前两篇文章讲解了Android的触控机制和手势操作，讲到多点触控时我们举了一个简单的手势缩放图片的例子，但是功能很有限。本篇我们通过解析一个国外大牛Mike Ortiz写的自定义TouchImageView的源码，来更加深入了解安卓触控和手势操作。
TouchImageView继承自ImageView具有ImageView的所有功能；除此之外，还有缩放、拖拽、双击放大等功能，支持viewpager和scaletype，并伴有动画效果。

• sharedConstructing

    private void sharedConstructing(Context context) {
        super.setClickable(true);
        this.context = context;
        mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
        mGestureDetector = new GestureDetector(context, new GestureListener());
        matrix = new Matrix();
        prevMatrix = new Matrix();
        m = new float[9];
        normalizedScale = 1;
        if (mScaleType == null) {
            mScaleType = ScaleType.FIT_CENTER;
        }
        minScale = 1;
        maxScale = 3;
        superMinScale = SUPER_MIN_MULTIPLIER * minScale;
        superMaxScale = SUPER_MAX_MULTIPLIER * maxScale;
        setImageMatrix(matrix);
        setScaleType(ScaleType.MATRIX);
        setState(State.NONE);
        onDrawReady = false;
        super.setOnTouchListener(new PrivateOnTouchListener());
    }

初始化，设置ScaleGestureDetector的监听器为ScaleListener，这是用来处理缩放手势的，设置GestureDetector的监听器为GestureListener，这是用来处理双击和fling手势的，前两个手势都会引起图片的缩放，而fling会引起图片的移动。

mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
mGestureDetector = new GestureDetector(context, new GestureListener());

最后设置自定义View的touch事件监听器为PrivateOnTouchListener，这是touch事件的入口。

super.setOnTouchListener(new PrivateOnTouchListener());

• PrivateOnTouchListener

private class PrivateOnTouchListener implements OnTouchListener {

        //
        // Remember last point position for dragging
        //
        private PointF last = new PointF();

        @Override
        public boolean onTouch(View v, MotionEvent event) {
            mScaleDetector.onTouchEvent(event);
            mGestureDetector.onTouchEvent(event);
            PointF curr = new PointF(event.getX(), event.getY());

            if (state == State.NONE || state == State.DRAG || state == State.FLING) {
                switch (event.getAction()) {
                    case MotionEvent.ACTION_DOWN:
                        last.set(curr);
                        if (fling != null)
                            fling.cancelFling();
                        setState(State.DRAG);
                        break;

                    case MotionEvent.ACTION_MOVE:
                        if (state == State.DRAG) {
                            float deltaX = curr.x - last.x;
                            float deltaY = curr.y - last.y;
                            float fixTransX = getFixDragTrans(deltaX, viewWidth, getImageWidth());
                            float fixTransY = getFixDragTrans(deltaY, viewHeight, getImageHeight());
                            matrix.postTranslate(fixTransX, fixTransY);
                            fixTrans();
                            last.set(curr.x, curr.y);
                        }
                        break;

                    case MotionEvent.ACTION_UP:
                    case MotionEvent.ACTION_POINTER_UP:
                        setState(State.NONE);
                        break;
                }
            }

            setImageMatrix(matrix);

            //
            // User-defined OnTouchListener
            //
            if(userTouchListener != null) {
                userTouchListener.onTouch(v, event);
            }

            //
            // OnTouchImageViewListener is set: TouchImageView dragged by user.
            //
            if (touchImageViewListener != null) {
                touchImageViewListener.onMove();
            }

            //
            // indicate event was handled
            //
            return true;
        }
    }

触摸时会走到PrivateOnTouchListener的onTouch，它又会将捕捉到的MotionEvent交给mScaleDetector和mGestureDetector来分析是否有合适的callback函数来处理用户的手势。

mScaleDetector.onTouchEvent(event);
mGestureDetector.onTouchEvent(event);

同时在当前状态是DRAG时将X、Y移动的距离赋值给变换矩阵

matrix.postTranslate(fixTransX, fixTransY);

给ImageView设置矩阵，完成X、Y的移动，即实现单指拖拽动作

setImageMatrix(matrix);

