通过查找文献和阅读代码发现: mViewpager.getAdapter.notifyDataSetChanged(); 一直返回POSITION_UNCHANGED. 默认pisition无变化即不刷新. 如果想viewpager在数据变化时候刷新,就需要让itemPosition 返回POSITION_NONE. 但是如果我们不对返回的条件进行判断.就会导致viewpager所有页面都会在调用 mViewpager.getAdapter.notifyDataSetChanged(); 强行刷新. 非常消耗内存和电量.
以下是源码和文献:
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 range newCurrItem = 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
在fragmentpageradapter中 可以对每一个object(实际上就是fragment)判断 他和当前页面的fragment是否属于同一个类，即是否属于同一个fragment来判断是否刷新该页面.
mViewpager.setAdapter(new FragmentPagerAdapter(getSupportFragmentManager()) {
Fragment mCurrentFragment;
@Override
public Fragment getItem(int position) {
return mIModel.getMainFragment(position);
}
@Override
public int getCount() {
return mTitles.length;
}
@Override
public CharSequence getPageTitle(int position) {
return mTitles[position];
}
@Override
public int getItemPosition(Object object) {
Fragment preFragment = (Fragment) object; //每个fragment
if (preFragment.getClass() == mCurrentView.getClass()) {
return POSITION_NONE;
} else {
return POSITION_UNCHANGED;
}
}
@Override
public void setPrimaryItem(ViewGroup container, int position, Object object) {
mCurrentFragment= (Fragment) object; //当前页面的fragmentg
super.setPrimaryItem(container, posi tion, object);
}
});
