When SyncAdapter runs synchronization on android?

Question

Let's say, my application implements SyncAdapter functionality and doesn't define periodic syncs. When synchronization will happen in such scenario? First scenario I may think about is local ContentProvided/database content change. What is about server changes? How SyncAdapter will know about that?

Answer 1

If you have no periodic sync setup, Sync will happen if your code explicitly calls ContentResolver.requestSync(Account account, String authority, Bundle extras) with your account and authority.

Also, if your ContentProvider insert or update or delete functions call ContentResolver.notifyChange(Uri uri, ContentObserver observer, boolean syncToNetwork), if the bool syncToNetwork is true (the default), it will also trigger a sync. There's a short delay induced here, to ensure that a batch of database changes only causes one sync, not one per change. Note that your code should be calling notifyChange because it's how Android signals your UI to update after Content that the UI is reflecting had been changed.

If the server database changes, your app won't know, because sync isn't happening. Two options:

1. Use periodic sync. This will be cleaner if your server API implements etags or the if-modified-since http headers to filter the data you sync so only the updates come down.
2. C2DM (Cloud 2 Device Messaging) Essentially, push notification for Android. Requires some server components -- You tie a device ID to an account on the server and when the server changes, it has to explicitly send a message to the device to tell it to update. This is custom code work on the server to support android specifically, but once you invest the time, it's great. C2DM is how Android gets gmail to show up on your device 10 seconds after it arrives in your inbox, rather than at the next 10 minute periodic sync. It's also more battery efficient since you only turn on the radio and sync when you know there's new data to get.