解析Android开发中多点触摸的实现方法

// import略  
public class ImageViewerActivity extends Activity implements OnTouchListener { 

    private ImageView mImageView; 

    private Matrix matrix = new Matrix(); 
    private Matrix savedMatrix = new Matrix(); 

    private static final int NONE = 0; 
    private static final int DRAG = 1; 
    private static final int ZOOM = 2; 
    private int mode = NONE; 

    // 第一个按下的手指的点  
    private PointF startPoint = new PointF(); 
    // 两个按下的手指的触摸点的中点  
    private PointF midPoint = new PointF(); 
    // 初始的两个手指按下的触摸点的距离  
    private float oriDis = 1f; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
        super.onCreate(savedInstanceState); 
        this.setContentView(R.layout.imageviewer); 
        mImageView = (ImageView) this.findViewById(R.id.imageView); 
        mImageView.setOnTouchListener(this); 
    } 

    @Override 
    public boolean onTouch(View v, MotionEvent event) { 
        ImageView view = (ImageView) v; 

        // 进行与操作是为了判断多点触摸  
        switch (event.getAction() & MotionEvent.ACTION_MASK) { 
        case MotionEvent.ACTION_DOWN: 
            // 第一个手指按下事件  
            matrix.set(view.getImageMatrix()); 
            savedMatrix.set(matrix); 
            startPoint.set(event.getX(), event.getY()); 
            mode = DRAG; 
            break; 
        case MotionEvent.ACTION_POINTER_DOWN: 
            // 第二个手指按下事件  
            oriDis = distance(event); 
            if (oriDis > 10f) { 
                savedMatrix.set(matrix); 
                midPoint = middle(event); 
                mode = ZOOM; 
            } 
            break; 
        case MotionEvent.ACTION_UP: 
        case MotionEvent.ACTION_POINTER_UP: 
            // 手指放开事件  
            mode = NONE; 
            break; 
        case MotionEvent.ACTION_MOVE: 
            // 手指滑动事件  
            if (mode == DRAG) { 
                // 是一个手指拖动  
                matrix.set(savedMatrix); 
                matrix.postTranslate(event.getX() - startPoint.x, event.getY() 
                        - startPoint.y); 
            } else if (mode == ZOOM) { 
                // 两个手指滑动  
                float newDist = distance(event); 
                if (newDist > 10f) { 
                    matrix.set(savedMatrix); 
                    float scale = newDist / oriDis; 
                    matrix.postScale(scale, scale, midPoint.x, midPoint.y); 
                } 
            } 
            break; 
        } 

        // 设置ImageView的Matrix  
        view.setImageMatrix(matrix); 
        return true; 
    } 

    // 计算两个触摸点之间的距离  
    private float distance(MotionEvent event) { 
        float x = event.getX(0) - event.getX(1); 
        float y = event.getY(0) - event.getY(1); 
        return FloatMath.sqrt(x * x + y * y); 
    } 

    // 计算两个触摸点的中点  
    private PointF middle(MotionEvent event) { 
        float x = event.getX(0) + event.getX(1); 
        float y = event.getY(0) + event.getY(1); 
        return new PointF(x / 2, y / 2); 
    } 

} 

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout 
  xmlns:android="http://schemas.android.com/apk/res/android" 
  android:layout_width="fill_parent" 
  android:layout_height="fill_parent"> 
    <ImageView  
        android:id="@+id/imageView" 
        android:layout_width="fill_parent" 
        android:layout_height="fill_parent" 
        android:src="@drawable/example" 
        android:scaleType="matrix" > 
    </ImageView> 
</RelativeLayout>