接著上一節(jié) Android手勢ImageView三部曲（二）的往下走，我們講到了github上的GestureDetector框架，
先附上github鏈接：
https://github.com/Almeros/android-gesture-detectors
其實把這個框架的主體思想也是參考的Android自帶的ScaleGestureDetector工具類，ScaleGestureDetector估計是參考的GestureDetector工具類，不管誰參考誰的，既然被我們遇到了，我們就要變成自己的東西，真不能全變成自己的東西的話，至少
我們要了解下它的思想。
我們先了解一下android自帶的ScaleGestureDetector（縮放手勢監(jiān)測器）：
ScaleGestureDetector跟GestureDetector構(gòu)造都差不多，但是ScaleGestureDetector只能用于監(jiān)測縮放的手勢，而GestureDetector監(jiān)測的手勢就比較多了，我們上一節(jié)內(nèi)容中有提到。
ScaleGestureDetector的一些用法跟api，小伙伴自行去查看官網(wǎng)文檔：
https://developer.android.google.cn/reference/android/view/ScaleGestureDetector.html
我們怎么使用它呢（我以第一節(jié)中最后一個demo為例）？
首先創(chuàng)建一個ScaleGestureDetector對象：
private void initView() { .... mScaleDetector = new ScaleGestureDetector(getContext(), new ScaleListener()); .... }然后傳遞一個叫ScaleListener的回調(diào)接口給它，ScaleListener里面有三個回調(diào)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); } }小伙伴應(yīng)該看得懂哈，就是onScale放縮時回調(diào)，onScaleBegin縮放開始時回調(diào)，onScaleEnd縮放完畢后回調(diào)。
最后在view的onTouchEvent方法中把事件給ScaleGestureDetector對象：
@Override public boolean onTouchEvent(MotionEvent event) { //把縮放事件給mScaleDetector mScaleDetector.onTouchEvent(event); return true; }好啦～！！上一節(jié)最后的時候，我寫了一個小demo去實現(xiàn)了圖片的位移，下面我們繼續(xù)加上圖片的縮放：
public class MatrixImageView extends ImageView { private Matrix currMatrix; private GestureDetector detector; private ScaleGestureDetector scaleDetector; private float currX;//當前圖片的x坐標值 private float currY;//當前圖片的y坐標值 private float scaleFactor=1f;//當前圖片的縮放值 public MatrixImageView(Context context, AttributeSet attrs) { super(context, attrs); initView(); detector=new GestureDetector(context,onGestureListener); //創(chuàng)建一個縮放手勢監(jiān)測器 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); //把事件給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(); /** * 因為getScaleFactor＝當前兩個手指之間的距離（preEvent）／手指按下時候兩個點的距離(currEvent) * 這里如果返回true的話，會在move操作的時候去更新之前的event, * 如果為false的話，不會去更新之前按下時候保存的event */ return true; } };}尷尬了，模擬器不太好用于兩個手指縮放的錄制，所以效果小伙伴自己拿到代碼運行一下哈～！！！
下面一起擼一擼ScaleGestureDetector的源碼：
我們知道，ScaleGestureDetector核心代碼也就是onTouchEvent,于是我們點開onTouchEvent：
@SuppressLint("NewApi") public boolean onTouchEvent(MotionEvent event) { //獲取當前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); } //獲取手指個數(shù) 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; //手指按下的時候 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; //處理多點之間距離 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; }一堆代碼，數(shù)學不太好的看起來還真比較艱難，大概就是根據(jù)多個觸碰點的x坐標算出一個x軸平均值，然后y軸也一樣，然后通過Math.hypot(spanX, spanY);算出斜邊長，斜邊長即為兩點之間的距離，然后保存當前的span跟移動過后的span。
最后當我們調(diào)用getScaleFactor獲取縮放比例的時候，即用現(xiàn)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; }這數(shù)學渣真的是硬傷啊～～～
有了android自帶的ScaleGestureDetector作為參考，我們能自己實現(xiàn)ScaleGestureDetector嗎？？ 當然github上大神已經(jīng)實現(xiàn)了，但是如果沒有的話，你能寫出來么？
先寫到這，未完待續(xù)。
以上就是本文的全部內(nèi)容，希望對大家的學習有所幫助，也希望大家多多支持VEVB武林網(wǎng)。
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