通常情况下我们在使用ScrollView嵌套ListView的时候，当出现问题的时候，相信绝大部分人都是在网上直接找别人的解决方案，都没有关心为什么会出现这种问题，为什么这样解决。

通常情况下ScrollView嵌套ListView会出现只会显示ListView的第一个item的情况，而网上大部分的解决方式是：

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

那么为什么会出现这样的情况呢？为什么这样解决呢？
我们来一起分析分析：

问题原因

出现问题的时候我们会发现只显示了ListView的第一个item，而其他item都是可滑动出来的，猜测是由于onMeasure的问题导致的。

所以我们先来看ScrollView的源码：

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

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

   if (getChildCount() > 0) {
       final View child = getChildAt(0);
       final int height = getMeasuredHeight();
       if (child.getMeasuredHeight() < height) {
           final int widthPadding;
           final int heightPadding;
           final FrameLayout.LayoutParams lp = (LayoutParams) child.getLayoutParams();
           final int targetSdkVersion = getContext().getApplicationInfo().targetSdkVersion;
           if (targetSdkVersion >= VERSION_CODES.M) {
               widthPadding = mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin;
               heightPadding = mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin;
           } else {
               widthPadding = mPaddingLeft + mPaddingRight;
               heightPadding = mPaddingTop + mPaddingBottom;
           }

           final int childWidthMeasureSpec = getChildMeasureSpec(
                   widthMeasureSpec, widthPadding, lp.width);
           final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                   height - heightPadding, MeasureSpec.EXACTLY);
           child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
       }
   }
}

通过源码可以看到ScrollView在测量的时候只拿了子View中的第一个，也就是ListView的高度。

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
   // Sets up mListPadding
   super.onMeasure(widthMeasureSpec, heightMeasureSpec);

   final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
   final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
   int widthSize = MeasureSpec.getSize(widthMeasureSpec);
   int heightSize = MeasureSpec.getSize(heightMeasureSpec);

   int childWidth = 0;
   int childHeight = 0;
   int childState = 0;

   mItemCount = mAdapter == null ? 0 : mAdapter.getCount();
   if (mItemCount > 0 && (widthMode == MeasureSpec.UNSPECIFIED
           || heightMode == MeasureSpec.UNSPECIFIED)) {
       final View child = obtainView(0, mIsScrap);

       // Lay out child directly against the parent measure spec so that
       // we can obtain exected minimum width and height.
       measureScrapChild(child, 0, widthMeasureSpec, heightSize);

       childWidth = child.getMeasuredWidth();
       childHeight = child.getMeasuredHeight();
       childState = combineMeasuredStates(childState, child.getMeasuredState());

       if (recycleOnMeasure() && mRecycler.shouldRecycleViewType(
               ((LayoutParams) child.getLayoutParams()).viewType)) {
           mRecycler.addScrapView(child, 0);
       }
   }
   // 代码省略

   if (heightMode == MeasureSpec.UNSPECIFIED) {
       heightSize = mListPadding.top + mListPadding.bottom + childHeight +
               getVerticalFadingEdgeLength() * 2;
   }

   if (heightMode == MeasureSpec.AT_MOST) {
       // TODO: after first layout we should maybe start at the first visible position, not 0
       heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
   }

   setMeasuredDimension(widthSize, heightSize);

   mWidthMeasureSpec = widthMeasureSpec;
}

通过ListView的onMeasure方法，我们看到：

1. 首先判断itemCount是否大于0，如果大于0，计算第一个item的高度
2. 由于heightMode是MeasureSpec.UNSPECIFIED，所以ListView的高度就是第一个item的高度加上相关的参数

所以这就得出了为什么ScrollView嵌套ListView会出现显示不全的问题。

如何解决

通过源码我们知道了问题所在的原因，那么如何解决呢？

这个时候就需要重写ListView，并覆盖onMeasure方法了

1. 首先我们知道系统的mode是MeasureSpec.UNSPECIFIED，而我们又发现：

   if (heightMode == MeasureSpec.AT_MOST) {
   // TODO: after first layout we should maybe start at the first visible position, not 0
   heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
   

   ListView当mode是MeasureSpec.AT_MOST的时候会依次累计计算每个item的高度，所以我们需要设置mode为MeasureSpec.AT_MOST。

2. 那么为什么要设置高度为Integer.MAX_VALUE >> 2

   final int measureHeightOfChildren(int widthMeasureSpec, int startPosition, int endPosition,
      int maxHeight, int disallowPartialChildPosition) {
       
       returnedHeight += child.getMeasuredHeight();
   
      if (returnedHeight >= maxHeight) {
          // We went over, figure out which height to return.  If returnedHeight > maxHeight,
          // then the i'th position did not fit completely.
          return (disallowPartialChildPosition >= 0) // Disallowing is enabled (> -1)
                      && (i > disallowPartialChildPosition) // We've past the min pos
                      && (prevHeightWithoutPartialChild > 0) // We have a prev height
                      && (returnedHeight != maxHeight) // i'th child did not fit completely
                  ? prevHeightWithoutPartialChild
                  : maxHeight;
      }
   }
   

   在计算高度的时候ListView会累加每个item的高度，并最终和maxHeight比较，也就是Integer.MAX_VALUE >> 2，当小于maxHeight的时候，就直接返回ListView的真实高度，这个时候问题也就解决了。