RocketData is a Uni-Directional UI Component Data Source

Nov 13th, 2016

I watched “Managing Consistency of Immutable Models” by Peter Livesey where Peter shows how RocketData works. Very worth the time!

I’d call RocketData a uni-directional data source. It’s uni-directional because you set up the DataProvider as the source once, wire update notifications to view updates via the delegate property, and you’re done with the setup. Update events include:

• Overwriting data, as you’d do to push the result of a successful network request to the DataProvider,
• Request old data from cache,
• or sync one DataProvider instance with another, maybe across different view controllers (think UISplitViewController!).

It’s all the same to the DataProvider.delegate, which you’ll most likely implement in the view controller. (Or in your Presenter object, if you have any. While we’re at it, the DataProvider behaves like a VIPER Interactor which you wire to a Presenter. If the task is trivial, Interactor, Presenter, and View collapse into a single component that uses RocketData.) The delegate’s dataProviderHasUpdatedData(_:, context:) callback will be triggered.

An example from Peter’s slides:

class MyViewController: UIViewController {
    let dataProvider = DataProvider<PersonModel>()

    func viewDidLoad() {
        super.viewDidLoad()
        dataProvider.fetchDataFromCache(cacheKey: self.id) { (_, _) in
            self.refreshView()
        }
        MyNetworkManager.fetchPerson(id: self.id) { (model, error) in
            if let model = model {
                self.dataProvider.setData(model)
                self.refreshView()
            }
        }
    }
}

Here you see that the DataProvider queries the cache (which is supposed to quickly find results on a background thread from a local key–value-store) and updates the view when finished, and that a network request is fired to fetch new data and push changes to the view, too.

A specialized CollectionDataProvider, your go-to table view data source, will uses a different delegate that passes change information along, essentially:

enum CollectionChangeInformation {
    case update(index: Int)
    case delete(index: Int)
    case insert(index: Int)
}

So it’s uni-directional because DataProvider updates are pushed to the delegate (your view component). In order to update the user interface, you have to push model changes either to the DataProvider directly or to a synced DataProvider instance. Models are designed to be immutable, so you cannot change data under the feet of your UI.

RocketData is a per-view component data source. The underlying principles are very similar to what I got to know from ReSwift. Bringing RocketData and ReSwift into context, you can say ReSwift is a DataProvider for all of your app. RocketData focuses on keeping single components consistent; ReSwift focuses on keeping all of your app’s state consistent.

I’d love to see their cache implementation, too. The combination of a key–value-cache and RocketData can be very powerful if your app relies on data from the network a lot.