android - What are the benefits of CursorLoaders?

Question

I use Cursors extensively in my app, to load and occasionally write information from and to a database. I have seen that Honeycomb and the Compatibility Package have new Loader classes designed to help with loading data in a "good" way.

Essentially, are these new classes (in particular CursorLoader) considerably better than previous methods of managing data? What is the benefit of a CursorLoader over managed Cursors for example?

And I use a ContentProvider to deal with data, which obviously takes Uris but how does this mesh with the initLoader() method? Must I set up each of my Fragments to use Loaders individually? And how unique does the id need to be for each loader, is it over the scope of my app or just a fragment? Is there any simple way of simply passing a Uri to a CursorLoader to query my data?

All I can see at the moment is that Loaders add an unnecessary extra step to getting my data into my app, so can someone explain them to me better?

Answer 1

There are two key benefits to using a CursorLoader in your app over Activity.managedQuery():

1. The query is handled on a background thread for you (courtesy of being build on AsyncTaskLoader) so large data queries do not block the UI. This is something the docs recommended you do for yourself when using a plain Cursor, but now it's done under the hood.
2. CursorLoader is auto-updating. In addition to performing the initial query, the CursorLoader registers a ContentObserver with the dataset you requested and calls forceLoad() on itself when the data set changes. This results in you getting async callbacks anytime the data changes in order to update the view.

Each Loader instance is also handled through the singular LoaderManager, so you still don't have to manage the cursor directly, and now the connection can persist even beyond a single Activity. LoaderManager.initLoader() and LoaderManager.restartLoader() allow you to reconnect with an existing Loader already set up for your query and, in some cases, instantly get the latest data if it is available.

Your Activity or Fragment will likely now implement the LoaderManager.Callback interface. Calling initLoader() will result in the onCreateLoader() method where you will construct the query and a new CursorLoader instance, if necessary. The onLoadFinished() method will be fired each time new data is available, and will include the latest Cursor for you to attach to the view or otherwise iterate through.

In addition, there is a pretty good example of all this fitting together on the LoaderManager class documentation page: http://developer.android.com/reference/android/app/LoaderManager.html

Hope that Helps!