Building responsive apps with Handler

The Handler class is fundamental to the infrastructure of Android apps—together with Looper. It underpins everything that the main thread does—including the invocation of the Activity lifecycle methods.

While Looper takes care of dispatching work on its message-loop thread, Handler provides the means to add work to the message queue belonging to a Looper.

We can create a Handler to submit work to be processed on the main thread simply by creating a new instance of Handler from an Activity lifecycle method such as onCreate, shown as follows:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Handler handler = new Handler();
    // …
}

We could also be explicit that we want to submit work to the main thread by passing a reference to the main Looper instance into the Handler constructor.

Handler handler = new Handler(Looper.getMainLooper());

Exactly what we mean by "work" can be described by subclasses of java.lang...