接着上一节 Android手势ImageView三部曲（二）的往下走，我们讲到了github上的GestureDetector框架，
先附上github链接：
https://github.com/Almeros/android-gesture-detectors
其实把这个框架的主体思想也是参考的Android自带的ScaleGestureDetector工具类，ScaleGestureDetector估计是参考的GestureDetector工具类，不管谁参考谁的，既然被我们遇到了，我们就要变成自己的东西，真不能全变成自己的东西的话，至少

我们要了解下它的思想。

我们先了解一下android自带的ScaleGestureDetector（缩放手势监测器）：

ScaleGestureDetector跟GestureDetector构造都差不多，但是ScaleGestureDetector只能用于监测缩放的手势，而GestureDetector监测的手势就比较多了，我们上一节内容中有提到。

ScaleGestureDetector的一些用法跟api，小伙伴自行去查看官网文档：
https://developer.android.google.cn/reference/android/view/ScaleGestureDetector.html

我们怎么使用它呢（我以第一节中最后一个demo为例）？

首先创建一个ScaleGestureDetector对象：

private void initView() { .... mScaleDetector = new ScaleGestureDetector(getContext(), new ScaleListener()); .... }

然后传递一个叫ScaleListener的回调接口给它，ScaleListener里面有三个回调方法：

private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener { @Override public boolean onScale(ScaleGestureDetector detector) { mScaleFactor *= detector.getScaleFactor(); // scale change since previous event // Don't let the object get too small or too large. mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor, 10.0f)); changeMatrix(); return true; } @Override public boolean onScaleBegin(ScaleGestureDetector detector) { return super.onScaleBegin(detector); } @Override public void onScaleEnd(ScaleGestureDetector detector) { super.onScaleEnd(detector); } }

小伙伴应该看得懂哈，就是onScale放缩时回调，onScaleBegin缩放开始时回调，onScaleEnd缩放完毕后回调。

最后在view的onTouchEvent方法中把事件给ScaleGestureDetector对象：

@Override public boolean onTouchEvent(MotionEvent event) { //把缩放事件给mScaleDetector mScaleDetector.onTouchEvent(event); return true; }好啦～！！上

一节最后的时候，我写了一个小demo去实现了图片的位移，下面我们继续加上图片的缩放：

public class MatrixImageView extends ImageView { private Matrix currMatrix; private GestureDetector detector; private ScaleGestureDetector scaleDetector; private float currX;//当前图片的x坐标值 private float currY;//当前图片的y坐标值 private float scaleFactor=1f;//当前图片的缩放值 public MatrixImageView(Context context, AttributeSet attrs) { super(context, attrs); initView(); detector=new GestureDetector(context,onGestureListener); //创建一个缩放手势监测器 scaleDetector=new ScaleGestureDetector(context,onScaleGestureListener); } private void initView() { currMatrix = new Matrix(); DisplayMetrics dm = getResources().getDisplayMetrics(); Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.mipmap.test); bitmap = Bitmap.createScaledBitmap(bitmap, dm.widthPixels, dm.heightPixels, true); setImageBitmap(bitmap); } @Override public boolean onTouchEvent(MotionEvent event) { detector.onTouchEvent(event); //把事件给scaleDetector scaleDetector.onTouchEvent(event); return true; } private void setMatrix(){ currMatrix.reset(); currMatrix.postTranslate(currX,currY); currMatrix.postScale(scaleFactor,scaleFactor,getMeasuredWidth()/2,getMeasuredHeight()/2); setImageMatrix(currMatrix); } private GestureDetector.SimpleOnGestureListener onGestureListener=new GestureDetector.SimpleOnGestureListener(){ @Override public boolean onDown(MotionEvent e) { return true; } @Override public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) { currX-=distanceX; currY-=distanceY; setMatrix(); return super.onScroll(e1, e2, distanceX, distanceY); } }; private ScaleGestureDetector.SimpleOnScaleGestureListener onScaleGestureListener=new ScaleGestureDetector.SimpleOnScaleGestureListener(){ @Override public boolean onScale(ScaleGestureDetector detector) { scaleFactor*=detector.getScaleFactor(); setMatrix(); /** * 因为getScaleFactor＝当前两个手指之间的距离（preEvent）／手指按下时候两个点的距离(currEvent) * 这里如果返回true的话，会在move操作的时候去更新之前的event, * 如果为false的话，不会去更新之前按下时候保存的event */ return true; } };}

尴尬了，模拟器不太好用于两个手指缩放的录制，所以效果小伙伴自己拿到代码运行一下哈～！！！

下面一起撸一撸ScaleGestureDetector的源码：
我们知道，ScaleGestureDetector核心代码也就是onTouchEvent,于是我们点开onTouchEvent：

