滑動會觸發(fā)onTouchEvent方法
MotionEvent.ACTION_MOVE中：

if (scrollByInternal(
        canScrollHorizontally ? dx : 0,
        canScrollVertically ? dy : 0,
        vtev)) {
    getParent().requestDisallowInterceptTouchEvent(true);
}

RecyclerView#scrollByInternal

當x方向或者y方向滑動的距離超過mTouchSlop時厕吉，會調用此方法滑動內部

垂直方向的滑動 —— scrollVerticallyBy->scrollBy

調用updateLayoutState(layoutDirection, absDy, true, state)集畅，計算在不添加子View的情況下還能滑多少距離

計算layoutDirection檩电，向上滑動時 = LAYOUT_START基公，向下滑動時 = LAYOUT_END

final int layoutDirection = dy > 0 ? LayoutState.LAYOUT_END : LayoutState.LAYOUT_START;

layoutDirection == LayoutState.LAYOUT_END销部，即向上滑動時

1. 獲得靠近底部的子View
   final View child = getChildClosestToEnd();
2. 將這個子View的底邊賦值給mLayoutState.mOffset
   mLayoutState.mOffset = mOrientationHelper.getDecoratedEnd(child);
3. 計算在不添加子View的情況下還能滑多少距離谓媒，在不考慮內外邊距的情況下就等于底部View的bottom-RecyclerView的高度

scrollingOffset = mOrientationHelper.getDecoratedEnd(child)
        - mOrientationHelper.getEndAfterPadding();
// 上面的計算實際上等于scrollingOffset = (mLayoutManager.getDecoratedBottom(view) 
+ params.bottomMargin) -(mLayoutManager.getHeight() - mLayoutManager.getPaddingBottom());

向上滑1.png

向下滑動時

1. 獲得靠近頭部的子View
   final View child = getChildClosestToStart();
2. 將這個子View的頂邊賦值給mLayoutState.mOffset
   mLayoutState.mOffset = mOrientationHelper.getDecoratedStart(child);
3. 計算在不添加子View的情況下還能滑多少距離夸楣，在不考慮內外邊距的情況下就等于頂部子View的top

scrollingOffset = -mOrientationHelper.getDecoratedStart(child)
        + mOrientationHelper.getStartAfterPadding();
// 上面的計算實際上等于scrollingOffset = (mLayoutManager.getDecoratedTop(view) - params.topMargin) 
- mLayoutManager.getPaddingTop();

向下滑1.png

設置 mLayoutState.mAvailable 和 mLayoutState.mScrollingOffset 的值

mLayoutState.mAvailable = requiredSpace;
if (canUseExistingSpace) {
    // mLayoutState.mAvailable實際上等于absDy-scrollingOffset
    mLayoutState.mAvailable -= scrollingOffset;
}
mLayoutState.mScrollingOffset = scrollingOffset;

調用fill方法箫攀，根據前面的計算得到的mAvailable，mAvailable>0才會添加子View

final int consumed = mLayoutState.mScrollingOffset + fill(recycler, mLayoutState, state, false);

根據RecyclerView測量和布局分析违崇，會根據remainingSpace(layoutState.mAvailable + layoutState.mExtra)來循環(huán)添加子View阿弃，而在循環(huán)過程中，添加完子View會執(zhí)行下列代碼來回收屏幕外的子View：

if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) 
    layoutState.mScrollingOffset += layoutChunkResult.mConsumed;
    if (layoutState.mAvailable < 0) {
        // 這里實際上mScrollingOffset = mScrollingOffset + mConsumed + mAvailable - mConsumed
        layoutState.mScrollingOffset += layoutState.mAvailable;
    }
    recycleByLayoutState(recycler, layoutState);
}

向上滑動時

recycleByLayoutState內部會調用recycleViewsFromStart方法

// limit 實際上等于mScrollingOffset
for (int i = 0; i < childCount; i++) {
    View child = getChildAt(i);
    if (mOrientationHelper.getDecoratedEnd(child) > limit
            || mOrientationHelper.getTransformedEndWithDecoration(child) > limit) {
        // stop here
        recycleChildren(recycler, 0, i);
        return;
    }
}

向上滑2.png

1. while循環(huán)第一次layoutChunk時伴箩，添加了子ViewF入愧，mConsumed等于子ViewF的高度，這時mAvailable -= mConsumed得到的mAvailable還是大于0的嗤谚，mScrollingOffset += mConsumed棺蛛，接著調用recycleByLayoutState，此時會回收recycleChildren(recycler, 0, 1);子ViewA巩步，將其添加到Cache緩存或RecyclerViewPool中旁赊，具體查看RecyclerView的緩存分析
2. 由于計算后的mAvailable還是大于0，再循環(huán)第二次layoutChunk椅野，添加了子ViewG终畅，mConsumed等于子ViewG的高度，這時mAvailable -= mConsumed得到的mAvailable是小于0的竟闪，mScrollingOffset = mScrollingOffset + mAvailable(這個mAvailable是-mConsumed之前的)相當于absDy离福，接著調用recycleByLayoutState，還是回收子ViewA

向下滑動時

recycleByLayoutState內部會調用recycleViewsFromEnd方法

for (int i = childCount - 1; i >= 0; i--) {
    View child = getChildAt(i);
    if (mOrientationHelper.getDecoratedStart(child) < limit
            || mOrientationHelper.getTransformedStartWithDecoration(child) < limit) {
        recycleChildren(recycler, childCount - 1, i);
        return;
    }
}

向下滑2.png

1. while循環(huán)一次layoutChunk時炼蛤，添加了子ViewA妖爷，mConsumed等于子ViewA的高度，這時mAvailable -= mConsumed得到的mAvailable等于0鲸湃，mScrollingOffset += mConsumed赠涮，接著調用recycleByLayoutState子寓，此時會回收recycleChildren(recycler, 5, 4);子ViewF暗挑，將其添加到Cache緩存或RecyclerViewPool中，具體查看RecyclerView的緩存分析

RecyclerView滑動的核心方法 —— offsetChildrenVertical

滑動的距離scroll的值等于final int scrolled = absDy > consumed ? layoutDirection * consumed : dy;

mOrientationHelper.offsetChildren(-scrolled);->mLayoutManager.offsetChildrenVertical(amount);->mRecyclerView.offsetChildrenVertical(dy)

遍歷ChildHelper中的子View斜友，讓所有子View都offsetTopAndBottom(dy)