MiddleDebuggingVery commonNot answered yet

`onAppear` fires twice — or never when you expect it. How does it differ from UIKit's lifecycle?

This screen loads its feed in onAppear, but the network fetch runs more than once — the list ends up with duplicated items after navigating away and back.

Requirements:

Explain why onAppear can run more than once for the same screen.
Make the load happen once and cancel itself when the view goes away.

struct FeedView: View {
    @State private var items: [Item] = []

    var body: some View {
        List(items) { RowView(item: $0) }
            .onAppear { Task { items = try await api.loadFeed() } }  // runs again on re-insert
    }
}

Find and fix the bug.

onAppear follows the view's structural identity, not a controller lifecycle, and fires each time the view enters the hierarchy. A re-inserted view calls it again; one never inserted never fires. It is not a once-per-screen viewDidAppear.

✗Treating onAppear as a once-per-screen viewDidAppear equivalent
✗Putting idempotency-sensitive setup in onAppear without a guard
✗Expecting onAppear to fire for a view that is never inserted

→Why does a re-identified view fire onAppear again?
→Where would you put load-once logic so a re-insertion doesn't repeat it?

onAppear is not viewDidAppear. It is tied to the view's structural identity and fires every time that view is inserted into the render tree — navigating away and back re-inserts FeedView, so onAppear runs again and appends a second copy of the feed. It can also never run for a view that is never inserted (behind a false condition or below the fold of a lazy container).

Prefer .task, which starts when the view appears and is automatically cancelled when it disappears — and give it an id so it does not repeat unless the identity you key on changes:

struct FeedView: View {
    @State private var items: [Item] = []

    var body: some View {
        List(items) { RowView(item: $0) }
            .task {                       // auto-cancelled on disappear
                guard items.isEmpty else { return }
                items = (try? await api.loadFeed()) ?? []
            }
    }
}

If you must stay on onAppear, guard the work with a stored flag or move ownership of the loaded data into a model that survives re-insertion. The rule: never assume onAppear runs exactly once.