Data Binding on iOS and macOS¶

iOS, Mac Catalyst, tvOS and macOS views come from UIKit or AppKit, neither of which gives a view a built-in way to raise a change notification. To bind a view to a view model, implement IViewFor<TViewModel> on the view. The core ReactiveUI package ships base classes that already do this. It builds them for the ios, maccatalyst, macos and tvos target frameworks, so there is no separate iOS package to install beyond ReactiveUI itself.

Across all four of those platforms, ReactiveViewController<TViewModel>, ReactiveView<TViewModel>, ReactiveControl<TViewModel>, ReactiveImageView<TViewModel> and ReactiveSplitViewController<TViewModel> wrap UIViewController, UIView, UIControl, UIImageView and UISplitViewController on Apple's platforms with a ViewModel property and its change notifications. ViewModelViewHost shows whatever view model you assign it, resolving the matching view through the view locator, the same way ViewModelControlHost does on other platforms.

iOS, Mac Catalyst and tvOS add a UIKit-specific layer on top. ReactiveTableViewController<TViewModel> and ReactiveCollectionViewController<TViewModel> pair with ReactiveTableViewSource and ReactiveCollectionViewSource to bind a collection of view models into a UITableView or a UICollectionView. ReactiveNavigationController<TViewModel>, ReactiveTabBarController<TViewModel> and ReactivePageViewController<TViewModel> wrap the corresponding UIKit containers. RoutedViewHost follows a router's navigation stack, the way RoutedControlHost does elsewhere. macOS instead adds ReactiveWindowController, an NSWindowController with a ViewModel property, since AppKit has no direct equivalent of a UITableViewController in this surface.

Derive your view controller, view or window controller from the matching base class and give it a ViewModel property. The base class raises PropertyChanged for you, the same way ReactiveObject does on a view model. From there, binding a control to a property works exactly as it does on any other platform.

Which views the hosts find¶

ViewModelViewHost and RoutedViewHost ask the view locator for a view in two steps, and neither step uses reflection. The first step is the view lookup the ReactiveUI.Binding source generator writes for every view class in your project that implements IViewFor<T>. The second is the views you add to the view locator with Map. So both hosts are safe to trim and to compile ahead of time, which matters on iOS, where apps are compiled ahead of time.

A view registered only in Splat's service locator, whose class the generator never sees, sits outside both steps. For that view, each host has an Unsafe twin. A twin asks both steps first, then the service locator for IViewFor<T> closed over the view model's run-time type. Building that type while the app runs needs code the compiler never generated, so each twin is marked [RequiresDynamicCode].

Prefer to keep the default hosts and bridge the registration with locator.CreateMappingBuilder().MapFromServiceLocator<TViewModel, IViewFor<TViewModel>>(). It adds a Map entry whose view comes from the service locator, so the default hosts find it in their second step. View location covers both lookups.