• ScaleListener

private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {
        @Override
        public boolean onScaleBegin(ScaleGestureDetector detector) {
            setState(State.ZOOM);
            return true;
        }

        @Override
        public boolean onScale(ScaleGestureDetector detector) {
            scaleImage(detector.getScaleFactor(), detector.getFocusX(), detector.getFocusY(), true);

            //
            // OnTouchImageViewListener is set: TouchImageView pinch zoomed by user.
            //
            if (touchImageViewListener != null) {
                touchImageViewListener.onMove();
            }
            return true;
        }

        @Override
        public void onScaleEnd(ScaleGestureDetector detector) {
            super.onScaleEnd(detector);
            setState(State.NONE);
            boolean animateToZoomBoundary = false;
            float targetZoom = normalizedScale;
            if (normalizedScale > maxScale) {
                targetZoom = maxScale;
                animateToZoomBoundary = true;

            } else if (normalizedScale < minScale) {
                targetZoom = minScale;
                animateToZoomBoundary = true;
            }

            if (animateToZoomBoundary) {
                DoubleTapZoom doubleTap = new DoubleTapZoom(targetZoom, viewWidth / 2, viewHeight / 2, true);
                compatPostOnAnimation(doubleTap);
            }
        }
    }

两指缩放动作会走到ScaleListener的回调，在它的onScale回调中会处理图片的缩放

scaleImage(detector.getScaleFactor(), detector.getFocusX(), detector.getFocusY(), true);

• scaleImage

    private void scaleImage(double deltaScale, float focusX, float focusY, boolean stretchImageToSuper) {

        float lowerScale, upperScale;
        if (stretchImageToSuper) {
            lowerScale = superMinScale;
            upperScale = superMaxScale;

        } else {
            lowerScale = minScale;
            upperScale = maxScale;
        }

        float origScale = normalizedScale;
        normalizedScale *= deltaScale;
        if (normalizedScale > upperScale) {
            normalizedScale = upperScale;
            deltaScale = upperScale / origScale;
        } else if (normalizedScale < lowerScale) {
            normalizedScale = lowerScale;
            deltaScale = lowerScale / origScale;
        }

        matrix.postScale((float) deltaScale, (float) deltaScale, focusX, focusY);
        fixScaleTrans();
    }

这里会将多次缩放的缩放比累积，并设置有最大和最小缩放比，不能超出范围

normalizedScale *= deltaScale;

最后把X、Y的缩放比和焦点传给变换矩阵，通过矩阵关联到ImageView，完成缩放动作

matrix.postScale((float) deltaScale, (float) deltaScale, focusX, focusY);

在onScaleEnd回调中，我们会判断是否当前缩放比超出最大缩放比或者小于最小缩放比，如果是，会有一个动画回到最大或最小缩放比状态

DoubleTapZoom doubleTap = new DoubleTapZoom(targetZoom, viewWidth / 2, viewHeight / 2, true);
compatPostOnAnimation(doubleTap);

这里的动画DoubleTapZoom就是双击动画，关于DoubleTapZoom我们下面会讲到。至此两指缩放动作就完成了，下面继续看双击缩放动作。

• GestureListener

private class GestureListener extends GestureDetector.SimpleOnGestureListener {

        @Override
        public boolean onSingleTapConfirmed(MotionEvent e)
        {
            if(doubleTapListener != null) {
                return doubleTapListener.onSingleTapConfirmed(e);
            }
            return performClick();
        }

        @Override
        public void onLongPress(MotionEvent e)
        {
            performLongClick();
        }

        @Override
        public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY)
        {
            if (fling != null) {
                //
                // If a previous fling is still active, it should be cancelled so that two flings
                // are not run simultaenously.
                //
                fling.cancelFling();
            }
            fling = new Fling((int) velocityX, (int) velocityY);
            compatPostOnAnimation(fling);
            return super.onFling(e1, e2, velocityX, velocityY);
        }

        @Override
        public boolean onDoubleTap(MotionEvent e) {
            boolean consumed = false;
            if(doubleTapListener != null) {
                consumed = doubleTapListener.onDoubleTap(e);
            }
            if (state == State.NONE) {
                float targetZoom = (normalizedScale == minScale) ? maxScale : minScale;
                DoubleTapZoom doubleTap = new DoubleTapZoom(targetZoom, e.getX(), e.getY(), false);
                compatPostOnAnimation(doubleTap);
                consumed = true;
            }
            return consumed;
        }

