Android View体系（四）从源码解析Scroller

在Android View体系（二）实现View滑动的六种方法这篇文章中我们讲到了用Scroller来实现View的滑动，所以这篇文章我们就不介绍Scroller是如何使用的了，本篇就从源码来分析下Scroller为何能够实现View的滑动。

1.Scroller的构造函数

要想使用Scroller，必须先调用new Scroller()，我们先来看看Scroller的构造函数：

/**
  * Create a Scroller with the default duration and interpolator.
  */
 public Scroller(Context context) {
     this(context, null);
 }

 /**
  * Create a Scroller with the specified interpolator. If the interpolator is
  * null, the default (viscous) interpolator will be used. "Flywheel" behavior will
  * be in effect for apps targeting Honeycomb or newer.
  */
 public Scroller(Context context, Interpolator interpolator) {
     this(context, interpolator,
             context.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.HONEYCOMB);
 }

 /**
  * Create a Scroller with the specified interpolator. If the interpolator is
  * null, the default (viscous) interpolator will be used. Specify whether or
  * not to support progressive "flywheel" behavior in flinging.
  */
 public Scroller(Context context, Interpolator interpolator, boolean flywheel) {
     mFinished = true;
     if (interpolator == null) {
         mInterpolator = new ViscousFluidInterpolator();
     } else {
         mInterpolator = interpolator;
     }
     mPpi = context.getResources().getDisplayMetrics().density * 160.0f;
     mDeceleration = computeDeceleration(ViewConfiguration.getScrollFriction());
     mFlywheel = flywheel;

     mPhysicalCoeff = computeDeceleration(0.84f); // look and feel tuning
 }

View Code

Scroller有三个构造函数，通常情况我们都用第一种，第二种需要传进去一个差值器Interpolator ，如果不传则采用默认的差值器（viscous）。

2.Scroller的startScroll方法

public void startScroll(int startX, int startY, int dx, int dy, int duration) {
      mMode = SCROLL_MODE;
      mFinished = false;
      mDuration = duration;
      mStartTime = AnimationUtils.currentAnimationTimeMillis();
      mStartX = startX;
      mStartY = startY;
      mFinalX = startX + dx;
      mFinalY = startY + dy;
      mDeltaX = dx;
      mDeltaY = dy;
      mDurationReciprocal = 1.0f / (float) mDuration;
  }

View Code

在startScroll()方法中并没有调用类似开启滑动的方法，而是保存了传进来的各种参数：startX和startY表示滑动开始的起点，dx和dy表示滑动的距离，duration则表示滑动持续的时间。所以startScroll()方法只是用来做前期准备的并不能使View进行滑动。关键是我们在startScroll()方法后调用了 invalidate()方法，这个方法会导致View的重绘，而View的重绘会调用View的draw()方法，draw()方法又会调用View的computeScroll()方法，我们重写computeScroll()方法：

@Override
public void computeScroll() {
      super.computeScroll();
      if(mScroller.computeScrollOffset()){
          ((View) getParent()).scrollTo(mScroller.getCurrX(),mScroller.getCurrY());
           //通过不断的重绘不断的调用computeScroll方法
          invalidate();
      }
  }

View Code

我们在computeScroll()方法中通过Scroller来获取当前的ScrollX和ScrollY然后调用scrollTo()方法来进行View的滑动，接着调用invalidate方法来让View进行重绘，重绘就会调用computeScroll()方法来实现View的滑动。这样我们就通过不断的移动一个小的距离并连贯起来就实现了平滑移动的效果。但是在Scroller中我们如何能获取当前的位置的ScrollX和ScrollY呢？我们忘了一点就是在调用scrollTo()方法前会调用Scroller的computeScrollOffset()方法，接下来我们就来看看computeScrollOffset()方法。

3.Scroller的computeScrollOffset方法

public boolean computeScrollOffset() {
       if (mFinished) {
           return false;
       }
       //动画持续的时间
       int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);
   
       if (timePassed < mDuration) {
           switch (mMode) {
           case SCROLL_MODE:
               final float x = mInterpolator.getInterpolation(timePassed * mDurationReciprocal);
               //根据插值器来算出timePassed这段时间移动的距离
               mCurrX = mStartX + Math.round(x * mDeltaX);
               mCurrY = mStartY + Math.round(x * mDeltaY);
               break;
           case FLING_MODE:
               final float t = (float) timePassed / mDuration;
               final int index = (int) (NB_SAMPLES * t);
               float distanceCoef = 1.f;
               float velocityCoef = 0.f;
               if (index < NB_SAMPLES) {
                   final float t_inf = (float) index / NB_SAMPLES;
                   final float t_sup = (float) (index + 1) / NB_SAMPLES;
                   final float d_inf = SPLINE_POSITION[index];
                   final float d_sup = SPLINE_POSITION[index + 1];
                   velocityCoef = (d_sup - d_inf) / (t_sup - t_inf);
                   distanceCoef = d_inf + (t - t_inf) * velocityCoef;
               }

               mCurrVelocity = velocityCoef * mDistance / mDuration * 1000.0f;
               
               mCurrX = mStartX + Math.round(distanceCoef * (mFinalX - mStartX));
               // Pin to mMinX <= mCurrX <= mMaxX
               mCurrX = Math.min(mCurrX, mMaxX);
               mCurrX = Math.max(mCurrX, mMinX);
               
               mCurrY = mStartY + Math.round(distanceCoef * (mFinalY - mStartY));
               // Pin to mMinY <= mCurrY <= mMaxY
               mCurrY = Math.min(mCurrY, mMaxY);
               mCurrY = Math.max(mCurrY, mMinY);

               if (mCurrX == mFinalX && mCurrY == mFinalY) {
                   mFinished = true;
               }

               break;
           }
       }
       else {
           mCurrX = mFinalX;
           mCurrY = mFinalY;
           mFinished = true;
       }
       return true;
   }

View Code

首先会计算动画持续的时间timePassed，如果动画持续时间小于我们设置的滑动持续时间mDuration，则执行Swich语句，因为在startScroll()方法中mMode为SCROLL_MODE所以执行分支语句SCROLL_MODE，然后根据插值器Interpolator来计算出在该时间段里面移动的距离，赋值给mCurrX和mCurrY，这样我们就能通过Scroller来获取当前的ScrollX和ScrollY了。另外，computeScrollOffset()的返回值如果为true则表示滑动未结束，false则表示滑动结束，所以如果滑动未结束我们就得持续的调用scrollTo()方法和invalidate()方法来进行View的滑动。