Clearly, one of the key rules of Android development is to never perform time-consuming operations on the main thread of an application. The second, equally important, rule is that the code within a separate thread must never, under any circumstances, directly update any aspect of the user interface.
Any changes to the user interface must always be performed from within the main thread. The reason for this is that the Android UI toolkit is not thread-safe. Attempts to work with non-thread-safe code from within multiple threads will typically result in intermittent problems and unpredictable application behavior.
In the event that the code executing in a thread needs to interact with the user interface, it must do so by synchronizing with the main UI thread. This is achieved by creating a handler within the main thread, which, in turn, receives messages from another thread and updates the user interface accordingly.
