Android -- 处理ViewPager的notifyDataSetChanged无刷新

Viewpager在调用notifyDataSetChanged()时，界面无刷新

Viewpager在调用notifyDataSetChanged()时，界面无刷新，它确实影响我们功能的实现了。可能选择为Viewpager重新设置一遍适配器adapter，达到刷新的目的。但是这种方法在大多数情况下，是有问题的。

查看方法

super.notifyDataSetChanged()调用的是PagerAdapter.notifyDataSetChanged()

/*** This method should be called by the application if the data backing this adapter has changed* and associated views should update.*/public void notifyDataSetChanged() {mObservable.notifyChanged();}

注释里说到，当附加在适配器上的数据发生变化时，应该调用该方法刷新数据。该方法调用了一个mObservable .notifyChanged()；

我们继续跟进这个方法，进入DataSetObservable类中，发现这样一段代码：

/*** Invokes {@link DataSetObserver#onChanged} on each observer.* Called when the contents of the data set have changed.  The recipient* will obtain the new contents the next time it queries the data set.*/public void notifyChanged() {synchronized(mObservers ) {// since onChanged() is implemented by the app, it could do anything, including// removing itself from {@link mObservers} - and that could cause problems if// an iterator is used on the ArrayList {@link mObservers}.// to avoid such problems, just march thru the list in the reverse order.for (int i = mObservers .size() - 1; i >= 0; i--) {mObservers.get(i).onChanged();}}}

这都不是重点，重点我们来看这个mObservers的类型是一个抽象类DataSetObserver，里面只有两个未实现的方法，都有谁使用了这个抽象类呢，其中我们发现了Viewpager的身影。进入viewpager，我们终于找到了viewpager中控制数据变更的重点方法dataSetChanged ，这个方法如下：

void dataSetChanged () {// This method only gets called if our observer is attached, so mAdapter is non-null.boolean needPopulate = mItems .size() < mOffscreenPageLimit * 2 + 1 &&mItems.size() < mAdapter.getCount();int newCurrItem = mCurItem ;boolean isUpdating = false;for (int i = 0; i < mItems.size(); i++) {final ItemInfo ii = mItems .get(i);final int newPos = mAdapter.getItemPosition(ii.object );if (newPos == PagerAdapter.POSITION_UNCHANGED ) {continue;}if (newPos == PagerAdapter.POSITION_NONE) {mItems.remove(i);i--;if (!isUpdating) {mAdapter.startUpdate( this);isUpdating = true;}mAdapter.destroyItem( this, ii.position , ii.object);needPopulate = true;if (mCurItem == ii.position ) {// Keep the current item in the valid rangenewCurrItem = Math. max(0, Math.min(mCurItem, mAdapter.getCount() - 1));needPopulate = true;}continue;}if (ii.position != newPos) {if (ii.position == mCurItem ) {// Our current item changed position. Follow it.newCurrItem = newPos;}ii. position = newPos;needPopulate = true;}}if (isUpdating) {mAdapter.finishUpdate( this);}Collections. sort(mItems, COMPARATOR);if (needPopulate) {// Reset our known page widths; populate will recompute them.final int childCount = getChildCount();for (int i = 0; i < childCount; i++) {final View child = getChildAt(i);final LayoutParams lp = (LayoutParams) child.getLayoutParams();if (!lp.isDecor ) {lp. widthFactor = 0.f;}}setCurrentItemInternal(newCurrItem, false, true);requestLayout();}}

重点看这样一行代码：

final int newPos = mAdapter.getItemPosition(ii.object );if (newPos == PagerAdapter.POSITION_UNCHANGED ) {continue ;}

Called when the host view is attempting to determine if an item’s position has changed. Returns POSITION_UNCHANGED if the position of the given item has not changed orPOSITION_NONE if the item is no longer present in the adapter.

The default implementation assumes that items will never change position and always returnsPOSITION_UNCHANGED.

意思是如果item的位置如果没有发生变化，则返回POSITION_UNCHANGED。如果返回了POSITION_NONE，表示该位置的item已经不存在了。默认的实现是假设item的位置永远不会发生变化，而返回POSITION_UNCHANGED

解决方案

所以我们可以尝试着修改适配器的写法，覆盖getItemPosition()方法，当调用notifyDataSetChanged时，让getItemPosition方法人为的返回POSITION_NONE，从而达到强迫viewpager重绘所有item的目的。

class SearchAdapter extends PagerAdapter {private int mChildCount = 0;@Overridepublic void notifyDataSetChanged() {         mChildCount = getCount();super.notifyDataSetChanged();}@Overridepublic int getItemPosition(Object object)   {          if ( mChildCount > 0) {mChildCount --;return POSITION_NONE;}return super.getItemPosition(object);}}

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本文转自我爱物联网博客园博客，原文链接：http://www.cnblogs.com/yydcdut/p/4232614.html，如需转载请自行联系原作者