Rebound动画框架简单介绍

菜鸟一枚,有不对的地方希望大家指出，谢谢。 

最近在接手了一个老项目，发现里面动画框架用的是facebook中的Rebound框架，由于以前没听说过，放假时闲得蛋痛，看看了源码，就顺手写这一篇吧。  写了一个小Demo，具体效果如下：   代码很简单，这是xml布局：

这是MainActivity：

package com.micro.mytest_button;import android.app.Activity;import android.os.Bundle;import android.view.Menu;import android.view.MotionEvent; import android.view.View; import android.view.View.OnTouchListener; import android.widget.ImageView; import com.facebook.rebound.Spring; import com.facebook.rebound.SpringListener; import com.facebook.rebound.SpringSystem; import com.nineoldandroids.view.ViewHelper; public class MainActivity extends Activity { private ImageView image; private Spring spring; private final float mScale = 1.0f; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); image = (ImageView) findViewById(R.id.image); SpringSystem springSystem = SpringSystem.create(); spring = springSystem.createSpring(); spring.addListener(new SpringListener() { @Override public void onSpringUpdate(Spring spring) { float value = (float) spring.getCurrentValue(); float scale = 1f - (value * mScale); System.out.println("the value is " + value + "--the scale is --" + scale); ViewHelper.setScaleX(image, scale); ViewHelper.setScaleY(image, scale); } @Override public void onSpringEndStateChange(Spring spring) { } @Override public void onSpringAtRest(Spring spring) { } @Override public void onSpringActivate(Spring spring) { } }); image.setOnTouchListener(new OnTouchListener() { @Override public boolean onTouch(View v, MotionEvent event) { switch (event.getAction()) { case MotionEvent.ACTION_DOWN: spring.setEndValue(1.0f); break; case MotionEvent.ACTION_UP: case MotionEvent.ACTION_CANCEL: spring.setEndValue(0.0f); break; } return true; } }); } @Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.main, menu); return true; } }

现在主要是来分析下Rebound的源码，看看里面到底怎么走的，用法很简单（这是最简单的用法），就三句话：

SpringSystem springSystem = SpringSystem.create();spring = springSystem.createSpring();spring.addListener(new SpringListener() {}

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主要是创建了三个对象，SpringSystem//SpringListener，对于动画的效果，就是这三个玩意搞出来的。具体的模式如下： 

SpringSystem:继承自BaseSpringSystem，其中包含了Spring对象引用的容器，控制Spring对象的操作，存在一个SpringLooper（是一个抽象方法，只有start()/stop()方法），这也是facebook自定义的类，与android.os.Looper无关，只是模拟了android.os.Looper的方法，内存start(),end()方法。其构造方法：

public static SpringSystem create() {    return new SpringSystem(AndroidSpringLooperFactory.createSpringLooper()); }

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打开SpringSystem(SpringLooper sl)源码：

public BaseSpringSystem(SpringLooper springLooper) {    if (springLooper == null) {      throw new IllegalArgumentException("springLooper is required"); } mSpringLooper = springLooper; mSpringLooper.setSpringSystem(this); }

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现在再看AndroidSpringLooperFactory.createSpringLooper()源码：

public static SpringLooper createSpringLooper() {    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN) {      return ChoreographerAndroidSpringLooper.create(); } else { return LegacyAndroidSpringLooper.create(); } }

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可以看到为了兼容JDK,高低版本创建的SpringLooper容器不经相同，这个不是考虑的重点。

现在来看第二句：

spring = springSystem.createSpring();

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翻开SpringSytem.createSpring()源码：

public Spring createSpring() {    Spring spring = new Spring(this);    registerSpring(spring);    return spring; }

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可以看到Spring类是一个对立的类，持有SpringSystem的引用，来看看Spring（SpringSystem ss)构造函数：

Spring(BaseSpringSystem springSystem) {    if (springSystem == null) {      throw new IllegalArgumentException("Spring cannot be created outside of a BaseSpringSystem"); } mSpringSystem = springSystem; mId = "spring:" + ID++; setSpringConfig(SpringConfig.defaultConfig); }

