Advanced Topics

Once you've grappled the fundamentals of creating a user interface, you can explore some advanced features for creating a more complex application interface.

• Define an event listener and register it with the View. More often than not, this is how you'll listen for events. The View class contains a collection of nested interfaces named On<something>Listener, each with a callback method called On<something> () . For example, View.OnClickListener (for handling "clicks" on a View), View.OnTouchListener (for handling touch screen events in a View), and View.OnKeyListener (for handling device key presses within a View). So if you want your View to be notified when it is "clicked" (such as when a button is selected), implement OnClickListener and define its onClick() callback method (where you perform the action upon click), and register it to the View with setOnClickListener() .

• Override an existing callback method for the View. This is what you should do when you've implemented your own View class and want to listen for specific events that occur within it. Example events you can handle include when the screen is touched (onTouchEvent() ), when the trackball is moved (onTrackballEvent()), or when a key on the device is pressed (onKeyDown()). This allows you to define the default behavior for each event inside your custom View and determine whether the event should be passed on to some other child

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