Книга: Beginning Android

Notify-on-Change Support

Notify-on-Change Support

An optional feature your content provider offers its clients is notify-on-change support. This means that your content provider will let clients know if the data for a given content Uri changes.

For example, suppose you have created a content provider that retrieves RSS and Atom feeds from the Internet based on the user’s feed subscriptions (via OPML, perhaps). The content provider offers read-only access to the contents of the feeds, with an eye toward several applications on the phone using those feeds versus everyone implementing their own feed-poll-fetch-and-cache system. You have also implemented a service that will get updates to those feeds asynchronously, updating the underlying data store. Your content provider could alert applications using the feeds that such-and-so feed was updated, so applications using that specific feed could refresh and get the latest data.

On the content-provider side, to do this call notifyChange() on your ContentResolver instance (available in your content provider via getContext().getContentResolver()). This takes two parameters: the Uri of the piece of content that changed, and the ContentObserver that initiated the change. In many cases, the latter will be null; a non-null value simply means the observer that initiated the change will not be notified of its own changes.

On the content-consumer side, an activity can call registerContentObserver() on its ContentResolver (via getContentResolver()). This ties a ContentObserver instance to a supplied Uri — the observer will be notified whenever notifyChange() is called for that specific Uri. When the consumer is done with the Uri, unregisterContentObserver() releases the connection.

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