On Android, there's more than one way to intercept the events from a user's interaction with your application.
When considering events within your user interface, the approach is to capture the events from
the specific View object that the user interacts with. The View class provides the means to do so.
Within the various View classes that you'll use to compose your layout, you may notice several public callback
methods that look useful for UI events. These methods are called by the Android framework when the
respective action occurs on that object. For instance, when a View (such as a Button) is touched,
the onTouchEvent() method is called on that object. However, in order to intercept this, you must extend
the class and override the method. Obviously, extending every View object
you want to use (just to handle an event) would be obsurd. This is why the View class also contains
a collection of nested interfaces with callbacks that you can much more easily define. These interfaces,
called
event listeners, are your ticket to capturing the user interaction with your UI.
While you will more commonly use the event listeners to listen for user interaction, there may
come a time when you do want to extend a View class, in order to build a custom component.
Perhaps you want to extend the Button
class to make something more fancy. In this case, you'll be able to define the default event behaviors for your
class using the class
event handlers.
Event Listeners
An event listener is an interface in the View class that contains a single
callback method. These methods will be called by the Android framework when the View to which the listener has
been registered is triggered by user interaction with the item in the UI.
Included in the event listener interfaces are the following callback methods:
onClick()- From View.OnClickListener.
This is called when the user either touches the item
(when in touch mode), or focuses upon the item with the navigation-keys or trackball and
presses the suitable "enter" key or presses down on the trackball.
onLongClick()- From View.OnLongClickListener.
This is called when the user either touches and holds the item (when in touch mode), or
focuses upon the item with the navigation-keys or trackball and
presses and holds the suitable "enter" key or presses and holds down on the trackball (for one second).
onFocusChange()- From View.OnFocusChangeListener.
This is called when the user navigates onto or away from the item, using the navigation-keys or trackball.
onKey()- From View.OnKeyListener.
This is called when the user is focused on the item and presses or releases a key on the device.
onTouch()- From View.OnTouchListener.
This is called when the user performs an action qualified as a touch event, including a press, a release,
or any movement gesture on the screen (within the bounds of the item).
onCreateContextMenu()- From View.OnCreateContextMenuListener.
This is called when a Context Menu is being built (as the result of a sustained "long click"). See the discussion
on context menus in Creating Menus for more information.
These methods are the sole inhabitants of their respective interface. To define one of these methods
and handle your events, implement the nested interface in your Activity or define it as an anonymous class.
Then, pass an instance of your implementation
to the respective View.set...Listener() method. (E.g., call
setOnClickListener()
and pass it your implementation of the OnClickListener.)
The example below shows how to register an on-click listener for a Button.
// Create an anonymous implementation of OnClickListener
private OnClickListener mCorkyListener = new OnClickListener() {
public void onClick(View v) {
// do something when the button is clicked
}
};
protected void onCreate(Bundle savedValues) {
...
// Capture our button from layout
Button button = (Button)findViewById(R.id.corky);
// Register the onClick listener with the implementation above
button.setOnClickListener(mCorkyListener);
...
}
You may also find it more conventient to implement OnClickListener as a part of your Activity.
This will avoid the extra class load and object allocation. For example:
public class ExampleActivity extends Activity implements OnClickListener {
protected void onCreate(Bundle savedValues) {
...
Button button = (Button)findViewById(R.id.corky);
button.setOnClickListener(this);
}
// Implement the OnClickListener callback
public void onClick(View v) {
// do something when the button is clicked
}
...
}
Notice that the onClick() callback in the above example has
no return value, but some other event listener methods must return a boolean. The reason
depends on the event. For the few that do, here's why:
onLongClick() -
This returns a boolean to indicate whether you have consumed the event and it should not be carried further.
That is, return true to indicate that you have handled the event and it should stop here;
return false if you have not handled it and/or the event should continue to any other
on-click listeners.onKey() -
This returns a boolean to indicate whether you have consumed the event and it should not be carried further.
