@Override
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    //测量ViewPager自身大小
    setMeasuredDimension(getDefaultSize(0, widthMeasureSpec),
        getDefaultSize(0, heightMeasureSpec));

    final int measuredWidth = getMeasuredWidth();

    // child的宽高，占满父控件
    int childWidthSize = measuredWidth - getPaddingLeft() - getPaddingRight();
    int childHeightSize = getMeasuredHeight() - getPaddingTop() - getPaddingBottom();

    //1.测量Decor
    int size = getChildCount();
    for (int i = 0; i < size; ++i) {
      final View child = getChildAt(i);
      if (child.getVisibility() != GONE) {
        final LayoutParams lp = (LayoutParams) child.getLayoutParams();
        if (lp != null && lp.isDecor) {//仅对Decor进行测量
          //省略若干代码，主要负责对Decor控件的测量
          ...
        }
      }
    }

    mChildWidthMeasureSpec = MeasureSpec.makeMeasureSpec(childWidthSize, MeasureSpec.EXACTLY);
    mChildHeightMeasureSpec = MeasureSpec.makeMeasureSpec(childHeightSize, MeasureSpec.EXACTLY);

    // 2.从Adapter中获取childView
    mInLayout = true;
    populate();
    mInLayout = false;

    // 3.测量非Decor的childView
    size = getChildCount();
    for (int i = 0; i < size; ++i) {
      final View child = getChildAt(i);
      if (child.getVisibility() != GONE) {
        final LayoutParams lp = (LayoutParams) child.getLayoutParams();
        if (lp == null || !lp.isDecor) {
          final int widthSpec = MeasureSpec.makeMeasureSpec(
              (int) (childWidthSize * lp.widthFactor), MeasureSpec.EXACTLY);
          child.measure(widthSpec, mChildHeightMeasureSpec);
        }
      }
    }
  }

#layout.xml
<ViewPager>
  <DecorView />
</ViewPager>

 @Override
  public void addView(View child, int index, ViewGroup.LayoutParams params) {
    if (!checkLayoutParams(params)) {
      params = generateLayoutParams(params);
    }
    final LayoutParams lp = (LayoutParams) params;
    lp.isDecor |= child instanceof Decor; //在此处给isDecor赋值

    //省略无关代码
    ...
  }

public class AutoScrollAdapter extends PagerAdapter {

  //省略构造方法代码
  ...

  @Override
  public void destroyItem(ViewGroup container, int position, Object object) {

  }

  @Override
  public int getCount() {
    return mData.size();
  }

  @Override
  public boolean isViewFromObject(View view, Object object) {
    return view == object;
  }

  @Override
  public Object instantiateItem(ViewGroup container, int position) {
    View itemView = new TextView(mContext); //通过各种方法新建一个childView
    container.addView(itemView);//将childView添加到ViewPager中
    return itemView;
  }
}

static class ItemInfo {
    Object object; //childView
    int position;  //childView在Adapter中的位置
    boolean scrolling; //是否在滚动
    float widthFactor; //宽度的倍数，默认情况下是1
    float offset;    //页面的偏移参数，粗暴的理解就是第几个页面
  }

  void populate(int newCurrentItem) {
    ItemInfo oldCurInfo = null;
    int focusDirection = View.FOCUS_FORWARD;
    if (mCurItem != newCurrentItem) {
      focusDirection = mCurItem < newCurrentItem ? View.FOCUS_RIGHT : View.FOCUS_LEFT;
      oldCurInfo = infoForPosition(mCurItem); //获取旧的ItemInfo对象
      mCurItem = newCurrentItem;  //更新mCurItem的值，就是在Adapter中的position
    }
    //省略无关代码
    ...
    //mOffscreenPageLimit就是setOffscreenPageLimit方法设置的值
    final int pageLimit = mOffscreenPageLimit;

    //根据下面三行代码可知：mItems的长度就是 2 * pageLimit + 1
    //这里声明的startPos和endPos在后面会起作用，大家注意一下
    final int startPos = Math.max(0, mCurItem - pageLimit);
    final int N = mAdapter.getCount();
    final int endPos = Math.min(N-1, mCurItem + pageLimit);

    // 遍历mItems列表，找出mCurItem对应的ItemInfo对象，是根据position来判断的
    int curIndex = -1;
    ItemInfo curItem = null;
    for (curIndex = 0; curIndex < mItems.size(); curIndex++) {
      final ItemInfo ii = mItems.get(curIndex);
      if (ii.position >= mCurItem) {
        if (ii.position == mCurItem) curItem = ii;
        break;
      }
    }
    // 如果mItems中还未保存该ItemInfo，则创建一个IntemInfo对象
    if (curItem == null && N > 0) {
      curItem = addNewItem(mCurItem, curIndex);
    }
    ...

ItemInfo addNewItem(int position, int index) {
    ItemInfo ii = new ItemInfo(); //新建一个ItemInfo对象
    ii.position = position;
    ii.object = mAdapter.instantiateItem(this, position);//用Adapter创建一个childView
    ii.widthFactor = mAdapter.getPageWidth(position);//默认返回1.0f
    if (index < 0 || index >= mItems.size()) { //添加到mItems中
      mItems.add(ii);
    } else {
      mItems.add(index, ii);
    }
    return ii;
  }

    if (curItem != null) {
      //1.调整curItem左边的对象
      float extraWidthLeft = 0.f;

