How to handle REST calls, data persistence, syncing and observing ContentProvider

Question

I know that this question has been asked too many times, but I think the issues I'm trying to target are a little bit different, maybe more complicated.

I am going to develop an application that uses a RESTful Web Service and needs to have the following requirements:

• the app should show some books, their authors and their editors in lists and in detail

• the app should also allow searching for a book

• books, authors and editors are fetched from a RESTful web service

• every entity has to be cached so that when I open an Activity I see the old data first (if any), while the new one updates from the network.

• every time an entity is updating, the interested parties should be notified (ContentObserver? A regular Listener implementation?)

• if a call is already executing (say to api/books/1337 or to api/editors) the caller should be notified that it is loading data and should be given the old one (if it exists), as if it was the original caller.

• some data (only books and authors) should be updated every N minutes (decided by the user) and the observers should be notified (SyncAdapter?)

Questions:

After watching and studying all of the components proposed by Virgil Dobjanschi at Google I/O 2010 here are my doubts:

1. How can I transparently handle the "entity-is-updating" concept for any caller? Should I use ContentObserver on a ContentProvider I will have to implement?

2. If I use a ContentObserver I can easily set a status-flag for the single entity (as suggested by Dobjanschi), for example UPDATING, INSERTING, and so on. But how should I handle list? Say I want a list of books, where should I put the status flag? Should I put it in a status table for lists only? If so, I should observe two Cursors, one for the status and one for the actual list (i.e., the table/Content URI). And what if the entity I'm asking for does not exists (yet) or the REST call returns a 404? How do I handle the callback?

3. If I put all of my REST methods in a **SyncAdapter**, can I "force" the SyncAdapter to update an entity/entity list from the network (and therefore put it into the proper table)? This way, the status flag would be useful.

4. Can the SyncAdapter work on multiple entities (actually, entity lists, as I want to update books and editors every now and then), since it only has a performSync method?

5. If my SyncAdapter implementation has been disabled by the user in the device settings it won't update anything (and that's fine). But if the user clicks on an "update books" button in an Activity, can I still call the performSync method, or will it be disabled as well?

Dourine

Answer 1

SyncAdapter is a design pattern involving five components:

1. An app. This uses a set of Activity along with Cursor and ContentObserver and maybe CursorAdapter and some to provide a UI to the locally stored data from a ContentProvider.
2. ContentProvider The local device's data-store. Handles CRUD calls, handles notifying SyncAdapter of the need to push an update to the server.
3. Account The user identity on the remote server.
4. SyncAdapter A background process which runs, and keeps the local datastore in sync with the server.
5. The server itself.

So. To the questions:

1. "Is-updating" means, "has local changes which have not yet been pushed to the server. It's a flag you set on a row in your database. It's set in ContentProvider when you Create/Update/Delete a row. When SyncAdapter runs, it sees the flag, pushes the update to the server, clears the flag. The flag itself does two things:
   a. Tells the user the app is busy saving the change, and when that's done.
   b. Marks the row as changed so SyncAdapter knows to push it to the server.
   Read here for more details.

2. If you're not syncing the entire catalog, then your client will directly query the server and cache the results by putting them into the ContentProvider. There is no status flag there, since they're coming from the server and therefore match the server state. Write your SyncAdapter to ignore them, or perhaps discard them after they've been cached a few days.

3. a. To ensure your local updates get sent to the server, you write your ContentProvider to notify the SyncAdapter during the ContentProvider's Create/Update/Delete calls. (Read here...)
   b. To ensure you get updates from the server periodically, you configure the account for automatic sync. (Read Here...)

4. Yes. performSync is just a function call. Write it to do what you want. Have it fetch table 1 from the server and put it into one table in your ContentProvider. Then have it fetch table 2, and put it into a different table. Etc.

5. a. You can force a sync by calling ContentResolver.RequestSync() with ContentResolver.SYNC_EXTRAS_MANUAL in the extras bundle.
   b. You can manually fetch something with client code and directly push it into the ContentProvider.

Horselaugh

Thank you very much for your valuable answer. I still haven't got the list-thing right: let's say the user clicks on an "Update" button. What should fetch the updated list (maybe using a since parameter)? The ContentProvider or a custom-defined service? And if the user exits the Activity and re-enters it when the fetching hasn't completed yet, how can I know that there are some entities updating? I thought I could use the status column to get and set the status for any event: get, delete, and so on. – Dourine

ContentProvider will NEVER fetch data for an update. It's just your local storage provider. You have two options there. 1. You can build a table in your ContentProvider for "requests for update", and then have SyncAdapter read that table to force a pull, and then once it has the results, push them into the ContentProvider. Second option, your app itself spawns a background thread, fetches the data, pushes into ContentProvider. Build your data fetching and parsing classes in isolation and make both the SyncAdapter and the application call them. Either way, you can flag a row as "updating". – Horselaugh

Thank you. This is so far the best answer I've ever got on StackOverflow. – Dourine

Any demo applications available on github that explains above mentioned best practice, building efficient mobile app? Currently I'm reverse engineering the Google I/O app! – Auntie

Answer 2

If you take a look at Running Sync Adapter section in Google Developers Training, you will find a whole detailed description of how to:

1. Run the Sync Adapter When Server Data Changes
2. Run the Sync Adapter When Content Provider Data Changes
3. Run the Sync Adapter After a Network Message
4. Run the Sync Adapter Periodically
5. Run the Sync Adapter On Demand

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