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();         }     } }}

然而并没有什么卵用~~

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

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

然并卵，还是没什么变化~~

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