Android™ Application Development - Bahar Ali Khan

Chapter 11: Advanced Android Development
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Finger-based touch makes interaction less precise and is often based more on movement than simple
contact. Android’s native applications make extensive use of fi nger-based touch-screen interfaces,
including the use of dragging motions to scroll lists or perform actions.
To create a View or Activity that uses touch-screen interaction, override the onTouchEvent handler as
shown in the skeleton code below:
@Override
public boolean onTouchEvent(MotionEvent event) {
return super.onTouchEvent(event);
}
Return true if you have handled the screen press; otherwise, return false to pass events through a
stack of Views and Activities until the touch has been successfully handled.
Processing Touch Events
The onTouchEvent handler is fi red when the user fi rst touches the screen, once each time the position
changes, and again when the contact ends.
The action property of the MotionEvent parameter refl ects which of these event types has triggered
the handler. To determine the initiating touch action, call getAction on the Motion Event parameter,
and compare it to the static MotionEvent action values, as shown in the following skeleton code:
@Override
public boolean onTouchEvent(MotionEvent event) {
}
int action = event.getAction();
switch (action) {
case (MotionEvent.ACTION_DOWN) : // Touch screen pressed
break;
case (MotionEvent.ACTION_UP) : // Touch screen touch ended
break;
case (MotionEvent.ACTION_MOVE) : // Contact has moved across screen
break;
case (MotionEvent.ACTION_CANCEL) : // Touch event cancelled
break;
}
return super.onTouchEvent(event);
The Motion Event also includes the coordinates of the current screen contact. You can access these
coordinates using the getX and getY methods. These methods return the coordinate relative to the
responding View or Activity; alternatively, getRawX and getRawY return the absolute screen coordinates.
Both techniques are shown in the following code snippet:
// Relative screen coordinates.
int xPos = (int)event.getX();
int yPos = (int)event.getY();