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How to respond to view lifecycle events: onAppear and onDisappear

SwiftUI gives us equivalents to UIKit’s viewDidAppear() and viewDidDisappear() in the form of onAppear() and onDisappear(). You can attach any code to these two events that you want, and SwiftUI will execute them when they occur.

As an example, this creates two views that use onAppear() and onDisappear() to print messages, with a navigation button to move between the two:

struct ContentView: View {
    var body: some View {
        NavigationView {
            VStack {
                NavigationLink(destination: DetailView()) {
                    Text("Hello World")
                }
            }
        }.onAppear {
            print("ContentView appeared!")
        }.onDisappear {
            print("ContentView disappeared!")
        }
    }
}

struct DetailView: View {
    var body: some View {
        VStack {
            Text("Second View")
        }.onAppear {
                print("DetailView appeared!")
        }.onDisappear {
                print("DetailView disappeared!")
        }
    }
}

When that code runs you should be able to move between the two views and see messages printed in your Xcode debug console.

Note: In the current SwiftUI beta onDisappear will never be called.

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