Rebound动画框架简单介绍

Rebound动画框架简单介绍

Android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：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/image"
        android:src="@drawable/a1" />

</RelativeLayout>

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() {}

• 1

主要是创建了三个对象，SpringSystem/spring/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Java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。

最后，代码地址