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How to fix “Modifying state during view update, this will cause undefined behavior”

Paul Hudson    @twostraws   

Updated for Xcode 12.0

This error occurs because you’re modifying the state of a SwiftUI view while the view is actually being rendered. Modifying the state of a view causes the view to be re-rendered, so SwiftUI gets confused – “undefined behavior” is Apple’s way of saying that whatever you see right now might change in the future because it’s not how SwiftUI should be used.

Here’s an example of the problem in action:

struct ContentView: View {
    @State private var name = ""

    var body: some View {
        if name.isEmpty {
            name = "Anonymous"
        }

        return Text("Hello, \(name)!")
    }
}

That creates a state property called name, and has a text view that shows it. However, inside body is some logic that attempts to modify name while the view is being rendered – SwiftUI is trying to figure out what’s in the view, and you’re changing it while that process is happening.

To fix the problem, move the code that changes your view’s state – in this case the name = "Anonymous" – to something that occurs outside of the view updates. For example, this sets the value when the text view first appears:

struct ContentView: View {
    @State var name = ""

    var body: some View {
        Text("Hello, \(name)!")
            .onAppear {
                if self.name.isEmpty {
                    self.name = "Anonymous"
                }
            }
    }
}

You could also move the logic into an onTapGesture() or similar – anywhere that isn’t directly inside the body of a view.

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