Android使用Canvas对象实现刮刮乐效果

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"   xmlns:tools="http://schemas.android.com/tools"   android:layout_width="match_parent"   android:layout_height="match_parent" >    <ImageView     android:id="@+id/after"     android:layout_width="wrap_content"     android:layout_height="wrap_content"     android:src="@drawable/a" />    <ImageView     android:id="@+id/before"     android:layout_width="wrap_content"     android:layout_height="wrap_content"     android:src="@drawable/b" />  </FrameLayout> 

public class MainActivity extends Activity implements OnTouchListener {    private ImageView imgafter;   private ImageView imgbefore;   private Canvas canvas;   private Paint paint;   private Bitmap bitmap;   private Bitmap before;   private Bitmap after;    @Override   protected void onCreate(Bundle savedInstanceState) {     super.onCreate(savedInstanceState);     setContentView(R.layout.activity_main);      imgafter = (ImageView) findViewById(R.id.after);     imgbefore = (ImageView) findViewById(R.id.before);      // 获得图片     after = BitmapFactory.decodeResource(getResources(), R.drawable.a);     before = BitmapFactory.decodeResource(getResources(), R.drawable.b);      imgafter.setImageBitmap(after);     imgbefore.setImageBitmap(before);     // 创建可以修改的空白的bitmap     bitmap = Bitmap.createBitmap(before.getWidth(), before.getHeight(),         before.getConfig());     imgbefore.setOnTouchListener(this);     paint = new Paint();     paint.setStrokeWidth(5);     paint.setColor(Color.BLACK);     // 创建画布     canvas = new Canvas(bitmap);     canvas.drawBitmap(before, new Matrix(), paint);   }    @Override   public boolean onTouch(View arg0, MotionEvent event) {     switch (event.getAction()) {     case MotionEvent.ACTION_MOVE:       int newX = (int) event.getX();       int newY = (int) event.getY();       // 将滑过的地方变为透明       for (int i = -10; i < 10; i++) {         for (int j = -10; j < 10; j++) {           if ((i + newX) >= before.getWidth()               || j + newY >= before.getHeight() || i + newX < 0               || j + newY < 0) {             return false;           }           bitmap.setPixel(i + newX, j + newY, Color.TRANSPARENT);         }       }       imgbefore.setImageBitmap(bitmap);       break;     }     return true;   } }