再看setSpringConfig(SpringConfig.defaultConfig)源码：

public Spring setSpringConfig(SpringConfig springConfig) {    if (springConfig == null) {      throw new IllegalArgumentException("springConfig is required"); } mSpringConfig = springConfig; return this; }

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也只是初始化一些参数，并没有有我们想要的源码(这里的意思就是我点击图片时，它为什么会有动画的意思)，再看第三个方法吧：

spring.addListener(new SpringListener() {}

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添加监听器，用的多的人就会有感觉，这个也不会有关键代码可以使我们图片有缩放效果，那么问题来了，看到facebook写的三句话，我们并没有找到动画缩放的想过函数啊，是不是有配置文件或者静态/动态代码块呢？？找了很久，没有啊。那只有接下来再看image的代码了，在看到Image的绑定的onTouch事件上，看到了这句话：

spring.setEndValue(1.0f);

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啥也不说，进去看看再说吧：

public Spring setEndValue(double endValue) {    if (mEndValue == endValue && isAtRest()) {      return this; } mStartValue = getCurrentValue(); mEndValue = endValue; mSpringSystem.activateSpring(this.getId()); for (SpringListener listener : mListeners) { listener.onSpringEndStateChange(this); } return this; }

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其他没啥好看的，看到了这句话：

mSpringSystem.activateSpring(this.getId());

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活了二十几年，立马感觉这句话有问题，进入看看：

void activateSpring(String springId) {    Spring spring = mSpringRegistry.get(springId);    if (spring == null) {      throw new IllegalArgumentException("springId " + springId + " does not reference a registered spring"); } mActiveSprings.add(spring); if (getIsIdle()) { mIdle = false; mSpringLooper.start(); } }

解释一下，刚才说的SpringSystem包含了Spring对象的引用，mSpringRegistry.get(springId)是找到SpringSystem中注册的Spring， mActiveSprings.add(spring)是将该spring添加到活动的spring队列中，那现在就看看这句话吧： mSpringLooper.start()，刚才说SpringLooper是一个抽象类，那好随便找个其子类看看里面的方法吧，

private static class LegacyAndroidSpringLooper extends SpringLooper {}

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刚才的兼容性代码中，JELLY_BEAN(也就是4.1之前的代码)，那我们就看看mSpringLooper.start()是个什么鬼了吧：

private static class LegacyAndroidSpringLooper extends SpringLooper { private final Handler mHandler; private final Runnable mLooperRunnable; private boolean mStarted; private long mLastTime; /** * @return an Android spring looper using a new {@link Handler} instance */ public static SpringLooper create() { return new LegacyAndroidSpringLooper(new Handler()); } public LegacyAndroidSpringLooper(Handler handler) { mHandler = handler; mLooperRunnable = new Runnable() { @Override public void run() { if (!mStarted || mSpringSystem == null) { return; } long currentTime = SystemClock.uptimeMillis(); mSpringSystem.loop(currentTime - mLastTime); mHandler.post(mLooperRunnable); } }; } @Override public void start() { if (mStarted) { return; } mStarted = true; mLastTime = SystemClock.uptimeMillis(); mHandler.removeCallbacks(mLooperRunnable); mHandler.post(mLooperRunnable); } @Override public void stop() { mStarted = false; mHandler.removeCallbacks(mLooperRunnable); } }

对，没错我们看到了 mHandler.post(mLooperRunnable);金典的方法啊，那我们看看这mLooperRunnable方法里面在干嘛吧，

long currentTime = SystemClock.uptimeMillis(); mSpringSystem.loop(currentTime - mLastTime);

废话不多说，立马去找我们想要看到的SpringSystem.loop方法了：

public void loop(double ellapsedMillis) {    for (SpringSystemListener listener : mListeners) { listener.onBeforeIntegrate(this); } advance(ellapsedMillis); if (mActiveSprings.isEmpty()) { mIdle = true; } for (SpringSystemListener listener : mListeners) { listener.onAfterIntegrate(this); } if (mIdle) { mSpringLooper.stop(); } }

