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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