其实在我们日常的编程中，对于缩放手势的使用并不是很经常，这一手势主要是用在图片浏览方面，比如下方例子。但是（敲重点），作为 Android 入门的基础来说，学习 ScaleGestureDetector 的使用，算是不得不过的一道坎，好在 ScaleGestureDetector 使用起来非常简单，就是源码分析上得花些功夫。
本文首先将简单的介绍下 ScaleGestureDetector 的使用，在重点给大家分析下源码（由于源码方面是我自己的理解，可能有偏差，希望各位大佬能在评论区指出，万分感谢～）
ScaleGestureDetector 包括一个监听器，以及它所有方法的空实现：
| 名称 | 用途 |
|---|---|
| ScaleGestureDetector | 缩放手势的监听器 |
| SimpleOnScaleGestureListener | 该监听器的空实现，在其中重写方法 |
| 名称 | 用途 |
|---|---|
| onScaleBegin | 当 >= 2 个手指碰触屏幕时调用，若返回 false 则忽略改事件调用 |
| onScale | 滑动（缩放）过程中调用，若成功处理，则用户返回 true，监听器继续记录下一个缩放等动作，若为 false 表明数据未处理，则监听器继续积累 |
| onScaleEnd | 全部手指离开屏幕，结束监听 |
通常情况下，手势监听会结合自定义 View 来讲，这里我给出一个最简单的使用，具体的使用实例，以后再结合自定义 View 讲讲。
private void iniScaleGestureListener(){
mListener = new ScaleGestureDetector.SimpleOnScaleGestureListener(){
@Override
public boolean onScaleBegin(ScaleGestureDetector detector) {
return super.onScaleBegin(detector);
}
@Override
public boolean onScale(ScaleGestureDetector detector) {
MyLog.d("X:" + detector.getFocusX());
MyLog.d("Y:" + detector.getFocusY());
MyLog.d("scale:" + detector.getScaleFactor());
return super.onScale(detector);
}
@Override
public void onScaleEnd(ScaleGestureDetector detector) {
super.onScaleEnd(detector);
}
};
detector = new ScaleGestureDetector(getContext(), mListener);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
detector.onTouchEvent(event);
return true;
}ScaleGestureDetector 在具体项目的使用有点复杂，我打算过段时间结合自定义 View 写一篇用来总结，所以这篇我们就先了解下 ScaleGestureDetector 的基本使用。
好了，现在我们进入本章重点，ScaleGestureDetector 源码分析，敲黑板敲黑板。首先，我们打开 ScaleGestureDetector 的源码可以看到，几乎所有的代码都集中在了 onTouchEvent 这个方法上，所以在这里，我就主要给大家介绍这个方法的实现。
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
mCurrTime = event.getEventTime();
final int action = event.getActionMasked();
// Forward the event to check for double tap gesture
if (mQuickScaleEnabled) {
mGestureDetector.onTouchEvent(event);
}
final int count = event.getPointerCount();
final boolean isStylusButtonDown =
(event.getButtonState() & MotionEvent.BUTTON_STYLUS_PRIMARY) != 0; if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
} mCurrTime = event.getEventTime(); final int action = event.getActionMasked(); if (mQuickScaleEnabled) {
mGestureDetector.onTouchEvent(event);
} final int count = event.getPointerCount();这个主要是由于判断手写笔是否按下
由于我们很少处理手写笔，所以这里不做过多说明
final boolean isStylusButtonDown =
(event.getButtonState() & MotionEvent.BUTTON_STYLUS_PRIMARY) != 0;## 第二部分：处理与手势变化
用户的缩放手势不总是一定的，就是说对于用户而言，随时可能有手指碰触或离开屏幕，这就使得缩放中心的（焦点）随时可能发生变化，这部分主要是用来处理这一变化，并做出响应。
final boolean anchoredScaleCancelled =
mAnchoredScaleMode == ANCHORED_SCALE_MODE_STYLUS && !isStylusButtonDown;
final boolean streamComplete = action == MotionEvent.ACTION_UP ||
action == MotionEvent.ACTION_CANCEL || anchoredScaleCancelled;
// 如果发生了上面这种小动作，或者说有一手指离开了屏幕，进行调用
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;
}
}### anchoredScaleCancelled
final boolean anchoredScaleCancelled =
mAnchoredScaleMode == ANCHORED_SCALE_MODE_STYLUS && !isStylusButtonDown; final boolean streamComplete = action == MotionEvent.ACTION_UP ||
action == MotionEvent.ACTION_CANCEL || anchoredScaleCancelled;if (action == MotionEvent.ACTION_DOWN || streamComplete) {...} 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;
}总结： 可以看到，当触发 down 或者触发 up，cancel 时，如果之前处于缩放计算的状态，会将其状态重置， 并调用 onScaleEnd 方法。
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;
} mAnchoredScaleStartX = event.getX();
mAnchoredScaleStartY = event.getY(); mAnchoredScaleMode = ANCHORED_SCALE_MODE_STYLUS; 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;
// Determine focal point
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;
} 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; // Determine focal point
float sumX = 0, sumY = 0;
// 如果是抬起手指，则当前手指数减1，否则不变
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
// 在锚定比例模式中，焦点pt始终是双击的位置，或按下手势开始
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 @Google translate
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;
// Span is the average distance between touch points through the focal point;
// i.e. the diameter of the circle with a radius of the average deviation from
// the focal point.
final float spanX = devX * 2;
final float spanY = devY * 2;
final float span;
if (inAnchoredScaleMode()) {
span = spanY;
} else {
span = (float) Math.hypot(spanX, spanY);
} 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);
} 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);
} 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;
}
} if (mInProgress) {
updatePrev = mListener.onScale(this);
}
if (updatePrev) {
mPrevSpanX = mCurrSpanX;
mPrevSpanY = mCurrSpanY;
mPrevSpan = mCurrSpan;
mPrevTime = mCurrTime;
}我要讲的所有内容，到这里就完全结束了
由于源码是按照我自己的理解来讲的，所以难免会有一些出入
希望大家能在评论区中帮我指出，谢谢～ ??
缩放手势 ScaleGestureDetector 源码解析，这一篇就够了
原文：https://www.cnblogs.com/yuanhao-1999/p/11102806.html