Communicating with the UI Thread with Handlers

Every app has its own special thread that runs UI objects such as View objects; this thread is called the UI thread. Only objects running on the UI thread have access to other objects on that thread. Because tasks that you run on a thread from a thread pool aren't running on your UI thread, they don't have access to UI objects. To move data from a background thread to the UI thread, use a Handler that's running on the UI thread

Define a Handler on the UI Thread

Handler is part of the Android system's framework for managing threads. A Handler object receives messages and runs code to handle the messages. Normally, you create a Handler for a new thread, but you can also create a Handler that's connected to an existing thread. When you connect a Handler to your UI thread, the code that handles messages runs on the UI thread.

Instantiate the Handler object in the constructor for the class that creates your thread pools, and store the object in a global variable. Connect it to the UI thread by instantiating it with the Handler(Looper) constructor. This constructor uses a Looper object, which is another part of the Android system's thread management framework. When you instantiate a Handler based on a particular Looper instance, the Handler runs on the same thread as the Looper. For example:

private PhotoManager() {
...
    // Defines a Handler object that's attached to the UI thread
    mHandler = new Handler(Looper.getMainLooper()) {
    ...

Inside the Handler, override the handleMessage() method. The Android system invokes this method when it receives a new message for a thread it's managing; all of the Handler objects for a particular thread receive the same message. For example:

        /*
         * handleMessage() defines the operations to perform when
         * the Handler receives a new Message to process.
         */
        @Override
        public void handleMessage(Message inputMessage) {
            // Gets the image task from the incoming Message object.
            PhotoTask photoTask = (PhotoTask) inputMessage.obj;
            ...
        }
    ...
    }
}
The next section shows how to tell the Handler to move data.