Global Loader (LoaderManager) for reuse in multiple Activities / Fragments
What I would like to achieve:
I have two different fragments. I would like them both to show the same data in two forms (in a list and on a map). I would like them to share one Loader (AsyncTaskLoader in particular). Everything works fine, but the Loader isn't re-used. Another one is created and the data is loaded twice.
What I do:
In the Fragments I use LoaderManager lm = getActivity().getSupportLoaderManager();
In both of them I implement LoaderCallbacks<ArrayList<Item>> and the required methods.
In both I use lm.initLoader(0, args, this);.
But when I output the lm.toString() it appears that these are two different Loaders. And the data is downloaded twice.
How to re-connect to the same Loader from a different Activity/Fragment than the one it was started in?
It should be possible since the context is attached to the Loader anyway on every onCreate(), e.g. on configuration change.
Answer
How to re-connect to the same Loader from a different Activity/Fragment than the one it was started in?
You should not reuse Loaders that are being managed by a LoaderManager instance across multiple Activitys and Fragments.
The LoaderManager will start/stop those Loaders with respect to the Activity/Fragment lifecycle, so there is no way of guaranteeing that those Loaders will exist once you are in another Activity.
From the documentation:
LoaderManager.LoaderCallbacks is a callback interface that lets a client interact with the LoaderManager.
Loaders, in particular CursorLoader, are expected to retain their data after being stopped. This allows applications to keep their data across the activity or fragment's onStop() and onStart() methods, so that when users return to an application, they don't have to wait for the data to reload. You use the LoaderManager.LoaderCallbacks methods when to know when to create a new loader, and to tell the application when it is time to stop using a loader's data.
In other words, it is often the case that your Loaders will be specific to some Activity (or Fragment). When you have your Activity implement the LoaderManager.LoaderCallbacks interface, your Activity is given type LoaderManager.LoaderCallbacks. Each time you call initLoader(int ID, Bundle args, LoaderCallbacks<D> callback), the LoaderManager either creates or reuses a Loader that is specific to some instance of the LoaderManager.LoaderCallbacks interface (which in this case is an instance of your Activity). This essentially binds your Activity with a Loader, and its callback methods will be called as the loader state changes.
That being said, unless you can find a way to have your two separate Activitys share the same callback methods, I doubt there is a clean way to do this (i.e. having an Activity and a Fragment share the same callbacks sounds like it would be tricky, if not impossible). I wouldn't worry about it too much though. In all of the sample code I have ever seen, I've never seen two Activitys and/or Fragments share the same callback methods. Further, given that Activitys and Fragments are both supposed to be designed for reuse, sharing Loaders in this way just doesn't seem like something that would be encouraged.