutter.embedding.engine.plugins.activity.ActivityPluginBinding;

public class FlutterCustomPlugin implements FlutterPlugin, MethodCallHandler, ActivityAware {

    private MethodChannel channel;
    private Activity activity;

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

  @Override
    public void onAttachedToActivity(ActivityPluginBinding activityPluginBinding) {
        // TODO: your plugin is now attached to an Activity
        this.activity = activityPluginBinding.getActivity();
    }

    @Override
    public void onDetachedFromActivityForConfigChanges() {
        // TODO: the Activity your plugin was attached to was destroyed to change configuration.
        // This call will be followed by onReattachedToActivityForConfigChanges().
    }

    @Override
    public void onReattachedToActivityForConfigChanges(ActivityPluginBinding activityPluginBinding) {
        // TODO: your plugin is now attached to a new Activity after a configuration change.
    }

    @Override
    public void onDetachedFromActivity() {
        // TODO: your plugin is no longer associated with an Activity. Clean up references.
    }

}

记住添加所有4个覆盖方法

在2023年，你可以这样做：
Kotlin代码：

class MainActivity: FlutterActivity() {
     override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
        super.configureFlutterEngine(flutterEngine)

        Global_Variable.current_activity = this.activity;
        Global_Variable.application_content = this.applicationContext;

        ...
    }
}

Java代码：

import io.flutter.embedding.android.FlutterActivity;
import io.flutter.embedding.engine.FlutterEngine;

public class MainActivity extends FlutterActivity {

    @Override
    public void configureFlutterEngine(FlutterEngine flutterEngine) {
        super.configureFlutterEngine(flutterEngine);

        // Get a reference to the Activity
        FlutterActivity mainActivity = (FlutterActivity) this;

        // Do something with the Activity
        // ...
    }
}

在我的情况下，alireza-easazade的分辨率不起作用。我从onAttachedToEngine删除了MethodChannel创建，但ActivityAware通知仍然没有出现。对我有用的是用flutterPluginBinding.getFlutterEngine().getDartExecutor()替换flutterPluginBinding.getBinaryMessenger()作为MethodChannel构造函数中的参数。有一次我做了这个onAttachedToActivity 在onAttachedToEngine之后立即调用。
下面是我的工作示例：

public class MyPlugin implements FlutterPlugin, MethodCallHandler, ActivityAware {

  private ActivityPluginBinding _activityBinding;
  private FlutterPluginBinding _flutterBinding;
  private MethodChannel _channel;

  // FlutterPlugin  overrides

  @Override
  public void onAttachedToEngine(@NonNull FlutterPluginBinding flutterPluginBinding) {
    _flutterBinding = flutterPluginBinding;
    _channel = new MethodChannel(flutterPluginBinding.getFlutterEngine().getDartExecutor(), "com.example.test/myplugin");
    _channel.setMethodCallHandler(this);
  }

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

  // ActivityAware  overrides

  @Override
  public void onAttachedToActivity​(ActivityPluginBinding binding) {
    _activityBinding = binding;
  }

  @Override
  public void   onDetachedFromActivity() {
    _activityBinding = null;
  }

  @Override
  public void   onReattachedToActivityForConfigChanges​(ActivityPluginBinding binding) {
    _activityBinding = binding;
  }

  @Override
  public void   onDetachedFromActivityForConfigChanges() {
    _activityBinding = null;
  }

  // MethodCallHandler overrides
  
  @Override
  public void onMethodCall(@NonNull MethodCall call, @NonNull Result result) {
    // Handle calls
  }

  // Implementation

  public Context getApplicationContext() {
    return (_flutterBinding != null) ? _flutterBinding.getApplicationContext() : null;
  }

  public Activity getActivity() {
    return (_activityBinding != null) ? _activityBinding.getActivity() : null;
  }
}