That is, return true to indicate that you have handled the event and it should stop here;
return false if you have not handled it and/or the event should continue to any other
on-key listeners.onTouch() -
This returns a boolean to indicate whether your listener consumes this event. The important thing is that
this event can have multiple actions that follow each other. So, if you return false when the
down action event is received, you indicate that you have not consumed the event and are also
not interested in subsequent actions from this event. Thus, you will not be called for any other actions
within the event, such as a fingure gesture, or the eventual up action event.
Remember that key events are always delivered to the View currently in focus. They are dispatched starting from the top
of the View hierarchy, and then down, until they reach the appropriate destination. If your View (or a child of your View)
currently has focus, then you can see the event travel through the dispatchKeyEvent() method. As an alternative to capturing key events through your View, you can also receive
all of the events inside your Activity with onKeyDown()
and onKeyUp().
Note: Android will call event handlers first and then the appropriate default
handlers from the class definition second. As such, returning true from these event listeners will stop
the propagation of the event to other event listeners and will also block the callback to the
default event handler in the View. So be certain that you want to terminate the event when you return true.
Event Handlers
If you're building a custom component from View, then you'll be able to define several callback methods
used as default event handlers.
In the document on Building Custom Components,
you'll learn see some of the common callbacks used for event handling, including:
There are some other methods that you should be awere of, which are not part of the View class,
but can directly impact the way you're able to handle events. So, when managing more complex events inside
a layout, consider these other methods:
Touch Mode
When a user is navigating a user interface with directional keys or a trackball, it is
necessary to give focus to actionable items (like buttons) so the user can see
what will accept input. If the device has touch capabilities, however, and the user
begins interacting with the interface by touching it, then it is no longer necessary to
highlight items, or give focus to a particular View. Thus, there is a mode
for interaction named "touch mode."
For a touch-capable device, once the user touches the screen, the device
will enter touch mode. From this point onward, only Views for which
isFocusableInTouchMode() is true will be focusable, such as text editing widgets.
Other Views that are touchable, like buttons, will not take focus when touched; they will
simply fire their on-click listeners when pressed.
Any time a user hits a directional key or scrolls with a trackball, the device will
exit touch mode, and find a view to take focus. Now, the user may resume interacting
with the user interface without touching the screen.
The touch mode state is maintained throughout the entire system (all windows and activities).
To query the current state, you can call
isInTouchMode() to see whether the device is currently in touch mode.
Handling Focus
The framework will handle routine focus movement in response to user input.
This includes changing the focus as Views are removed or hidden, or as new
Views become available. Views indicate their willingness to take focus
through the isFocusable() method. To change whether a View can take
focus, call setFocusable(). When in touch mode,
you may query whether a View allows focus with isFocusableInTouchMode().
You can change this with setFocusableInTouchMode().
Focus movement is based on an algorithm which finds the nearest neighbor in a
given direction. In rare cases, the default algorithm may not match the
intended behavior of the developer. In these situations, you can provide
explicit overrides with the following XML attributes in the layout file:
nextFocusDown, nextFocusLeft, nextFocusRight, and
nextFocusUp. Add one of these attributes to the View from which
the focus is leaving. Define the value of the attribute to be the id of the View
to which focus should be given. For example:
<LinearLayout
android:orientation="vertical"
... >
<Button android:id="@+id/top"
android:nextFocusUp="@+id/bottom"
... />
<Button android:id="@+id/bottom"
android:nextFocusDown="@+id/top"
... />
</LinearLayout>
Ordinarily, in this vertical layout, navigating up from the first Button would not go
anywhere, nor would navigating down from the second Button. Now that the top Button has
defined the bottom one as the nextFocusUp (and vice versa), the navigation focus will
cycle from top-to-bottom and bottom-to-top.
If you'd like to declare a View as focusable in your UI (when it is traditionally not),
add the android:focusable XML attribute to the View, in your layout declaration.
Set the value true. You can also declare a View
as focusable while in Touch Mode with android:focusableInTouchMode.
To request a particular View to take focus, call requestFocus().
To listen for focus events (be notified when a View receives or looses focus), use
onFocusChange(),
as discussed in the
Event Listeners section, above.
