ListView设置adapter时getView被多次调用

今天在做项目的时候，发现ListView在高度为wrap_content时，adater中的getView方法居然被调用了三次，于是在网上搜索

发现也有网友与我碰到的问题相同，但有的只是说把wrap_content改为match_parent即可，但是亲测不行，这里就不吐槽一些只是转发别人的博客的网友了

不过还是发现了一些问题，那就是ListView被指定为wrap_content时无法确定高度，所以才会调用多次getView

看了下ListView的代码

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);
}

这里就是当ListView高度被指定为wrap_content走的代码

下面先看下 measureHeightOfChildren 方法

/**
* Measures the height of the given range of children (inclusive) and
* returns the height with this ListView's padding and divider heights
* included. If maxHeight is provided, the measuring will stop when the
* current height reaches maxHeight.
*
* @param widthMeasureSpec The width measure spec to be given to a child's
*            {@link View#measure(int, int)}.
* @param startPosition The position of the first child to be shown.
* @param endPosition The (inclusive) position of the last child to be
*            shown. Specify {@link #NO_POSITION} if the last child should be
*            the last available child from the adapter.
* @param maxHeight The maximum height that will be returned (if all the
*            children don't fit in this value, this value will be
*            returned).
* @param disallowPartialChildPosition In general, whether the returned
*            height should only contain entire children. This is more
*            powerful--it is the first inclusive position at which partial
*            children will not be allowed. Example: it looks nice to have
*            at least 3 completely visible children, and in portrait this
*            will most likely fit; but in landscape there could be times
*            when even 2 children can not be completely shown, so a value
*            of 2 (remember, inclusive) would be good (assuming
*            startPosition is 0).
* @return The height of this ListView with the given children.
*/
final int measureHeightOfChildren(int widthMeasureSpec, int startPosition, int endPosition,
final 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 > 0) && mDivider != null) ? mDividerHeight : 0;
// 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);

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;
}

多的代码先不看了 先看这行

child = obtainView(i, isScrap);

调用了AbsListView中的obtainView方法

/**
* Get a view and have it show the data associated with the specified
* position. This is called when we have already discovered that the view is
* not available for reuse in the recycle bin. The only choices left are
* converting an old view or making a new one.
*
* @param position The position to display
* @param isScrap Array of at least 1 boolean, the first entry will become true if
*                the returned view was taken from the scrap heap, false if otherwise.
*
* @return A view displaying the data associated with the specified position
*/
View obtainView(int position, boolean[] isScrap) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");

isScrap[0] = false;

// Check whether we have a transient state view. Attempt to re-bind the
// data and discard the view if we fail.
final View transientView = mRecycler.getTransientStateView(position);
if (transientView != null) {
final LayoutParams params = (LayoutParams) transientView.getLayoutParams();

// If the view type hasn't changed, attempt to re-bind the data.
if (params.viewType == mAdapter.getItemViewType(position)) {
final View updatedView = mAdapter.getView(position, transientView, this);

// If we failed to re-bind the data, scrap the obtained view.
if (updatedView != transientView) {
setItemViewLayoutParams(updatedView, position);
mRecycler.addScrapView(updatedView, position);
}
}

// Scrap view implies temporary detachment.
isScrap[0] = true;
return transientView;
}

final View scrapView = mRecycler.getScrapView(position);
final View child = mAdapter.getView(position, scrapView, this);
if (scrapView != null) {
if (child != scrapView) {
// Failed to re-bind the data, return scrap to the heap.
mRecycler.addScrapView(scrapView, position);
} else {
isScrap[0] = true;

child.dispatchFinishTemporaryDetach();
}
}

if (mCacheColorHint != 0) {
child.setDrawingCacheBackgroundColor(mCacheColorHint);
}

if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);
}

setItemViewLayoutParams(child, position);

if (AccessibilityManager.getInstance(mContext).isEnabled()) {
if (mAccessibilityDelegate == null) {
mAccessibilityDelegate = new ListItemAccessibilityDelegate();
}
if (child.getAccessibilityDelegate() == null) {
child.setAccessibilityDelegate(mAccessibilityDelegate);
}
}

Trace.traceEnd(Trace.TRACE_TAG_VIEW);

return child;
}

这里调用了adapter 的getView方法

final View updatedView = mAdapter.getView(position, transientView, this);

好了，下面看下obtainView 在ListView中被调用了多次，都是什么时候被调用的，上面这次是指定高度为wrap_content时 在onMeasure方法中调用 的

/**
* Obtain the view and add it to our list of children. The view can be made
* fresh, converted from an unused view, or used as is if it was in the
* recycle bin.
*
* @param position Logical position in the list
* @param y Top or bottom edge of the view to add
* @param flow If flow is true, align top edge to y. If false, align bottom
*        edge to y.
* @param childrenLeft Left edge where children should be positioned
* @param selected Is this position selected?
* @return View that was added
*/
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
boolean selected) {
View child;

if (!mDataChanged) {
// Try to use an existing view for this position
child = mRecycler.getActiveView(position);
if (child != null) {
// Found it -- we're using an existing child
// This just needs to be positioned
setupChild(child, position, y, flow, childrenLeft, selected, true);

return child;
}
}

// Make a new view for this position, or convert an unused view if possible
child = obtainView(position, mIsScrap);

// This needs to be positioned and measured
setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);

return child;
}

这个看注释是将子view添加到list中调用的

别的不看了，还有好多，到这里就是说，当ListView的高度被指定为wrap_content时getView应该至少会被调用两次

解决方法在网上搜到一个

参考：/article/1914154.html

亲测在ListView外部套一层RelativeLayout，ListView 高度设置为match_parent 生效