接着上面一篇文章。
当GridView不知道parent高度的时候，也就是MeasureSpec是UNSPECIFIED，这个时候，GridView高度为第一个child的高度，并显示滚动条。
mItemCount = mAdapter == null ? 0 : mAdapter.getCount();
final int count = mItemCount;
if (count > 0) {
final View child = obtainView(0, mIsScrap); AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
if (p == null) {
p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
child.setLayoutParams(p);
}
p.viewType = mAdapter.getItemViewType(0);
p.forceAdd = true; int childHeightSpec = getChildMeasureSpec(
MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED), 0, p.height);
int childWidthSpec = getChildMeasureSpec(
MeasureSpec.makeMeasureSpec(mColumnWidth, MeasureSpec.EXACTLY), 0, p.width);
child.measure(childWidthSpec, childHeightSpec); childHeight = child.getMeasuredHeight();
childState = combineMeasuredStates(childState, child.getMeasuredState()); if (mRecycler.shouldRecycleViewType(p.viewType)) {
mRecycler.addScrapView(child, -1);
}
} if (heightMode == MeasureSpec.UNSPECIFIED) {
heightSize = mListPadding.top + mListPadding.bottom + childHeight +
getVerticalFadingEdgeLength() * 2;
} if (heightMode == MeasureSpec.AT_MOST) {
int ourSize = mListPadding.top + mListPadding.bottom; final int numColumns = mNumColumns;
for (int i = 0; i < count; i += numColumns) {
ourSize += childHeight;
if (i + numColumns < count) {
ourSize += mVerticalSpacing;
}
if (ourSize >= heightSize) {
ourSize = heightSize;
break;
}
}
heightSize = ourSize;
}
我们可以发现，确实是只设置为第一个child的高度，当MeasureSpec是AT_MOST的时候，才会读取所有child的高度。
再网上，搜到了下面的办法。
heightMeasureSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE, MeasureSpec.AT_MOST);
其实这是不对的，MeasureSpec的前两个bits是mode，size的大小不能超过30位。于是，稍微修改，size设置为1000000，应该是足够了，当不足够的时候，你的app如果还没更新早就该被淘汰了。
heightMeasureSpec = MeasureSpec.makeMeasureSpec(1000000, MeasureSpec.AT_MOST);