        @Override
        public boolean onDoubleTapEvent(MotionEvent e) {
            if(doubleTapListener != null) {
                return doubleTapListener.onDoubleTapEvent(e);
            }
            return false;
        }
    }

在onDoubleTap回调中，设置双击缩放比，如果当前无缩放，则设置缩放比为最大值，如果已经是最大值，则设置为无缩放

float targetZoom = (normalizedScale == minScale) ? maxScale : minScale;

然后将当前点击坐标做为缩放中心，连同缩放比一起交给DoubleTapZoom，完成缩放动画

DoubleTapZoom doubleTap = new DoubleTapZoom(targetZoom, e.getX(), e.getY(), false);
compatPostOnAnimation(doubleTap);

• DoubleTapZoom

    private class DoubleTapZoom implements Runnable {

        private long startTime;
        private static final float ZOOM_TIME = 500;
        private float startZoom, targetZoom;
        private float bitmapX, bitmapY;
        private boolean stretchImageToSuper;
        private AccelerateDecelerateInterpolator interpolator = new AccelerateDecelerateInterpolator();
        private PointF startTouch;
        private PointF endTouch;

        DoubleTapZoom(float targetZoom, float focusX, float focusY, boolean stretchImageToSuper) {
            setState(State.ANIMATE_ZOOM);
            startTime = System.currentTimeMillis();
            this.startZoom = normalizedScale;
            this.targetZoom = targetZoom;
            this.stretchImageToSuper = stretchImageToSuper;
            PointF bitmapPoint = transformCoordTouchToBitmap(focusX, focusY, false);
            this.bitmapX = bitmapPoint.x;
            this.bitmapY = bitmapPoint.y;

            //
            // Used for translating image during scaling
            //
            startTouch = transformCoordBitmapToTouch(bitmapX, bitmapY);
            endTouch = new PointF(viewWidth / 2, viewHeight / 2);
        }

        @Override
        public void run() {
            float t = interpolate();
            double deltaScale = calculateDeltaScale(t);
            scaleImage(deltaScale, bitmapX, bitmapY, stretchImageToSuper);
            translateImageToCenterTouchPosition(t);
            fixScaleTrans();
            setImageMatrix(matrix);

            //
            // OnTouchImageViewListener is set: double tap runnable updates listener
            // with every frame.
            //
            if (touchImageViewListener != null) {
                touchImageViewListener.onMove();
            }

            if (t < 1f) {
                //
                // We haven‘t finished zooming
                //
                compatPostOnAnimation(this);

            } else {
                //
                // Finished zooming
                //
                setState(State.NONE);
            }
        }

        /**
         * Interpolate between where the image should start and end in order to translate
         * the image so that the point that is touched is what ends up centered at the end
         * of the zoom.
         * @param t
         */
        private void translateImageToCenterTouchPosition(float t) {
            float targetX = startTouch.x + t * (endTouch.x - startTouch.x);
            float targetY = startTouch.y + t * (endTouch.y - startTouch.y);
            PointF curr = transformCoordBitmapToTouch(bitmapX, bitmapY);
            matrix.postTranslate(targetX - curr.x, targetY - curr.y);
        }

        /**
         * Use interpolator to get t
         * @return
         */
        private float interpolate() {
            long currTime = System.currentTimeMillis();
            float elapsed = (currTime - startTime) / ZOOM_TIME;
            elapsed = Math.min(1f, elapsed);
            return interpolator.getInterpolation(elapsed);
        }

        /**
         * Interpolate the current targeted zoom and get the delta
         * from the current zoom.
         * @param t
         * @return
         */
        private double calculateDeltaScale(float t) {
            double zoom = startZoom + t * (targetZoom - startZoom);
            return zoom / normalizedScale;
        }
    }

DoubleTapZoom其实是一个线程，实现了Runnable，我们直接看它的Run方法吧，这里定义了一个时间t

float t = interpolate();