      // curIndex是curItem在mItems中的索引
      // itemIndex就是curItem左边的ItemInfo的索引
      int itemIndex = curIndex - 1; 
      //获取左边的ItemInfo对象
      ItemInfo ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
      final int clientWidth = getClientWidth();
      //curItem左边需要的宽度，默认情况下为1.0f
      final float leftWidthNeeded = clientWidth <= 0 ? 0 :
          2.f - curItem.widthFactor + (float) getPaddingLeft() / (float) clientWidth;
      //遍历mItems左半部分，即curIndex左边的对象
      //只有在pos < startPos时才能退出循环，否则会一直遍历到pos=0
      for (int pos = mCurItem - 1; pos >= 0; pos--) {
        // 建议大家先从下面的else if开始看，因为这里的逻辑是准备退出循环了
        if (extraWidthLeft >= leftWidthNeeded && pos < startPos) {
          //当pos < startPos，说明mItems左边部分已经调整完毕了
          //此时的ii代表的是，startPos左边的对象了
          if (ii == null) { 
            break;
          }
          //如果startPos左边还有对象，需要从mItems中移除
          if (pos == ii.position && !ii.scrolling) {
            mItems.remove(itemIndex);
            mAdapter.destroyItem(this, pos, ii.object);
            itemIndex--;
            curIndex--;
            ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
          }

        //如果curIndex左边的ItemInfo对象不为null
        } else if (ii != null && pos == ii.position) {
          extraWidthLeft += ii.widthFactor; //累加curItem左边需要的宽度
          itemIndex--;           //再往curIndex左边移一个位置
          ii = itemIndex >= 0 ? mItems.get(itemIndex) : null; //取出ItemInfo对象

        //如果curIndex左边的ItemInfo为null
        } else {
          //新建一个ItemInfo对象，添加到itemIndex的右边
          ii = addNewItem(pos, itemIndex + 1); 
          extraWidthLeft += ii.widthFactor;  //累加左边宽度
          curIndex++;  //由于往mItems中插入了一个对象，故curIndex需要加1
          ii = itemIndex >= 0 ? mItems.get(itemIndex) : null; //去除ItemInfo
        }
      }

      //2.调整curItem右边的对象，逻辑与上面类似
      //代码省略
      ...
      // 3.计算mItems中的偏移参数
      calculatePageOffsets(curItem, curIndex, oldCurInfo);
    }

    // 将ItemInfo的内容更新到childView的LayoutParams中
    final int childCount = getChildCount();
    for (int i = 0; i < childCount; i++) {
      final View child = getChildAt(i);
      final LayoutParams lp = (LayoutParams) child.getLayoutParams();
      lp.childIndex = i;
      if (!lp.isDecor && lp.widthFactor == 0.f) {
        final ItemInfo ii = infoForChild(child);
        if (ii != null) {
          lp.widthFactor = ii.widthFactor;
          lp.position = ii.position;
        }
      }
    }
    //根据lp.position的大小对所有childView进行排序，另外DecorView是排在其他child之前的
    sortChildDrawingOrder();

  @Override
  protected void onLayout(boolean changed, int l, int t, int r, int b) {
    final int count = getChildCount();
    int width = r - l;
    int height = b - t;

    //1.布局Decor,根据lp.isDecor来筛选DecorView
    //代码略
    ...

    final int childWidth = width - paddingLeft - paddingRight;

    for (int i = 0; i < count; i++) {
      final View child = getChildAt(i);
      if (child.getVisibility() != GONE) {
        final LayoutParams lp = (LayoutParams) child.getLayoutParams();
        ItemInfo ii;
        //此处将DecorView过滤掉，并且根据view从mItems中查找ItemInfo对象
        //如果ViewPager布局中添加了未实现Decor接口的控件，将不会被布局
        //因为无法从mItems中查找到ItemInfo对象
        if (!lp.isDecor && (ii = infoForChild(child)) != null) {
          //计算当前page的左边界偏移值，此处的offset会随着页面增加而增加
          int loff = (int) (childWidth * ii.offset);
          int childLeft = paddingLeft + loff;
          int childTop = paddingTop;
          if (lp.needsMeasure) {//如果需要重新测量，则重新测量之
            lp.needsMeasure = false;
            final int widthSpec = MeasureSpec.makeMeasureSpec(
                (int) (childWidth * lp.widthFactor),
                MeasureSpec.EXACTLY);
            final int heightSpec = MeasureSpec.makeMeasureSpec(
                (int) (height - paddingTop - paddingBottom),
                MeasureSpec.EXACTLY);
            child.measure(widthSpec, heightSpec);
          }
          //child调用自己的layout方法来布局自己
          child.layout(childLeft, childTop,
              childLeft + child.getMeasuredWidth(),
              childTop + child.getMeasuredHeight());
        }
      }
    }
    mTopPageBounds = paddingTop;
    mBottomPageBounds = height - paddingBottom;
    mDecorChildCount = decorCount;
    //如果是首次布局，则会调用scrollToItem方法
    if (mFirstLayout) {
      scrollToItem(mCurItem, false, 0, false);
    }
    mFirstLayout = false;
  }

  private void scrollToItem(int item, boolean smoothScroll, int velocity,
      boolean dispatchSelected) {
    final ItemInfo curInfo = infoForPosition(item);
    int destX = 0;
    if (curInfo != null) {
      final int width = getClientWidth();
      destX = (int) (width * Math.max(mFirstOffset,
          Math.min(curInfo.offset, mLastOffset)));
    }
    if (smoothScroll) {
      smoothScrollTo(destX, 0, velocity);
      if (dispatchSelected) {
        dispatchOnPageSelected(item);
      }
    } else {
      if (dispatchSelected) { //是否需要分发OnPageSelected回调
        dispatchOnPageSelected(item);
      }
      completeScroll(false);
      scrollTo(destX, 0);
      pageScrolled(destX);
    }
  }