Maintaining State in SwiftUI

Written by Team Kodeco

In traditional MVC (Model-View-Controller) paradigms, state management tends to be distributed across both the model and the controller. The model holds the data or state, while the controller manipulates this data and updates the view. This approach can sometimes lead to bloated controllers with too many responsibilities and it can make it harder to track where and how state is being changed.

In contrast, SwiftUI uses a more declarative approach to state management. With property wrappers like @State, @Binding, @ObservedObject, @StateObject, @EnvironmentObject and others, state is typically encapsulated within the views themselves. When the state changes, SwiftUI automatically rerenders the affected views to reflect those changes. This automatic and localized management of state makes it easier to understand and predict how state changes affect your user interface.

This shift in state management approach is a key aspect of why SwiftUI can make UI development more straightforward and efficient. It’s part of a broader trend in software development towards declarative and reactive programming models, which focus on describing what the UI should look like for a given state rather than how to transition between states.

Let’s consider a simple example:

struct ContentView: View {
  @State private var counter = 0  // Declare a private @State property called counter

  var body: some View {
    VStack {
      Text("Counter: \(counter)")
      Button("Increment") {
        counter += 1  // Increment the counter when the button is tapped
      }
    }
  }
}

Click the “Increment” button 5 times and your preview should look like:

In this example, ContentView displays a counter and a button to increment it. The counter value is managed by the @State property wrapper. When the button is tapped, the counter value is incremented by 1, and the view automatically re-renders with the updated value. By declaring counter as private, we ensure it can only be modified within ContentView, promoting better data encapsulation.

The @State property wrapper is designed for state that is local to a single view. For state that needs to be shared across multiple views, SwiftUI offers @EnvironmentObject or @ObservedObject with a view model. These tools enable you to propagate state changes across your entire SwiftUI app, maintaining consistency and responsiveness.

Lastly, SwiftUI employs a unidirectional data flow. This means that state changes always flow from parent views down to child views, rather than the other way around.

In conclusion, SwiftUI provides a powerful and simplified way to handle state management within your user interfaces. By understanding and properly using the @State property wrapper, you’ll be able to create responsive views that automatically update when their state changes. As you continue developing with SwiftUI, remember to consider how state is managed — it’s a critical aspect of building effective UIs.