从android2.2开始引入了一个ScaleGestureDetector类，可以用来实现多点触摸缩放图片。
ScaleGestureDetector detector ;
ImageView imageView;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        imageView = (ImageView) findViewById(R.id.imageView1);
        detector = new ScaleGestureDetector(this, listener);
    }
    private OnScaleGestureListener listener = new OnScaleGestureListener() {
Matrix matrix = new Matrix();
@Override
public void onScaleEnd(ScaleGestureDetector detector) {

}

@Override
public boolean onScaleBegin(ScaleGestureDetector detector) {
return true;
}
//记得在该方法中使用getScaleFactor方法，在onscaleEnd方法中得到的值基本都是1
@Override
public boolean onScale(ScaleGestureDetector detector) {
float factor = detector.getScaleFactor();
matrix.setScale(factor,factor);
imageView.setImageMatrix(matrix);
return false;
}
};
public boolean onTouchEvent(android.view.MotionEvent event) {
return detector.onTouchEvent(event);
}