Android 6.0 RecyclerView嵌套在ScrollView中顯示不全

ScrollView嵌套RecyclerView在Android6.0以下能正常顯示，但是在6.0以上就會出現RecyclerView顯示不全的bug。嘗試多種方法之后終於找到解決辦法，特在此記錄下。

無效方法之一

ScrollView嵌套RecyclerView時出現的各種沖突一般是靠重寫RecyclerView的LayoutManager來解決的。如下，重寫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 動態添加時報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();
         }
     }
 }
}

無效方法之二

於是又想到當初ListView在ScrollView中顯示不全時，重寫ListView的onMeasure()方法可以完美解決。試試

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

終極解決方案

終極解決辦法是在RecyclerView的外部套上一層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>

發現RecyclerView完全顯示出來了，完美解決！

隱隱感覺只是android:descendantFocusability="blocksDescendants"一句在起作用。於是嘗試在根布局上加這句，再把這個RelativeLayout刪除。。問題又重現了。。可是后來發現不加android:descendantFocusability="blocksDescendants"同樣能完美解決，好奇怪的解決方案！！先不求甚解地用上吧。。

android:descendantFocusability屬性的值有三種： 
beforeDescendants：viewgroup會優先其子類控件而獲取到焦點 
blocksDescendants：viewgroup會覆蓋子類控件而直接獲得焦點 
afterDescendants：viewgroup只有當其子類控件不需要獲取焦點時才獲取焦點