How to manage lifecycle events in SwiftUI iOS applications

When working with views in an iOS application, we often need to trigger side effects when certain lifecycle events occur - for example loading or refreshing some data when a view appears.

In UIKit, we did that by overriding methods like viewDidAppear(_:) or viewDidDisappear(_:).

SwiftUI also provides methods like onAppear(perform:), onDisappear(perform:) and task(priority:_:) to react to lifecycle events. Let's jump in and look at those methods in more detail.

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The onAppear(perform:) method

The onAppear(perform:) method can be used to do some work right before the view appears - for example to setup some initial state.

struct SomeView: View {

    var body: some View {
        VStack {
            ...
        }
        .onAppear {
            viewModel.update()
        }
    }
}

The onDisappear(perform:) method

The onDisappear(perform:) method can be used to do some work right after the view disappears - for example to cancel running tasks.

struct SomeView: View {

    var body: some View {
        VStack {
            ...
        }
        .onDisappear {
            viewModel.cancelLoading()
        }
    }
}

The task(priority:_:) method

The task(priority:_:) method can be used to start some asynchronious work right before the view appears.

struct SomeView: View {

    var body: some View {
        VStack {
            ...
        }
        .task {
            do {
                try await viewModel.load()
            } catch {
                ...
            }
        }
    }
}

Additionally, SwiftUI automatically cancels the task when the view disappears.

The onReceive(_:perform:) method

Not a SwiftUI view lifecycle method per se, the onReceive(_:perform:) method can be useful to react to application lifecycle events - for example to update a view when the app enters foreground.

struct SomeView: View {

    var body: some View {
        VStack {
            ...
        }
        .onReceive(NotificationCenter.default.publisher(for: UIApplication.willEnterForegroundNotification)) { _ in
            viewModel.update()
        }
    }
}

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Check out my new book on preparing for a technical iOS job interview with over 200 questions & answers. Test your knowledge on iOS topics such as Swift & Objective-C, SwiftUI & UIKit, Combine, HTTP Networking, Authentication, Core Data, Concurrency with async/await, Security, Automated Testing and more.

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