How to Define Callbacks in Android

Question

During the most recent Google IO  there was a presentation about implementing restful client applications  Unfortunately  it was only a high level discussion with no source code of the implementation   In this diagram  on the return path there are various different callbacks to other methods      How do I declare what these methods are    I understand the idea of a callback - a piece of code that gets called after a certain event has happened  but I don t know how to implement it  The only way I ve implemented callbacks so far have been overriding various methods  onActivityResult for example    I feel like I have a basic understanding of the design pattern  but I keep on getting tripped up on how to handle the return path

User · Answer

No need to define a new interface when you can use an existing one  android os Handler Callback  Pass an object of type Callback  and invoke callback s handleMessage Message msg

User · Answer

You can also use LocalBroadcast for this purpose  Here is a quick guide Create a broadcast receiver     LocalBroadcastManager getInstance this  registerReceiver              mMessageReceiver  new IntentFilter  quot speedExceeded quot      private BroadcastReceiver mMessageReceiver   new BroadcastReceiver          Override     public void onReceive Context context  Intent intent            String action   intent getAction            Double currentSpeed   intent getDoubleExtra  quot currentSpeed quot   20           Double currentLatitude   intent getDoubleExtra  quot latitude quot   0           Double currentLongitude   intent getDoubleExtra  quot longitude quot   0                   react to local broadcast message        This is how you can trigger it Intent intent   new Intent  quot speedExceeded quot    intent putExtra  quot currentSpeed quot   currentSpeed   intent putExtra  quot latitude quot   latitude   intent putExtra  quot longitude quot   longitude   LocalBroadcastManager getInstance this  sendBroadcast intent    unRegister receiver in onPause  protected void onPause       super onPause      LocalBroadcastManager getInstance this  unregisterReceiver mMessageReceiver

User · Answer

Example to implement callback method using interface   Define the interface  NewInterface java   package javaapplication1   public interface NewInterface       void callback        Create a new class  NewClass java  It will call the callback method in main class   package javaapplication1   public class NewClass        private NewInterface mainClass       public NewClass NewInterface mClass           mainClass   mClass             public void calledFromMain              Do somthing               call back main         mainClass callback              The main class  JavaApplication1 java  to implement the interface NewInterface - callback   method  It will create and call NewClass object  Then  the NewClass object will callback it s callback   method in turn   package javaapplication1  public class JavaApplication1 implements NewInterface       NewClass newClass       public static void main String   args             System out println  test                JavaApplication1 myApplication   new JavaApplication1            myApplication doSomething                private void doSomething            newClass   new NewClass this           newClass calledFromMain                Override     public void callback             System out println  callback

User · Answer

In many cases  you have an interface and pass along an object that implements it  Dialogs for example have the OnClickListener   Just as a random example      The callback interface interface MyCallback       void callbackCall          The class that takes the callback class Worker      MyCallback callback      void onEvent           callback callbackCall               Option 1   class Callback implements MyCallback      void callbackCall              callback code goes here         worker callback   new Callback        Option 2   worker callback   new MyCallback         void callbackCall              callback code goes here           I probably messed up the syntax in option 2  It s early

User · Answer

When something happens in my view I fire off an event that my activity is listening for      DECLARED IN  CUSTOM  VIEW      private OnScoreSavedListener onScoreSavedListener      public interface OnScoreSavedListener           public void onScoreSaved                 ALLOWS YOU TO SET LISTENER  amp  amp  INVOKE THE OVERIDING METHOD         FROM WITHIN ACTIVITY     public void setOnScoreSavedListener OnScoreSavedListener listener            onScoreSavedListener   listener             DECLARED IN ACTIVITY      MyCustomView slider    MyCustomView  view findViewById R id slider      slider setOnScoreSavedListener new OnScoreSavedListener              Override         public void onScoreSaved                 Log v     EVENT FIRED                        If you want to know more about communication  callbacks  between fragments see here  http   developer android com guide components fragments html CommunicatingWithActivity

User · Answer

to clarify a bit on dragon s answer  since it took me a while to figure out what to do with Handler Callback    Handler can be used to execute callbacks in the current or another thread  by passing it Messages  the Message holds data to be used from the callback  a Handler Callback can be passed to the constructor of Handler in order to avoid extending Handler directly  thus  to execute some code via callback from the current thread   Message message   new Message     lt set data to be passed to callback - eg message obj  message arg1 etc - here gt   Callback callback   new Callback         public boolean handleMessage Message msg             lt code to be executed during callback gt            Handler handler   new Handler callback   handler sendMessage message     EDIT  just realized there s a better way to get the same result  minus control of exactly when to execute the callback    post new Runnable          Override     public void run              lt code to be executed during callback gt

[android] How to Define Callbacks in Android?

Examples related to android

Examples related to callback