Android手势ImageView三部曲第三部

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。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持脚本之家。

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