#notifydatasetchanged

Fun bug from this afternoon: let's say you have a custom ListAdapter you're working with to fill in data into a ListView, and you're trying to implement a method which allows you to insert a new element into that list. You're competent with the use of Adapters, so you're confident it works - in fact, the model for your add function matches most of the recommendations on StackOverflow:

So, you're confident everything is looking good, yet whenever you actually go to add the new string, the list doesn't update! It comes up when you leave and return to the activity (because you're properly serializing elsewhere in code), but it won't update on the spot.

So you go hunting further into StackOverflow to see if there was something subtle you missed about how to hook up a ListAdapter properly in your ListFragment. Maybe using setListAdapter doesn't properly set up your fragment's ListView as an observer for the adapter? That wouldn't make much sense, but you aren't sure. Perhaps it was something else...

Regardless, you spend a few hours hunting, and nothing comes up. All the errors with NotifyDataSetChanged are pretty dumb errors regarding simply misusing the class, which you've confirmed many times over by now that you haven't done.

So what's the problem?

Well, in case you had the same issues I did this afternoon, it may be that you accidentally let Eclipse fill in your functions for you, so among them you overwrote registerDataSetObserver and unregisterDataSetObserver, and they're filled in with TODO messages rather than calling the superclass's versions. WHOOPS.

在stackoverflow有一篇文章的問題是"ViewPager PagerAdapter not updating the View"，在觀看這篇文章時，剛好我的app應用也有類似的問題，就是該如果更新ViewPager裡某一個View的內容，而不是全部砍掉重建。後來在欣賞外國佬的回答後，有位大大rui.araujo提出一個十分容易解決的方式，就是改寫getItemPosition()

Override getItemPosition in your PagerAdapter like this:

public int getItemPosition(Object object) {     return POSITION_NONE; }

This way, when you call notifyDataSetChanged(), the view pager will remove all views and reload them all. As so the reload effect is obtained.

讓每次呼叫PagerAdapter時，都會被誤判會POSITION_NONE並重新建置所有View。如此一來會造成效能上很大的問題，而且邏輯上也是不太正確。我一開始也是使用這方法來解決我碰到的問題，但後來另一位大大alvarolb為此種錯誤的解法方式做了一些解釋，當中提到為何改寫getItemPosition為永遠回POSITION_NONE是錯誤的邏輯，原文如下：

The notifyDataSetChanged() method on the PagerAdapter will only notify the ViewPager that the underlying pages have changed. For example, if you have create/deleted dynamically pages (adding or removing items from your list) the ViewPager should take care about that. In this case i think that the ViewPager determines if a new view should be deleted or instantiated using the getItemPosition() and getCount() methods.

就連官方SDK文件也說明了ViewPager Adapter notifyDataSetChanged的用意為

PagerAdapter supports data set changes. Data set changes must occur on the main thread and must end with a call to notifyDataSetChanged() similar to AdapterView adapters derived from BaseAdapter. A data set change may involve pages being added, removed, or changing position. The ViewPager will keep the current page active provided the adapter implements the method getItemPosition(Object).

原文看完後，大概是說明Adapter會自動控管View Pager每一頁(Item)的狀態，而notifyDataSetChanged()是用在當View Pager要新增一頁、刪除一頁或改變各個頁面的排列的時候。所以ViewPager Adapter的notifyDataSetChanged自然就不適用於只要更新View Pager裡面某個View的內容的需求啦。所以alvarolb也針對此向需求做了一翻解釋

My approach is to use the setTag() method for any instantiated view in the instantiateItem() method. So when you want to change the data or invalidate the view that you need, you can call the findViewWithTag() method on the viewPager to retrieve the previously instantiated view and modify/use it as you want without having to delete create any new view each time you want to update some value.

關鍵點就是在instantiateItem時，將有需要在後續進行內容更新的view來setTag()，並且在需要更新的時候，使用viewpager的findViewWithTag()，來找到要更新的View並進行更新。講了這麼長的文章，還是來用程式碼來說明會來的直覺一點，我有省略很多細節，主要把精隨分享給大家。

@Override public Object instantiateItem(ViewGroup container, int position) { View view = null; view = mInflater.inflate(R.layout.record_list_layout, null); TextView tvRecord = (TextView) view.findViewById(R.id.tv_record); String key = "tvRecord" + position; // 關鍵點，針對要更新的View來設定Tag，主要是在後續使用ViewPager.findViewWithTag()時，可以找到要更新的View tvRecord.setTag(key); container.addView(view); return view; } // 在後續的應用當中，如果要開始更新View Pager當中某個View的內容時，需要進行下列動作 TextView tvRecord = myViewPager.findViewWithTag("tvRecord1"); // 假設要更新第2頁的TextView // 進行內容更新 if (tvRecord != null ) { tvRecord.setText("update"); } /* end of if */

=======下列為最簡單且暫時能夠解決nofifyDataSetChanged無作用的問題，但如果view pager有很多複雜的view所組成，會有效能上的問題，請注意。=======

如果沒有用過Android ViewPager的開發者們，可以先參考官方的介紹與文件說明(雖然現在官方因為推廣Fragment更推荐使用FragmentPagerAdapter，看來又要花時間了解一下了XDD)。除了官方說明之外，我之前也有介紹一些相關文章，像是如何使用Vertical View Pager、如何取得View Pager當中的child view、如何讓ViewPager有彈回效果。

回歸正題，為何直接呼叫PagerAdapter.notifyDataSetChanged()會無作用呢？主要是因為在運作當中會呼叫到getItemPosition (Object object)，sdk文件說明當中也提到

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 or POSITION_NONE if the item is no longer present in the adapter.

如果有參考sdk文件說明的人，眼尖的人會看到這一行

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

預設值是POSITION_UNCHANGED，代表此item內容並沒有更新

所以這時就要對你的ViewPagerAdapter動一些手腳，這時需要改寫getItemPosition()：

@Override public int getItemPosition(Object object) { // 回傳POSITION_NONE告知此item已經不在Adapter裡 return POSITION_NONE; }

因為已經改寫getItemPosition為強制回傳POSITION_NONE，如果一來，每次呼叫notifyDataSetChanged，就會重新繪製ViewPager。