Syncing PDF Annotations on iOS

By default, PSPDFKit Instant automatically synchronizes annotations with your Document Engine instance in real time. This is configurable, and you can instead choose for it to sync manually when your app requests it.

Using a network, especially a cellular network, is one of the most energy-intensive tasks on mobile devices. While we do our best to minimize the energy impact of Instant, it can be reduced further by disabling listening for changes from the server or by syncing less often after local changes are made.

Syncing after making local changes and listening for server changes can be configured separately. However, syncing always sends all local changes and fetches all changes from the server. It isn’t possible to fetch remote changes without pushing local changes or to push local changes without fetching remote changes.

Automatic Syncing

By default, when you show a document managed by Instant in an InstantViewController, real-time syncing of annotations is fully automatic: Instant will push local changes to the server as they happen and listen for changes from the server.

If you don’t show the document in an InstantViewController, you can enable listening for changes using startListeningForServerChanges() and stopListeningForServerChanges() on the InstantDocumentDescriptor.

Instant uses the network efficiently by coalescing changes with a one-second delay. You can reduce energy consumption at the cost of less immediate syncing by increasing the delayForSyncingLocalChanges property of a document descriptor. Unless syncing after local changes is disabled entirely (see below), if the app enters the background and there are local changes that haven’t been synced, Instant will immediately attempt to sync.

Disabling Listening for Server Changes

If your app doesn’t require real-time syncing, it may be better to manually fetch changes from the server to reduce energy consumption — for example, when the view showing the document appears, or with a button the user can tap.

If you show the document in an InstantViewController, you can disable listening by using the shouldListenForServerChangesWhenVisible property . Then, sync manually whenever you want to by calling syncChanges(_:). This method is marked with IBAction for easy setup in the Interface Builder.

If you use a custom view controller or something else, you don’t have to do anything: Just sync manually when required by calling sync() on the document descriptor.

Disabling Syncing after Local Changes

In addition to disabling listening for changes from the server (see above), you can disable syncing after local changes are made by setting a document descriptor’s delayForSyncingLocalChanges property to the special InstantSyncingLocalChangesDisabled constant . This means that Instant will never sync annotations with the server unless your app requests this by calling sync() (on the document descriptor) or syncChanges(_:) (on the Instant view controller) — these do the same thing, so use whichever is more convenient for you.

It’s possible to configure Instant to not sync after local changes are made but to listen for changes. However, this combination makes little sense and isn’t recommended, as the local changes would be pushed at seemingly random times when changes are received from the server.