其实t在500ms内通过一个加速差值器从0到1加速增长

private float interpolate() {
    long currTime = System.currentTimeMillis();
    float elapsed = (currTime - startTime) / ZOOM_TIME;
    elapsed = Math.min(1f, elapsed);
    return interpolator.getInterpolation(elapsed);
}

通过t计算出当前缩放比

double deltaScale = calculateDeltaScale(t);

实现缩放

scaleImage(deltaScale, bitmapX, bitmapY, stretchImageToSuper);

然后根据当前t的值判断动画是否结束，如果t小于1，表示动画还未结束，重新执行本线程，否则设置状态完成。这里就是通过在这500ms内多次执行线程，多次重绘ImageView实现动画效果的。

if (t < 1f) {
    compatPostOnAnimation(this);
 } else {
    setState(State.NONE);
 }

同时在GestureListener的onFling回调中，设置Fling的X、Y速度，然后执行Fling的位移动画

fling = new Fling((int) velocityX, (int) velocityY);
compatPostOnAnimation(fling);

• Fling

    private class Fling implements Runnable {

        CompatScroller scroller;
        int currX, currY;

        Fling(int velocityX, int velocityY) {
            setState(State.FLING);
            scroller = new CompatScroller(context);
            matrix.getValues(m);

            int startX = (int) m[Matrix.MTRANS_X];
            int startY = (int) m[Matrix.MTRANS_Y];
            int minX, maxX, minY, maxY;

            if (getImageWidth() > viewWidth) {
                minX = viewWidth - (int) getImageWidth();
                maxX = 0;

            } else {
                minX = maxX = startX;
            }

            if (getImageHeight() > viewHeight) {
                minY = viewHeight - (int) getImageHeight();
                maxY = 0;

            } else {
                minY = maxY = startY;
            }

            scroller.fling(startX, startY, (int) velocityX, (int) velocityY, minX,
                    maxX, minY, maxY);
            currX = startX;
            currY = startY;
        }

        public void cancelFling() {
            if (scroller != null) {
                setState(State.NONE);
                scroller.forceFinished(true);
            }
        }

        @Override
        public void run() {

            //
            // OnTouchImageViewListener is set: TouchImageView listener has been flung by user.
            // Listener runnable updated with each frame of fling animation.
            //
            if (touchImageViewListener != null) {
                touchImageViewListener.onMove();
            }

            if (scroller.isFinished()) {
                scroller = null;
                return;
            }

            if (scroller.computeScrollOffset()) {
                int newX = scroller.getCurrX();
                int newY = scroller.getCurrY();
                int transX = newX - currX;
                int transY = newY - currY;
                currX = newX;
                currY = newY;
                matrix.postTranslate(transX, transY);
                fixTrans();
                setImageMatrix(matrix);
                compatPostOnAnimation(this);
            }
        }
    }

Fling其实也是一个线程，实现了Runnable，根据Fling手势的X、Y速度我们会执行Scroller的fling函数，并且将当前位置设置为起始位置

scroller.fling(startX, startY, (int) velocityX, (int) velocityY, minX,maxX, minY, maxY);
currX = startX;
currY = startY;

再来看看Run函数，根据scroller当前滚动位置计算出新的位置信息，与旧位置相减得出在X、Y轴平移距离，实现平移

if (scroller.computeScrollOffset()) {
     int newX = scroller.getCurrX();
     int newY = scroller.getCurrY();
     int transX = newX - currX;
     int transY = newY - currY;
     currX = newX;
     currY = newY;
     matrix.postTranslate(transX, transY);
     fixTrans();
     setImageMatrix(matrix);
     compatPostOnAnimation(this);
}

最后延时一段时间再次调用线程完成新的平移绘图，如此往复，直到scroller停止滚动，多次重绘ImageView实现了fling动画效果。

    private void compatPostOnAnimation(Runnable runnable) {
        if (VERSION.SDK_INT >= VERSION_CODES.JELLY_BEAN) {
            postOnAnimation(runnable);

        } else {
            postDelayed(runnable, 1000/60);
        }
    }

下面看一看显示效果吧：

• 单个图片
  

• 图片加载到ViewPager中
  

• 镜像图片
  

• 点击可改变图片
  

• 点击可改变ScaleType
  

Demo下载地址