Android 6.0 RecyclerView嵌套在ScrollView中显示不全

ScrollView嵌套RecyclerView在Android6.0以下能正常显示，但是在6.0以上就会出现RecyclerView显示不全的bug。尝试多种方法之后终于找到解决办法，特在此记录下。

无效方法之一

ScrollView嵌套RecyclerView时出现的各种冲突一般是靠重写RecyclerView的LayoutManager来解决的。如下，重写GridLayoutManager。

public class FullyGridLayoutManager extends GridLayoutManager {
 public FullyGridLayoutManager(Context context, int spanCount) {
     super(context, spanCount);
 }

 public FullyGridLayoutManager(Context context, int spanCount, int orientation, boolean reverseLayout) {
     super(context, spanCount, orientation, reverseLayout);
 }

 private int[] mMeasuredDimension = new int[2];

 @Override
 public void onMeasure(RecyclerView.Recycler recycler, RecyclerView.State state, int widthSpec, int heightSpec) {
     final int widthMode = View.MeasureSpec.getMode(widthSpec);
     final int heightMode = View.MeasureSpec.getMode(heightSpec);
     final int widthSize = View.MeasureSpec.getSize(widthSpec);
     final int heightSize = View.MeasureSpec.getSize(heightSpec);

     int width = 0;
     int height = 0;
     int count = getItemCount();
     int span = getSpanCount();
     for (int i = 0; i < count; i++) {
         measureScrapChild(recycler, i,
                 View.MeasureSpec.makeMeasureSpec(i, View.MeasureSpec.UNSPECIFIED),
                 View.MeasureSpec.makeMeasureSpec(i, View.MeasureSpec.UNSPECIFIED),
                 mMeasuredDimension);

         if (getOrientation() == HORIZONTAL) {
             if (i % span == 0) {
                 width = width + mMeasuredDimension[0];
             }
             if (i == 0) {
                 height = mMeasuredDimension[1];
             }
         } else {
             if (i % span == 0) {
                 height = height + mMeasuredDimension[1];
             }
             if (i == 0) {
                 width = mMeasuredDimension[0];
             }
         }
     }

     switch (widthMode) {
         case View.MeasureSpec.EXACTLY:
             width = widthSize;
         case View.MeasureSpec.AT_MOST:
         case View.MeasureSpec.UNSPECIFIED:
     }

     switch (heightMode) {
         case View.MeasureSpec.EXACTLY:
             height = heightSize;
         case View.MeasureSpec.AT_MOST:
         case View.MeasureSpec.UNSPECIFIED:
     }

     setMeasuredDimension(width, height);
 }

 private void measureScrapChild(RecyclerView.Recycler recycler, int position, int widthSpec,
                                int heightSpec, int[] measuredDimension) {
     if (position < getItemCount()) {
         try {
             View view = recycler.getViewForPosition(0);//fix 动态添加时报IndexOutOfBoundsException
             if (view != null) {
                 RecyclerView.LayoutParams p = (RecyclerView.LayoutParams) view.getLayoutParams();
                 int childWidthSpec = ViewGroup.getChildMeasureSpec(widthSpec,
                         getPaddingLeft() + getPaddingRight(), p.width);
                 int childHeightSpec = ViewGroup.getChildMeasureSpec(heightSpec,
                         getPaddingTop() + getPaddingBottom(), p.height);
                 view.measure(childWidthSpec, childHeightSpec);
                 measuredDimension[0] = view.getMeasuredWidth() + p.leftMargin + p.rightMargin;
                 measuredDimension[1] = view.getMeasuredHeight() + p.bottomMargin + p.topMargin;
                 recycler.recycleView(view);
             }
         } catch (Exception e) {
             e.printStackTrace();
         }
     }
 }
}

无效方法之二

于是又想到当初ListView在ScrollView中显示不全时，重写ListView的onMeasure()方法可以完美解决。试试

@Override
public void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int expandSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2, MeasureSpec.AT_MOST);
    super.onMeasure(widthMeasureSpec, expandSpec);
}

终极解决方案

终极解决办法是在RecyclerView的外部套上一层RelativeLayout

<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:descendantFocusability="blocksDescendants">

        <android.support.v7.widget.RecyclerView
            android:id="@+id/menuRv"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginLeft="@dimen/margin_16"
            android:layout_marginRight="@dimen/margin_16"/>

</RelativeLayout>

发现RecyclerView完全显示出来了，完美解决！

隐隐感觉只是android:descendantFocusability="blocksDescendants"一句在起作用。于是尝试在根布局上加这句，再把这个RelativeLayout删除。。问题又重现了。。可是后来发现不加android:descendantFocusability="blocksDescendants"同样能完美解决，好奇怪的解决方案！！先不求甚解地用上吧。。

android:descendantFocusability属性的值有三种： 
beforeDescendants：viewgroup会优先其子类控件而获取到焦点 
blocksDescendants：viewgroup会覆盖子类控件而直接获得焦点 
afterDescendants：viewgroup只有当其子类控件不需要获取焦点时才获取焦点