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The Importance of Synchronize

About synchronize()

Controllers are lightweight objects and they only fetch data when needed. When a controller is created, it doesn't fetch local or remote data until it needs it.

synchronize() makes sure StreamChat's local database and backend is in sync. It queries the backend for the latest state of the Channel and updates the database. In addition, synchronize() call starts actually observing the Channel for changes, so you will start getting live updates of the changes to the Channel, including it's messages.

If you only need the local data, you can just access it after creation, like so:

let channelController = chatClient.channelController(for: ChannelId(type: .messaging, id: "general"))
let channel =
let messages = channelController.messages

In addition, you don't need to call synchronize() to be able to call actions on the channel, such as freezeChannel:

let channelController = chatClient.channelController(for: ChannelId(type: .messaging, id: "general"))

But as stated in the table above, if the Channel is not created in the backend yet, you'll need to call synchronize() first, else the action calls will fail.

Also, if you create a channel without passing a ChannelID then you must call synchronize() every time.

synchronize() is automatically called within the setUp() function on each View or ViewController which will automatically update both local and remote data for you. It's important to note that if you implement your own custom implementation that you call super.setUp() so this function is called. If you implement your own UI completely then make sure you call synchronize() on the custom UI lifecycle.

Additionally, as with all StreamChat Controllers, ChannelController has state and a delegate function to observe it's state:

func controller(_ controller: DataController, didChangeState state: DataController.State)

You can use this delegate function to show any error states you might see. For more information, see DataControllerStateDelegate Overview.

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