When Sync in Background is enabled, a silent push is sent to all devices that are linked to the same iCloud/Dropbox account as soon as changes have been uploaded.
When this push notification is received on an iOS device, this silent push notification requests iOS to wake Due up in the background to fetch changes, cancel any unneeded notifications and schedule new ones.
When received on a Mac, Due will automatically fetch the changes from Dropbox if Due is currently running.
If background sync does not seem to be working for you on your iOS devices, do make sure that the ‘Sync in Background' switch is enabled in Due > Settings > Sync
That said, iOS can choose not to wake up an app despite receiving a silent push. There are a few reasons:
- If you manually force quit Due (as described in http://support.apple.com/en-us/ht5137), iOS will not wake up Due.
- If you disable 'Background App Refresh' in iOS Settings > General > Background App Refresh > Due, background sync cannot work.
- Finally, a more complicated reason is that iOS have the final say on whether, and how soon an app is woken up even after receiving the push notification. This depends on a variety of factors that is not explicitly specified by Apple, but in our experience can include your battery life, whether you're plugged into power, whether you're on Wi-Fi or cellular, etc.
To determine whether or not the problem is because of factor #3, you can leave Due running on one iOS device (device A) while you make a change on another device (device B, iOS or Mac), then sync and wait for upload to complete. Within seconds after upload has completed, if the push is correctly sent and received, Due on device A will automatically indicate that it is checking for updates and sync the new data.
If this is happening as described, this indicates that Due has correctly pushed the update out to all devices, but iOS has chosen not to wake up Due.
In our experience, it can take minutes or even up to an hour before iOS decides to wake up an app, usually in response to other push notifications or other scheduled background app refresh from other apps, in order to conserve battery on your device.
On our end, we'll be further optimizing the sync in future updates to make it even more energy-efficient and use lesser bandwidth in order to encourage iOS to wake up Due in a timely manner.
But ultimately, the final call on when and whether to wake up Due to complete the background sync depends on iOS.