很久不寫blog了，今天遇到問題了記錄一下別人的解決方法。
Viewpager在調用notifyDataSetChanged()時，界面無刷新。
相信很多做過Viewpager的同學肯定遇到過這個問題，這個是bug還是android就是如此設計的，我們不做討論。總之，它確實影響我們功能的實現了。
可能不少同學選擇爲Viewpager重新設置一遍適配器adapter，達到刷新的目的。但是這種方法在大多數情況下，是有問題的。
追蹤源代碼：
爲什麼調用數據更新的方法，Viewpager卻沒有更新呢，我們跟進該方法的源代碼看一下。
首先查看適配器調用的super.notifyDataSetChanged()，該方法調到抽象基類PagerAdapter.notifyDataSetChanged()中：
/**
* 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();
}
註釋裏說到，當附加在適配器上的數據發生變化時，應該調用該方法刷新數據。該方法調用了一個mObservable .notifyChanged()；
我們繼續跟進這個方法，進入DataSetObservable類中，發現這樣一段代碼：
/**
* 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();
}
}
}
這都不是重點，重點我們來看這個mObservers的類型是一個抽象類DataSetObserver，裏面只有兩個未實現的方法，都有誰使用了這個抽象類呢，快捷鍵 ctrl + alt + H ,在衆多的調用者當中，我們發現了Viewpager的身影
進入viewpager，我們終於找到了viewpager中控制數據變更的重點方法dataSetChanged ，這個方法如下：
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();
}
}
重點看這樣一行代碼：
final int newPos = mAdapter.getItemPosition(ii.object );
if (newPos == PagerAdapter.POSITION_UNCHANGED ) {
continue ;
}
官方對getItemPosition()的解釋是：
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 returns POSITION_UNCHANGED.
意思是如果item的位置如果沒有發生變化，則返回POSITION_UNCHANGED。如果返回了POSITION_NONE，表示該位置的item已經不存在了。默認的實現是假設item的位置永遠不會發生變化，而返回POSITION_UNCHANGED
解決方案：
所以我們可以嘗試着修改適配器的寫法，覆蓋getItemPosition()方法，當調用notifyDataSetChanged時，讓getItemPosition方法人爲的返回POSITION_NONE，從而達到強迫viewpager重繪所有item的目的。
具體代碼如下：
class SearchAdapter extends PagerAdapter {
private int mChildCount = 0;
@Override
public void notifyDataSetChanged() {
mChildCount = getCount();
super.notifyDataSetChanged();
}
@Override
public int getItemPosition(Object object) {
if ( mChildCount > 0) {
mChildCount --;
return POSITION_NONE;
}
return super.getItemPosition(object);
}
}