对了，我们好像看到了advance(ellapsedMillis)啊，这个看起来比较吊，不说了进去看看再说啊：

void advance(double deltaTime) {    for (Spring spring : mActiveSprings) {      // advance time in seconds      if (spring.systemShouldAdvance()) { spring.advance(deltaTime / 1000.0); } else { mActiveSprings.remove(spring); } } }

尼玛啊，我看到了

spring.advance(deltaTime / 1000.0);

哈哈，我感觉快要找到了，不说，赶快进去找：

void advance(double realDeltaTime) {    boolean isAtRest = isAtRest();    if (isAtRest && mWasAtRest) {      /* begin debug Log.d(TAG, "bailing out because we are at rest:" + getName()); end debug */ return; } // clamp the amount of realTime to simulate to avoid stuttering in the UI. We should be able // to catch up in a subsequent advance if necessary. double adjustedDeltaTime = realDeltaTime; if (realDeltaTime > MAX_DELTA_TIME_SEC) { adjustedDeltaTime = MAX_DELTA_TIME_SEC; } /* begin debug long startTime = System.currentTimeMillis(); int iterations = 0; end debug */ mTimeAccumulator += adjustedDeltaTime; double tension = mSpringConfig.tension; double friction = mSpringConfig.friction; double position = mCurrentState.position; double velocity = mCurrentState.velocity; double tempPosition = mTempState.position; double tempVelocity = mTempState.velocity; double aVelocity, aAcceleration; double bVelocity, bAcceleration; double cVelocity, cAcceleration; double dVelocity, dAcceleration; double dxdt, dvdt; // iterate over the true time while (mTimeAccumulator >= SOLVER_TIMESTEP_SEC) { /* begin debug iterations++; end debug */ mTimeAccumulator -= SOLVER_TIMESTEP_SEC; if (mTimeAccumulator < SOLVER_TIMESTEP_SEC) { // This will be the last iteration. Remember the previous state in case we need to // interpolate mPreviousState.position = position; mPreviousState.velocity = velocity; } // Perform an RK4 integration to provide better detection of the acceleration curve via // sampling of Euler integrations at 4 intervals feeding each derivative into the calculation // of the next and taking a weighted sum of the 4 derivatives as the final output. // This math was inlined since it made for big performance improvements when advancing several // springs in one pass of the BaseSpringSystem. // The initial derivative is based on the current velocity and the calculated acceleration aVelocity = velocity; aAcceleration = (tension * (mEndValue - tempPosition)) - friction * velocity; // Calculate the next derivatives starting with the last derivative and integrating over the // timestep tempPosition = position + aVelocity * SOLVER_TIMESTEP_SEC * 0.5; tempVelocity = velocity + aAcceleration * SOLVER_TIMESTEP_SEC * 0.5; bVelocity = tempVelocity; bAcceleration = (tension * (mEndValue - tempPosition)) - friction * tempVelocity; tempPosition = position + bVelocity * SOLVER_TIMESTEP_SEC * 0.5; tempVelocity = velocity + bAcceleration * SOLVER_TIMESTEP_SEC * 0.5; cVelocity = tempVelocity; cAcceleration = (tension * (mEndValue - tempPosition)) - friction * tempVelocity; tempPosition = position + cVelocity * SOLVER_TIMESTEP_SEC; tempVelocity = velocity + cAcceleration * SOLVER_TIMESTEP_SEC; dVelocity = tempVelocity; dAcceleration = (tension * (mEndValue - tempPosition)) - friction * tempVelocity; // Take the weighted sum of the 4 derivatives as the final output. dxdt = 1.0/6.0 * (aVelocity + 2.0 * (bVelocity + cVelocity) + dVelocity); dvdt = 1.0/6.0 * (aAcceleration + 2.0 * (bAcceleration + cAcceleration) + dAcceleration); position += dxdt * SOLVER_TIMESTEP_SEC; velocity += dvdt * SOLVER_TIMESTEP_SEC; } mTempState.position = tempPosition; mTempState.velocity = tempVelocity; mCurrentState.position = position; mCurrentState.velocity = velocity; if (mTimeAccumulator > 0) { interpolate(mTimeAccumulator / SOLVER_TIMESTEP_SEC); } // End the spring immediately if it is overshooting and overshoot clamping is enabled. // Also make sure that if the spring was considered within a resting threshold that it's now // snapped to its end value. if (isAtRest() || (mOvershootClampingEnabled && isOvershooting())) { // Don't call setCurrentValue because that forces a call to onSpringUpdate mStartValue = mEndValue; mCurrentState.position = mEndValue; setVelocity(0); isAtRest = true; } /* begin debug long endTime = System.currentTimeMillis(); long elapsedMillis = endTime - startTime; Log.d(TAG, "iterations:" + iterations + " iterationTime:" + elapsedMillis + " position:" + mCurrentState.position + " velocity:" + mCurrentState.velocity + " realDeltaTime:" + realDeltaTime + " adjustedDeltaTime:" + adjustedDeltaTime + " isAtRest:" + isAtRest + " wasAtRest:" + mWasAtRest); end debug */ // NB: do these checks outside the loop so all listeners are properly notified of the state // transition boolean notifyActivate = false; if (mWasAtRest) { mWasAtRest = false; notifyActivate = true; } boolean notifyAtRest = false; if (isAtRest) { mWasAtRest = true; notifyAtRest = true; } for (SpringListener listener : mListeners) { // starting to move if (notifyActivate) { listener.onSpringActivate(this); } // updated listener.onSpringUpdate(this); // coming to rest if (notifyAtRest) { listener.onSpringAtRest(this); } } }

