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.

Answer 1

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.