NestedScrollView嵌套RecyclerView导致

为什么会失效呢，该怎样去分析。

这一块还得看NestedScrollView的代码及RecyclerView的复用原理。

NestedScrollView

@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        if (!mFillViewport) {
            return;
        }

        final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        if (heightMode == MeasureSpec.UNSPECIFIED) {
            return;
        }

        if (getChildCount() > 0) {
            View child = getChildAt(0);
            final NestedScrollView.LayoutParams lp = (LayoutParams) child.getLayoutParams();

            int childSize = child.getMeasuredHeight();
            int parentSpace = getMeasuredHeight()
                    - getPaddingTop()
                    - getPaddingBottom()
                    - lp.topMargin
                    - lp.bottomMargin;
            //如果孩子高度小于父布局的高度，则对孩子的布局进行精确测量
            if (childSize < parentSpace) {
                int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                        getPaddingLeft() + getPaddingRight() + lp.leftMargin + lp.rightMargin,
                        lp.width);
                int childHeightMeasureSpec =
                        MeasureSpec.makeMeasureSpec(parentSpace, MeasureSpec.EXACTLY);
                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
            }
        }
    }

看下super.onMeasure(widthMeasureSpec, heightMeasureSpec)方法,NestedScrollView继承自FrameLayout，我们看FrameLayout的onMeasure方法。

measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);

measureChildWithMargins这个方法是个关键，
NestedScrollView重写了measureChild、measureChildWithMargins两个方法。

 @Override
    protected void measureChild(View child, int parentWidthMeasureSpec,
            int parentHeightMeasureSpec) {
        ViewGroup.LayoutParams lp = child.getLayoutParams();

        int childWidthMeasureSpec;
        int childHeightMeasureSpec;

        childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, getPaddingLeft()
                + getPaddingRight(), lp.width);

        childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

    @Override
    protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                getPaddingLeft() + getPaddingRight() + lp.leftMargin + lp.rightMargin
                        + widthUsed, lp.width);
        final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                lp.topMargin + lp.bottomMargin, MeasureSpec.UNSPECIFIED);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

在这两个方法中，测量模式设置为MeasureSpec.UNSPECIFIED。当测量模式为MeasureSpec.UNSPECIFIED时，父容器没有对子View有任何限制，子View可以任意取尺寸，即大小有子View决定。

RecyclerView

来分析下RecyclerView中的代码,以LinearLayoutManager为例，RecyclerView是将绘制流程交给LayoutManager处理。

LinearLayoutManager#onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) 

    ...
     mLayoutState.mInfinite = resolveIsInfinite();
    ....
     fill(recycler, mLayoutState, state, false);

我们分析下resolveIsInfinite()方法，
当父View是View.MeasureSpec.UNSPECIFIED的时候，则返回为true。

boolean resolveIsInfinite() {
        return mOrientationHelper.getMode() == View.MeasureSpec.UNSPECIFIED
                && mOrientationHelper.getEnd() == 0;
    }

mLayoutState.mInfinite的值为true，我们定位
fill的方法。

//这里是关键，因为layoutState.mInfinite为true，因此会将getItemCount()得到的count数据都加载出来
while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {
            layoutChunkResult.resetInternal();
            layoutChunk(recycler, state, layoutState, layoutChunkResult);
            if (layoutChunkResult.mFinished) {
                break;
            }
            layoutState.mOffset += layoutChunkResult.mConsumed * layoutState.mLayoutDirection;
            /**
             * Consume the available space if:
             * * layoutChunk did not request to be ignored
             * * OR we are laying out scrap children
             * * OR we are not doing pre-layout
             */
            if (!layoutChunkResult.mIgnoreConsumed || mLayoutState.mScrapList != null
                    || !state.isPreLayout()) {
                layoutState.mAvailable -= layoutChunkResult.mConsumed;
                // we keep a separate remaining space because mAvailable is important for recycling
                remainingSpace -= layoutChunkResult.mConsumed;
            }

            if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
                layoutState.mScrollingOffset += layoutChunkResult.mConsumed;
                if (layoutState.mAvailable < 0) {
                    layoutState.mScrollingOffset += layoutState.mAvailable;
                }
                recycleByLayoutState(recycler, layoutState);
            }
            if (stopOnFocusable && layoutChunkResult.mFocusable) {
                break;
            }
        }
        

layoutChunk方法

 //得到View,这里会走相关的缓存策略
  View view = layoutState.next(recycler);
  ...
  //添加View
if (layoutState.mScrapList == null) {
            if (mShouldReverseLayout == (layoutState.mLayoutDirection
                    == LayoutState.LAYOUT_START)) {
                addView(view);
            } else {
                addView(view, 0);
            }
        } else {
            if (mShouldReverseLayout == (layoutState.mLayoutDirection
                    == LayoutState.LAYOUT_START)) {
                addDisappearingView(view);
            } else {
                addDisappearingView(view, 0);
            }
        }

总结

• 首先NestedScrollView对measureChild、measureChildWithMargins两个方法进行重写，测量模式为MeasureSpec.UNSPECIFIED，当测量模式为MeasureSpec.UNSPECIFIED时，父容器没有对子View有任何限制，子View可以任意取尺寸，即大小有子View决定。
• RecyclerView中onMeasure时，调用LayoutManager的onLayoutChildren方法中，将mLayoutState.mInfinite设置为true。
• LayoutManager调用fill方法绘制时，会根据layoutState.mInfinite来判断,如果为true的话，会将会将getItemCount()得到的count数据都加载出来。