1.问题
scrollView嵌套ListView会导致ListView只显示一行item的高度，显示不全。
2.原因
通过查看源码发现：
ScrollView和ListView的heightMeasureSpec都是MeasureSpec.UNSPECIFIED
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
// MeasureSpec由父级传进来，因为ScrollView为MeasureSpec.UNSPECIFIED，所以这里的heightMode == MeasureSpec.UNSPECIFIED
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);
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);
}
}
if (widthMode == MeasureSpec.UNSPECIFIED){
widthSize = mListPadding.left + mListPadding.right + childWidth + getVerticalScrollbarWidth();
} else {
widthSize |= (childState & MEASURED_STATE_MASK);
}
// 因为ListView的模式也是MeasureSpec.UNSPECIFIED，所以导致进入if
if (heightMode ==MeasureSpec.UNSPECIFIED){
// 高度就为一个的高度childHeight
heightSize = mListPadding.top + mListPadding.bottom + childHeight + getVerticalFadingEdgeLength() * 2;
}
if (heightMode == MeasureSpec.AT_MOST) {
heightSize = measureHeightOfChildren( widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
}
setMeasuredDimension(widthSize, heightSize);
mWidthMeasureSpec = widthMeasureSpec;
}
根据源码可以发现，导致显示不全的原因是heightMode == MeasureSpec.UNSPECIFIED进入了：
if (heightMode == MeasureSpec.UNSPECIFIED) {
// 高度就为一个的高度childHeight
heightSize = mListPadding.top + mListPadding.bottom + childHeight +
getVerticalFadingEdgeLength() * 2;
}
因此导致heightSize 被重新赋值为了一个childHeight 的高度。
3.解决方法
自己继承一个ListView重写onMeasure，通过MeasureSpec.makeMeasureSpec()方法来设置heightMode为MeasureSpec.AT_MOST。
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// 将模式设置为MeasureSpec.AT_MOST
heightMeasureSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE>>2,MeasureSpec.AT_MOST);
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
Integer.MAX_VALUE>>2 ：因为MeasureSpec 一个值表示了两个值，最高两位代表模式，后面代表的是尺寸。而这边makeMeasureSpec传的第一个参数是给子级的最大尺寸，所以右移两位就是能给子级最大的高度，所以解决了显示不全的问题。