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代码精简一下，就变成这样了：

void advance(double realDeltaTime) {    boolean isAtRest = isAtRest();    if (isAtRest && mWasAtRest) {      return; } double adjustedDeltaTime = realDeltaTime; if (realDeltaTime > MAX_DELTA_TIME_SEC) { adjustedDeltaTime = MAX_DELTA_TIME_SEC; } mTimeAccumulator += adjustedDeltaTime; while (mTimeAccumulator >= SOLVER_TIMESTEP_SEC) { /** inner change **/ } mTempState.position = tempPosition; mTempState.velocity = tempVelocity; mCurrentState.position = position; mCurrentState.velocity = velocity; for (SpringListener listener : mListeners) { // starting to move if (notifyActivate) { listener.onSpringActivate(this); } // updated listener.onSpringUpdate(this); // coming to rest if (notifyAtRest) { listener.onSpringAtRest(this); } } }

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这下看得十分清楚了吧，在while循环里面变换之后，我们得到了 ：

mCurrentState.position = position; mCurrentState.velocity = velocity;

然后在接口回调中我们使用到了这些参数，

spring.addListener(new SpringListener() {            @Override            public void onSpringUpdate(Spring spring) { float value = (float) spring.getCurrentValue(); float scale = 1f - (value * mScale); ViewHelper.setScaleX(image, scale); ViewHelper.setScaleY(image, scale); } @Override public void onSpringEndStateChange(Spring spring) {} @Override public void onSpringAtRest(Spring spring) {} @Override public void onSpringActivate(Spring spring) {} });

就这样我么的变换终于就成功了，我们也就走通了相关的流程了，是不是很好玩呢？？？主要是facebook这个Rebound框架很小，类很少，我们可以在很短的时间对它通读，是不是感觉到分析源码很好玩呢？我也是一名android菜鸟，很多时候不敢去分析源码，很为很多的东西都看不懂看不透，那就慢慢来吧，先从简单的开始吧，呵呵。

    本文转自 一点点征服   博客园博客，原文链接：http://www.cnblogs.com/ldq2016/p/7079770.html，如需转载请自行联系原作者