Android for Game Developers - Beginning Android Games - page 132

An OnTouchListener can be registered with any View implementation via the
View.setOnTouchListener() method. The OnTouchListener will be called before the MotionEvent
is dispatched to the View itself. We can signal to the View in our implementation of the onTouch()
method that we have already processed the event by returning true from the method. If we
return false , the View itself will process the event.
The MotionEvent instance has three methods that are relevant to us:
ï?® MotionEvent.getX() and MotionEvent.getY() : These methods report the
x and y coordinates of the touch event relative to the View . The coordinate
system is defined with the origin in the top left of the view, with the x axis
pointing to the right and the y axis pointing downward. The coordinates are
given in pixels. Note that the methods return floats, and thus the coordinates
have subpixel accuracy.
ï?® MotionEvent.getAction() : This method returns the type of the touch event.
It is an integer that takes on one of the values MotionEvent.ACTION_DOWN ,
MotionEvent.ACTION_MOVE , MotionEvent.ACTION_CANCEL , or
MotionEvent.ACTION_UP .
Sounds simple, and it really is. The MotionEvent.ACTION_DOWN event happens when the finger
touches the screen. When the finger moves, events with type MotionEvent.ACTION_MOVE are fired.
Note that you will always get MotionEvent.ACTION_MOVE events, as you can't hold your finger still
enough to avoid them. The touch sensor will recognize the slightest change. When the finger
is lifted up again, the MotionEvent.ACTION_UP event is reported. MotionEvent.ACTION_CANCEL
events are a bit of a mystery. The documentation says they will be fired when the current gesture
is canceled. We have never seen that event in real life yet. However, we'll still process it and
pretend it is a MotionEvent.ACTION_UP event when we start implementing our first game.
Let's write a simple test activity to see how this works in code. The activity should display the
current position of the finger on the screen as well as the event type. Listing 4-3 shows what we
came up with.
Listing 4-3. SingleTouchTest.java; Testing Single-Touch Handling
package com.badlogic.androidgames;
import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;
import android.widget.TextView;
public class SingleTouchTest extends Activity implements OnTouchListener {
StringBuilder builder = new StringBuilder();
TextView textView;
public void onCreate(Bundle savedInstanceState) {
super .onCreate(savedInstanceState);
textView = new TextView( this );