@SuppressLint("NewApi") public boolean onTouchEvent(MotionEvent event) { //获取当前event事件 mCurrTime = event.getEventTime(); final int action = event.getActionMasked(); // Forward the event to check for double tap gesture if (mQuickScaleEnabled) { mGestureDetector.onTouchEvent(event); } //获取手指个数 final int count = event.getPointerCount(); final boolean isStylusButtonDown = (event.getButtonState() & MotionEvent.BUTTON_STYLUS_PRIMARY) != 0; final boolean anchoredScaleCancelled = mAnchoredScaleMode == ANCHORED_SCALE_MODE_STYLUS && !isStylusButtonDown; final boolean streamComplete = action == MotionEvent.ACTION_UP || action == MotionEvent.ACTION_CANCEL || anchoredScaleCancelled; //手指按下的时候 if (action == MotionEvent.ACTION_DOWN || streamComplete) { // Reset any scale in progress with the listener. // If it's an ACTION_DOWN we're beginning a new event stream. // This means the app probably didn't give us all the events. Shame on it. if (mInProgress) { mListener.onScaleEnd(this); mInProgress = false; mInitialSpan = 0; mAnchoredScaleMode = ANCHORED_SCALE_MODE_NONE; } else if (inAnchoredScaleMode() && streamComplete) { mInProgress = false; mInitialSpan = 0; mAnchoredScaleMode = ANCHORED_SCALE_MODE_NONE; } if (streamComplete) { return true; } } if (!mInProgress && mStylusScaleEnabled && !inAnchoredScaleMode() && !streamComplete && isStylusButtonDown) { // Start of a button scale gesture mAnchoredScaleStartX = event.getX(); mAnchoredScaleStartY = event.getY(); mAnchoredScaleMode = ANCHORED_SCALE_MODE_STYLUS; mInitialSpan = 0; } final boolean configChanged = action == MotionEvent.ACTION_DOWN || action == MotionEvent.ACTION_POINTER_UP || action == MotionEvent.ACTION_POINTER_DOWN || anchoredScaleCancelled; final boolean pointerUp = action == MotionEvent.ACTION_POINTER_UP; final int skipIndex = pointerUp ? event.getActionIndex() : -1; //处理多点之间距离 float sumX = 0, sumY = 0; final int div = pointerUp ? count - 1 : count; final float focusX; final float focusY; if (inAnchoredScaleMode()) { // In anchored scale mode, the focal pt is always where the double tap // or button down gesture started focusX = mAnchoredScaleStartX; focusY = mAnchoredScaleStartY; if (event.getY() < focusY) { mEventBeforeOrAboveStartingGestureEvent = true; } else { mEventBeforeOrAboveStartingGestureEvent = false; } } else { for (int i = 0; i < count; i++) { if (skipIndex == i) continue; sumX += event.getX(i); sumY += event.getY(i); } focusX = sumX / div; focusY = sumY / div; } // Determine average deviation from focal point float devSumX = 0, devSumY = 0; for (int i = 0; i < count; i++) { if (skipIndex == i) continue; // Convert the resulting diameter into a radius. devSumX += Math.abs(event.getX(i) - focusX); devSumY += Math.abs(event.getY(i) - focusY); } final float devX = devSumX / div; final float devY = devSumY / div; final float spanX = devX * 2; final float spanY = devY * 2; final float span; if (inAnchoredScaleMode()) { span = spanY; } else { span = (float) Math.hypot(spanX, spanY); } // Dispatch begin/end events as needed. // If the configuration changes, notify the app to reset its current state by beginning // a fresh scale event stream. final boolean wasInProgress = mInProgress; mFocusX = focusX; mFocusY = focusY; if (!inAnchoredScaleMode() && mInProgress && (span < mMinSpan || configChanged)) { mListener.onScaleEnd(this); mInProgress = false; mInitialSpan = span; } if (configChanged) { mPrevSpanX = mCurrSpanX = spanX; mPrevSpanY = mCurrSpanY = spanY; mInitialSpan = mPrevSpan = mCurrSpan = span; } final int minSpan = inAnchoredScaleMode() ? mSpanSlop : mMinSpan; if (!mInProgress && span >= minSpan && (wasInProgress || Math.abs(span - mInitialSpan) > mSpanSlop)) { mPrevSpanX = mCurrSpanX = spanX; mPrevSpanY = mCurrSpanY = spanY; mPrevSpan = mCurrSpan = span; mPrevTime = mCurrTime; mInProgress = mListener.onScaleBegin(this); } // Handle motion; focal point and span/scale factor are changing. if (action == MotionEvent.ACTION_MOVE) { mCurrSpanX = spanX; mCurrSpanY = spanY; mCurrSpan = span; boolean updatePrev = true; if (mInProgress) { updatePrev = mListener.onScale(this); } if (updatePrev) { mPrevSpanX = mCurrSpanX; mPrevSpanY = mCurrSpanY; mPrevSpan = mCurrSpan; mPrevTime = mCurrTime; } } return true; }

一堆代码，数学不太好的看起来还真比较艰难，大概就是根据多个触碰点的x坐标算出一个x轴平均值，然后y轴也一样，然后通过Math.hypot(spanX, spanY);算出斜边长，斜边长即为两点之间的距离，然后保存当前的span跟移动过后的span。

最后当我们调用getScaleFactor获取缩放比例的时候，即用现在的span／之前的span：

public float getScaleFactor() { if (inAnchoredScaleMode()) { // Drag is moving up; the further away from the gesture // start, the smaller the span should be, the closer, // the larger the span, and therefore the larger the scale final boolean scaleUp = (mEventBeforeOrAboveStartingGestureEvent && (mCurrSpan < mPrevSpan)) || (!mEventBeforeOrAboveStartingGestureEvent && (mCurrSpan > mPrevSpan)); final float spanDiff = (Math.abs(1 - (mCurrSpan / mPrevSpan)) * SCALE_FACTOR); return mPrevSpan <= 0 ? 1 : scaleUp ? (1 + spanDiff) : (1 - spanDiff); } return mPrevSpan > 0 ? mCurrSpan / mPrevSpan : 1; }

这数学渣真的是硬伤啊～～～

有了android自带的ScaleGestureDetector作为参考，我们能自己实现ScaleGestureDetector吗？？ 当然github上大神已经实现了，但是如果没有的话，你能写出来么？

先写到这，未完待续。

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