Implementing a ListView is probably one of the first things you do when learning Android development. At In the Pocket, we use them a lot. To make everything work as smooth as possible, all Android developer at In the Pocket agreed to use CursorAdapters as much as possible. You probably wonder why we do this?
Well, in 90% of the cases, the following flow is applicable for us:
- start a CursorLoader (pointing to one of our ContentProviders) and bind it to a list adapter
- http request to a web service
- parse http request response data
- bulk insert data into ContentProvider
- ContentProvider notifies all observers (in this case, the list adapter)
- CursorLoader automagically reloads the Cursor and updates the ListView
If you want to see a detailed example of this kind of implementation, take a look at the following training on the Android developers website:
http://developer.android.com/guide/topics/ui/layout/listview.html
Now, the important part of this article is about 2 similar methods in the CursorAdapter class: swapCursor and changeCursor. Judging by their name, it looks like the 2 methods are doing the same, but it’s important that you know how they differ from each other.
When you are using a CursorLoader, the Cursor is managed for you. The only thing you have to do is implement the following three methods:
// Called when a new Loader needs to be created public Loader<Cursor> onCreateLoader(int id, Bundle args) { // Now create and return a CursorLoader that will take care of // creating a Cursor for the data being displayed. return new CursorLoader(this, ContactsContract.Data.CONTENT_URI, PROJECTION, SELECTION, null, null); } // Called when a previously created loader has finished loading public void onLoadFinished(Loader<Cursor> loader, Cursor data) { // Swap the new cursor in. (The framework will take care of closing the // old cursor once we return.) mAdapter.swapCursor(data); } // Called when a previously created loader is reset, making the data unavailable public void onLoaderReset(Loader<Cursor> loader) { // This is called when the last Cursor provided to onLoadFinished() // above is about to be closed. We need to make sure we are no // longer using it. mAdapter.swapCursor(null); }
You don’t have to open and close the Cursor yourself, the loader will do this for you. This is the most important reason why you have to use swapCursor, it doesn’t close the Cursor when you swap it with another Cursor.
public Cursor swapCursor(Cursor newCursor) { if (newCursor == mCursor) { return null; } Cursor oldCursor = mCursor; if (oldCursor != null) { if (mChangeObserver != null) oldCursor.unregisterContentObserver(mChangeObserver); if (mDataSetObserver != null) oldCursor.unregisterDataSetObserver(mDataSetObserver); } mCursor = newCursor; if (newCursor != null) { if (mChangeObserver != null) newCursor.registerContentObserver(mChangeObserver); if (mDataSetObserver != null) newCursor.registerDataSetObserver(mDataSetObserver); mRowIDColumn = newCursor.getColumnIndexOrThrow("_id"); mDataValid = true; // notify the observers about the new cursor notifyDataSetChanged(); } else { mRowIDColumn = -1; mDataValid = false; // notify the observers about the lack of a data set notifyDataSetInvalidated(); } return oldCursor; }
ChangeCursor on the other hand, first swaps the current Cursor with the new one and then closes it for you. If you use this method with your CursorLoader, your app will crash.
public void changeCursor(Cursor cursor) { Cursor old = swapCursor(cursor); if (old != null) { old.close(); } }
Great explanation, Cliff. I was trying to figure out the difference, and this question is really related to the question about the benefits of using a cursor loader. Thank you!
nice explanation thank you mate
In my humble opinion this is really wrong. You’ll end up with a large number of open cursors and will run out of memory.
changeCursor must be used instead of swapCursor and no, your app will not crash.
I know the samples are misleading. However what I am saying is now confirmed by more recent Android documentation and some Google discussions.
Cheers.
Alex
Hi Alex
I don’t agree with what you are saying, but nevertheless thanks for your input on this topic. I appreciate your honest opinion!
Thanks Cliff!!!! While ChangeCursor did end up crashing my app I was able to use your blog to find that that was what I needed to use for my app instead of SwapCursor. Then after a few minutes of research I called a few destroyLoaders() on previously loaded loaders and I fixed a nasty bug in my app 😀
I don’t know if fusiller is right, but it won’t crash your app if you run the same code with changeCursor() instead. I just ran it.
It will if you actually change cursors and come back to original one. At least it did on my app
Thanks for this explanation.
Another pointer here :
https://code.google.com/p/android/issues/detail?id=42945