What is the lifecycle of a SwiftUI view? When do onAppear and task run?

#TL;DR

A SwiftUI View is a cheap struct. SwiftUI creates it many times. Its init and body can run often. The real lifetime is the view's identity. @State and .task live as long as that identity. onAppear runs when the view appears. .task runs async work and cancels it automatically when the view goes away.

#Three different things

So do not put side effects in init or body.

struct ProfileView: View {
    init() {
        print("init")   // prints often, not a good place to load data
    }
    var body: some View {
        Text("Profile")
    }
}

#onAppear and onDisappear

Text("Hello")
    .onAppear { analytics.track("hello_shown") }
    .onDisappear { analytics.track("hello_hidden") }

• Runs before the first frame is shown.
• Can run more than once. For example, after you go back from a pushed screen, or when a row in a lazy list scrolls back in.

#.task: async work tied to the view

struct UserScreen: View {
    let userID: User.ID
    @State private var user: User?

    var body: some View {
        Group {
            if let user { UserCard(user: user) } else { ProgressView() }
        }
        .task {
            user = try? await api.user(id: userID)
        }
    }
}

When the view disappears, SwiftUI cancels the task. No leaks and no work after the screen is gone. The task runs on the main actor, because View is @MainActor.

#.task(id:): restart when a value changes

.task(id: searchText) {
    try? await Task.sleep(for: .milliseconds(300)) // simple debounce
    results = await search(searchText)
}

When searchText changes, the old task is cancelled and a new one starts. This is a clean way to debounce search.

#onChange(of:)

.onChange(of: selectedTab) { oldValue, newValue in
    print("Tab moved from \(oldValue) to \(newValue)")
}

The two-parameter form is the iOS 17 version. The one-parameter version is deprecated. Add initial: true to also run it when the view first appears.

#Scene phase

For app-level events, read scenePhase:

@Environment(\.scenePhase) private var scenePhase

.onChange(of: scenePhase) { _, phase in
    if phase == .background { store.save() }
}

#Compared with UIKit

#Common mistakes

• Loading data in init. It runs often, even for screens that are never shown.
• Expecting onAppear to run only once. Use a flag or load in the model.
• Starting a Task { } inside onAppear. It is not cancelled on disappear. Use .task instead.
• Expecting a @State object's init to run once. The expression may run on every struct init, but SwiftUI keeps the first value.

#Common follow-up questions

• When does .task run compared to onAppear? At about the same time, before the view appears. Both are fine for kicking off work.
• Does onDisappear run when the app goes to the background? No. Use scenePhase for that.
• How do you run something only once? Load in an @Observable model and guard with a flag, or use .task on a view that does not reappear.
• What happens to .task in a List row? It starts when the row appears and is cancelled when the row scrolls away.