在之前的开发中(没有recyclerview的时候)有时候会碰到ScrollView和ListView的嵌套使用，这个时候就会出现ListView只能显示一个条目，当时直接在网上找了解决方法，做法是自定义一个ListView重写它的onMeasure方法，也没有去研究为什么会这样，最近在看到一位大牛的资料的时候，其中就讲到为什么会出现这样的现象，所以就记录在这里。
ScrollView和ListView都是系统帮我们已经实现了的控件，都是继承自ViewGroup，那么他们在测量的时候都会调用onMeasure方法，下面是
ListView的onMeasure源码:
@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);
}
}
if (widthMode == MeasureSpec.UNSPECIFIED) {
widthSize = mListPadding.left + mListPadding.right + childWidth +
getVerticalScrollbarWidth();
} else {
widthSize |= (childState & MEASURED_STATE_MASK);
}
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;
}
一开始我们就看到会调用MeasureSpec.getMode方法去获取父布局的宽高的模式，在这里的会有三种模式：
MeasureSpec.AT_MOST 相当于wrap_content
MeasureSpec.EXACTLY 相当于在布局中指定了值 或者match_parent 或者fill_parent
MeasureSpec.UNSPECIFIED 尽可能的大
前两种在开发中使用的比较多，后面那一种的话就使用的比较少，而对于ScrollView来说，它的模式就是MeasureSpec.UNSPECIFIED，所以ListView在获取宽高模式的时候获取到的就是MeasureSpec.UNSPECIFIED，而在ListView的onMeasure源码中对宽高模式做了判断：
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);
}
如果获取到的宽高模式是MeasureSpec.UNSPECIFIED的时候，ListView的高度就是：
heightSize = mListPadding.top + mListPadding.bottom + childHeight +
getVerticalFadingEdgeLength() * 2;
所以就要重写ListView的onMeasure方法，改变宽高模式；
public class MyListView extends ListView{
public MyListView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
public MyListView(Context context) {
super(context);
}
public MyListView(Context context, AttributeSet attrs) {
super(context, attrs);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//解决显示不全的问题
heightMeasureSpec=MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE>>2,MeasureSpec.AT_MOST);
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
}
在这里将宽高模式改为MeasureSpec.AT_MOST这样就会走源码中的这里去测量计算所有条目的高度：
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);
}
在重写的时候我们还传入了一个Integer.MAX_VALUE>>2(Integer的最大值右移两位)，在ListView的measureHeightOfChildren方法中对最大高度做了判断；
measureHeightOfChildren()方法源码:
final int measureHeightOfChildren(int widthMeasureSpec, int startPosition, int endPosition,
int maxHeight, int disallowPartialChildPosition) {
final ListAdapter adapter = mAdapter;
if (adapter == null) {
return mListPadding.top + mListPadding.bottom;
}
// Include the padding of the list
int returnedHeight = mListPadding.top + mListPadding.bottom;
final int dividerHeight = mDividerHeight;
// The previous height value that was less than maxHeight and contained
// no partial children
int prevHeightWithoutPartialChild = 0;
int i;
View child;
// mItemCount - 1 since endPosition parameter is inclusive
endPosition = (endPosition == NO_POSITION) ? adapter.getCount() - 1 : endPosition;
final AbsListView.RecycleBin recycleBin = mRecycler;
final boolean recyle = recycleOnMeasure();
final boolean[] isScrap = mIsScrap;
for (i = startPosition; i <= endPosition; ++i) {
child = obtainView(i, isScrap);
measureScrapChild(child, i, widthMeasureSpec, maxHeight);
if (i > 0) {
// Count the divider for all but one child
returnedHeight += dividerHeight;
}
// Recycle the view before we possibly return from the method
if (recyle && recycleBin.shouldRecycleViewType(
((LayoutParams) child.getLayoutParams()).viewType)) {
recycleBin.addScrapView(child, -1);
}
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;
}
if ((disallowPartialChildPosition >= 0) && (i >= disallowPartialChildPosition)) {
prevHeightWithoutPartialChild = returnedHeight;
}
}
// At this point, we went through the range of children, and they each
// completely fit, so return the returnedHeight
return returnedHeight;
}
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;
}
这样子的话就解决了ScrollView嵌套ListView显示不全的问题，如上面有写的不对的地方，欢迎交流。
