android 两指缩放图片 event.getActionMasked() 方法的使用

想实现一个缩放图片的效果，是那种两指触摸滑动缩放的。我并没有去重新ImageView 而是重写了View然后调用canvas.drawBitmap的方法，然后通过matrix去实现缩放效果的。(重写ImageView会更麻烦的)这里的实现比较简单。

代码如下：

public class ScaleImage extends BaseView {    private Matrix matrix;    private Bitmap bitmap;    private Paint paint;    private int fingerSize = 0;    private float downX0;    private float downY0;    private float downX1;    private float downY1;    public ScaleImage(Context context) {        super(context);    }    public ScaleImage(Context context, @Nullable AttributeSet attrs) {        super(context, attrs);    }    public ScaleImage(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);    }    public ScaleImage(Context context, @Nullable AttributeSet attrs, int defStyleAttr, int defStyleRes) {        super(context, attrs, defStyleAttr, defStyleRes);    }    @Override    protected void onSizeChanged(int w, int h, int oldw, int oldh) {        super.onSizeChanged(w, h, oldw, oldh);        paint = new Paint();        paint.setAntiAlias(true);        matrix = new Matrix();        Bitmap temp = BitmapFactory.decodeResource(getResources(), R.drawable.yellow);        int bw = temp.getWidth();        int bh = temp.getHeight();        float scale = Math.min(1f * mWidth / bw, 1f * mHeight / bh);        LogUtils.w("scale==" + scale);        bitmap = scaleBitmap(temp, scale);        // compute init left, top        int bbw = bitmap.getWidth();        int bbh = bitmap.getHeight();        Point center = new Point(mWidth / 2, mHeight / 2);        Point bmpCenter = new Point(bbw / 2, bbh / 2); // 图片原来的中心点//        matrix.postScale(0.9f, 0.9f, center.x, center.y); // 中心点参数是有用的        matrix.postTranslate(center.x - bmpCenter.x, center.y - bmpCenter.y); // 移动到当前view 的中心    }    @Override    protected void onDraw(Canvas canvas) {        super.onDraw(canvas);        canvas.drawBitmap(bitmap, matrix, paint);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        // todo: event.getPointerCount() 这个值并不准确，第一个离开的手指没有去计算！        switch (event.getActionMasked()) {            case MotionEvent.ACTION_DOWN:                fingerSize = 1;                LogUtils.i("当前手指数量：" + fingerSize);                downX0 = event.getX(0);                downY0 = event.getY(0);                break;            case MotionEvent.ACTION_MOVE:                if (fingerSize >= 2) {                    // 计算两指触摸到的时候，中心点的位置                    float center2x = downX0 + (downX1 - downX0) / 2;                    float center2y = downX0 + (downX1 - downX0) / 2;                    float x0 = event.getX(0);                    float y0 = event.getY(0);                    float x1 = event.getX(1);                    float y1 = event.getY(1);                    // 缩放比例：移动后的两点距离 / 移动前的两点距离                    float scaleX = distance(x0, y0, x1, y1) / distance(downX0, downY0, downX1, downY1);                    LogUtils.w("缩放比例：" + scaleX);                    if (Math.abs(scaleX) <= 2) {                        matrix.postScale(scaleX, scaleX, center2x, center2y);                    }                    downX0 = x0;                    downY0 = y0;                    downX1 = x1;                    downY1 = y1;                } else if (fingerSize == 1) {                    // 移动图片                    float x0 = event.getX(0);                    float y0 = event.getY(0);                    float dx = x0 - downX0;                    float dy = y0 - downY0;                    matrix.postTranslate(dx, dy);                    downX0 = x0;                    downY0 = y0;                }                break;            case MotionEvent.ACTION_POINTER_DOWN:                fingerSize += 1;                LogUtils.i("当前手指数量：" + event.getPointerCount());                if (fingerSize == 2) {                    downX1 = event.getX(0);                    downY1 = event.getY(0);                }                break;            case MotionEvent.ACTION_POINTER_UP:                fingerSize -= 1;                LogUtils.i("当前手指数量：" + event.getPointerCount());                break;            case MotionEvent.ACTION_CANCEL:                break;            case MotionEvent.ACTION_UP:                fingerSize = 0;                LogUtils.i("当前手指数量：" + event.getPointerCount());                break;        }        postInvalidate();        return true;    }    private float distance(float x1, float y1, float x2, float y2) {        float dx = x2 - x1;        float dy = y2 - y1;        return (float) Math.pow(dx * dx + dy * dy, 0.5);    }    /**     * 根据给定的宽和高进行拉伸     *     * @param origin 原图     * @param scale  缩放比例     * @return new Bitmap     */    private Bitmap scaleBitmap(Bitmap origin, float scale) {        if (origin == null) {            return null;        }        int height = origin.getHeight();        int width = origin.getWidth();        Matrix matrix = new Matrix();        matrix.postScale(scale, scale);// 使用后乘        Bitmap newBM = Bitmap.createBitmap(origin, 0, 0, width, height, matrix, false);        if (!origin.isRecycled()) {            origin.recycle();        }        return newBM;    }}

由于这个需要动图才能看到效果，我就不上图了。

以上，就实现了单指移动，双指缩放的效果了。~

参考资料：
安卓–多指触控
getAction、getActionMask、getActionIndex区别
安卓开发——MotionEvent中getAction、getActionMask、getActionIndex区别

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