Android
Schnittstellen

Benutzerdefinierter Listener

Schnittstelle definieren

//In this interface, you can define messages, which will be send to owner.
public interface MyCustomListener {
    //In this case we have two messages, 
    //the first that is sent when the process is successful.
    void onSuccess(List<Bitmap> bitmapList);
    //And The second message, when the process will fail.
    void onFailure(String error);
}

Listener erstellen

Im nächsten Schritt müssen wir eine Instanzvariable im Objekt definieren, die den Callback über MyCustomListener . Und fügen Sie Setter für unseren Hörer hinzu.

public class SampleClassB {
    private MyCustomListener listener;

    public void setMyCustomListener(MyCustomListener listener) {
        this.listener = listener;
    }
}

Listener implementieren

In einer anderen Klasse können wir jetzt eine Instanz von SampleClassB erstellen.

public class SomeActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        SampleClassB sampleClass = new SampleClassB();
    }
}

Als nächstes können wir unseren Listener auf zwei Arten auf sampleClass :

implementiert in unserer Klasse MyCustomListener :

public class SomeActivity extends Activity implements MyCustomListener {
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        SampleClassB sampleClass = new SampleClassB();
        sampleClass.setMyCustomListener(this);
    }

    @Override
    public void onSuccess(List<Bitmap> bitmapList) {

    }

    @Override
    public void onFailure(String error) {

    }
}

oder einfach eine anonyme innere Klasse instanziieren:

public class SomeActivity extends Activity {
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        SampleClassB sampleClass = new SampleClassB();
        sampleClass.setMyCustomListener(new MyCustomListener() {

            @Override
            public void onSuccess(List<Bitmap> bitmapList) {

            }

            @Override
            public void onFailure(String error) {

            }
        });
    }
}

Trigger Listener

public class SampleClassB {
    private MyCustomListener listener;

    public void setMyCustomListener(MyCustomListener listener) {
        this.listener = listener;
    }

    public void doSomething() {
        fetchImages();
    }

    private void fetchImages() {
        AsyncImagefetch imageFetch = new AsyncImageFetch();
        imageFetch.start(new Response<Bitmap>() {
            @Override
            public void onDone(List<Bitmap> bitmapList, Exception e) {
                //do some stuff if needed

                //check if listener is set or not.
                if(listener == null)
                    return;
                //Fire proper event. bitmapList or error message will be sent to
                //class which set listener.
                if(e == null)
                    listener.onSuccess(bitmapList);
                else
                    listener.onFailure(e.getMessage());
            }
        });
    }
}

Grundlegender Hörer

Das "Listener" - oder "Observer" -Muster ist die häufigste Strategie zum Erstellen asynchroner Rückrufe in der Android-Entwicklung.

public class MyCustomObject {       
  
  //1 - Define the interface 
  public interface MyCustomObjectListener {
      public void onAction(String action);
  }

  //2 - Declare your listener object
  private MyCustomObjectListener listener;

  // and initialize it in the costructor
  public MyCustomObject() {        
    this.listener = null; 
 }

 //3 - Create your listener setter
 public void setCustomObjectListener(MyCustomObjectListener listener) {
    this.listener = listener;
 }

 // 4 - Trigger listener event
 public void makeSomething(){
    if (this.listener != null){
       listener.onAction("hello!");
 }        
}

Nun zu Ihrer Aktivität:

public class MyActivity extends Activity {
   public final String TAG = "MyActivity";

  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.main_activity);
        

      MyCustomObject mObj = new MyCustomObject();
    
      //5 - Implement listener callback
      mObj.setCustomObjectListener(new MyCustomObjectListener() {
        @Override
          public void onAction(String action) {
              Log.d(TAG, "Value: "+action);
          }
      });
   }
}