Manage a Sync Session - Flutter SDK

Prerequisites

Before you can manage your sync session state, you must perform the following:

1. Configure Flexible Sync on the Atlas App Services backend

2. Add Device Sync to your app

Wait for Changes to Upload and Download

To asynchronously wait for all changes to upload to Atlas from your synced realm, call Session.waitForUpload(). To asynchronously wait for all changes on Atlas to download to your synced realm, call Session.waitForDownload().

// Wait to download all pending changes from Atlas
await realm.syncSession.waitForDownload();
// Add data locally
realm.write(() {
  realm.addAll<Car>([
    Car(ObjectId(), "Hyundai"),
    Car(ObjectId(), "Kia"),
    Car(ObjectId(), "Lincoln")
  ]);
});
// Wait for changes to upload to Atlas before continuing execution.
await realm.syncSession.waitForUpload();

You can add an optional CancellationToken to waitForUpload() and waitForDownload().

final cancellationToken = CancellationToken();
final waitForDownloadFuture =
    realm.syncSession.waitForDownload(cancellationToken);
cancellationToken.cancel();
final waitForUploadFuture =
    realm.syncSession.waitForUpload(cancellationToken);
cancellationToken.cancel();

Pause and Resume a Sync Session

To pause syncing for a session, call Session.pause(). The realm will not sync changes with Atlas while the session is paused.

To resume syncing a changes, call Session.resume().

You must manually call Session.pause() and Session.resume() for each realm whose Sync session you want to pause and restart. The sync state of one session has no impact on other open sessions.

The following code block demonstrates calling these methods:

// Pause the sync session
realm.syncSession.pause();
// Data that you add while the sync session is paused does not sync to Atlas.
// However, the data is still added to the realm locally.
realm.write(() {
  realm.addAll<Car>([
    Car(ObjectId(), "Volvo"),
    Car(ObjectId(), "Genesis"),
    Car(ObjectId(), "VW")
  ]);
});
// Resume sync session. Now, the data you wrote to the realm
// syncs to Atlas.
realm.syncSession.resume();

Monitor Sync Upload and Download Progress

To monitor Sync progress, call SyncSession.getProgressStream(). This method returns a Stream of SyncProgress objects that provide a progressEstimate for the current upload or download.

The provided progressEstimate is a double whose value ranges from 0.0 to 1.0. At 1.0, the progress stream is complete.

SyncSession.getProgressStream() takes two arguments:

• A ProgressDirection enum that can be set to upload or download. Specifies whether the progress stream monitors upload or download progress.

• A ProgressMode enum that can be set to one of the following:

  • reportIndefinitely: Sets notifications to continue until the callback is unregistered.

  • forCurrentlyOutstandingWork: Sets notifications to continue until the progressEstimate reaches 1.0.

final stream = realm.syncSession.getProgressStream(
    ProgressDirection.upload, ProgressMode.forCurrentlyOutstandingWork);
double progressEstimate = -1;
late StreamSubscription streamListener;
streamListener = stream.listen((syncProgressEvent) {
  progressEstimate = syncProgressEvent.progressEstimate;
  if (progressEstimate < 1.0) {
    print('Upload progress: ${progressEstimate * 100}%');
  }
}, onDone: () {
  print('Upload progress: ${progressEstimate * 100}%');
  print("Upload complete");
}, onError: (error) {
  print("An error occurred: $error");
  streamListener.cancel();
});

Monitor Network Connection

You can get the state of the current network connection with Session.connectionState. This returns a ConnectionState enum that contains the network connection's state: connected, disconnected, or connecting.

if (realm.syncSession.connectionState == ConnectionState.connected) {
  // ... do stuff
}

Monitor the state of the network connection with Session.connectionStateChanges. This property returns a Stream of ConnectionStateChange objects that updates when the network connection changes. You can access the current and previous ConnectionState from ConnectionStateChange.

final connectionStream = realm.syncSession.connectionStateChanges;
late StreamSubscription streamListener;
streamListener = connectionStream.listen((connectionStateChange) {
  if (connectionStateChange.current == ConnectionState.connected) {
    print("Connected to Atlas Device Sync server");
    streamListener.cancel();
  }
});

Manually Reconnect All Sync Sessions

The Flutter SDK automatically detects when a device regains connectivity after being offline and attempts to reconnect using an incremental backoff strategy.

You can choose to manually trigger a reconnect attempt with the App.reconnect() instead of waiting for the duration of the incremental backoff. This is useful if you have a more accurate understanding of the network conditions and don't want to rely on automatic reconnect detection.

app.reconnect();

When you call this method, the SDK forces all sync sessions to attempt to reconnect immediately and resets any timers used for incremental backoff.