View Lifecycle (onAppear, onDisappear)

Relax. We'll talk through this in plain words — no textbook voice.

What you'll walk away with

Understand View Lifecycle (onAppear, onDisappear) with nothing to be intimidated by
Get comfortable running Xcode/SwiftUI code yourself
Be able to apply this concept in a real project right away

Let's think about it this way for a second

A SwiftUI View struct doesn't have a big set of lifecycle callbacks like UIKit's Activity/ViewController lifecycle (`onCreate`, `onResume`, etc.) — the two most commonly used modifiers are `onAppear { }` (runs every time the View appears on screen) and `onDisappear { }` (runs every time the View disappears from screen). Data fetching (API calls) is typically triggered inside `onAppear`.

Let's connect this to a real-world scenario

If `WeatherScreen` has `.onAppear { viewModel.fetchWeather() }` written on it — the weather data gets automatically fetched every time the user lands on this screen. If you want to cancel an ongoing network task when the user leaves the screen, you can add `.onDisappear { viewModel.cancelFetch() }`.

onAppear { } = a modifier that runs every time the View appears on screen
onDisappear { } = a modifier that runs every time the View disappears from screen
Data fetching (API calls) is typically triggered inside onAppear

Let's look at it together

swift

struct WeatherScreen: View {
    @State private var weather: WeatherResponse?

    var body: some View {
        VStack {
            if let weather {
                Text("\(weather.tempCelsius)°C")
            } else {
                ProgressView()
            }
        }
        .onAppear {
            Task {
                weather = try? await fetchWeather(city: "Yangon")
            }
        }
    }
}

You should see

Every time you land on the screen, you'll see a loading spinner followed by the weather data being automatically fetched and displayed.

5-minute try-it

Run `WeatherScreen` and test it by adding `print("appeared")` to the fetch logic inside `onAppear` — confirm that 'appeared' shows up again in the console every time you return to the screen.

A quick heads-up

`onAppear` isn't the same as 'the View being created for the first time' — watch out, because `onAppear` can fire repeatedly every time you enter/leave a screen inside a NavigationStack.

Easy traps

Using `onAppear` on a View inside a NavigationStack without realizing data gets re-fetched over and over every time the user navigates in/out of the screen — you should think about a caching strategy
Running a heavy/blocking operation synchronously inside `onAppear` — this can freeze the UI (you should run it asynchronously inside a `Task { }`)

Now try it yourself

Run `WeatherScreen` and test it by adding `print("appeared")` to the fetch logic inside `onAppear` — confirm that 'appeared' shows up again in the console every time you return to the screen.

You'll know it worked when: Every time you land on the screen, you'll see a loading spinner followed by the weather data being automatically fetched and displayed.