【Android】ScrollView巢狀ListView只顯示第一行問題原理分析

指間沙似流年發表於2018-01-05
AndroidView巢狀

通常情況下我們在使用ScrollView巢狀ListView的時候，當出現問題的時候，相信絕大部分人都是在網上直接找別人的解決方案，都沒有關心為什麼會出現這種問題，為什麼這樣解決。

通常情況下ScrollView巢狀ListView會出現只會顯示ListView的第一個item的情況，而網上大部分的解決方式是：

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

那麼為什麼會出現這樣的情況呢？為什麼這樣解決呢？ 我們來一起分析分析：

問題原因

出現問題的時候我們會發現只顯示了ListView的第一個item，而其他item都是可滑動出來的，猜測是由於onMeasure的問題導致的。

所以我們先來看ScrollView的原始碼：

@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);
       }
   }
}
複製程式碼

通過原始碼可以看到ScrollView在測量的時候只拿了子View中的第一個，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);
       }
   }
   // 程式碼省略

   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;
}
複製程式碼

通過ListView的onMeasure方法，我們看到：

  1. 首先判斷itemCount是否大於0，如果大於0，計算第一個item的高度
  2. 由於heightMode是MeasureSpec.UNSPECIFIED，所以ListView的高度就是第一個item的高度加上相關的引數

所以這就得出了為什麼ScrollView巢狀ListView會出現顯示不全的問題。

如何解決

通過原始碼我們知道了問題所在的原因，那麼如何解決呢？

這個時候就需要重寫ListView，並覆蓋onMeasure方法了

  1. 首先我們知道系統的mode是MeasureSpec.UNSPECIFIED，而我們又發現：

    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);
複製程式碼

    ListView當mode是MeasureSpec.AT_MOST的時候會依次累計計算每個item的高度，所以我們需要設定mode為MeasureSpec.AT_MOST。

  2. 那麼為什麼要設定高度為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;
   }
}
複製程式碼

    在計算高度的時候ListView會累加每個item的高度，並最終和maxHeight比較，也就是Integer.MAX_VALUE >> 2，當小於maxHeight的時候，就直接返回ListView的真實高度，這個時候問題也就解決了。

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