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How to find which data change is causing a SwiftUI view to update

Paul Hudson    @twostraws   

Updated for Xcode 13.0

SwiftUI provides a special, debug-only method call we can use to identify what change caused a view to reload itself. The method is specifically for debugging, and should not be shipped in a real app, but it’s extremely helpful for the times when you can see a view is reinvoking its body property but you’re not sure why.

The method is Self._printChanges(), and should be called inside the body property. This means you will temporarily need to add an explicit return to send back your regular view code.

To demonstrate this method in action, here’s some sample code where a view relies on an observable object that randomly issues change notifications:

class EvilStateObject: ObservableObject {
    var timer: Timer?

    init() {
        timer = Timer.scheduledTimer(
            withTimeInterval: 1,
            repeats: true
        ) { _ in
            if Int.random(in: 1...5) == 1 {
                self.objectWillChange.send()
            }
        }
    }
}

struct ContentView: View {
    @StateObject private var evilObject = EvilStateObject()

    var body: some View {
        print(Self._printChanges())
        return Text("What could possibly go wrong?")
    }
}

Peter Steinberger has a helpful tip for discovering when the body property of a view is being reinvoked: assign a random background color to one of its views. This will be re-evaluated along with the rest of the body, so if body is being called a lot then your views will flicker as they change background.

To use this in your own projects, first add the following Color extension to get random colors:

extension ShapeStyle where Self == Color {
    static var random: Color {
        Color(
            red: .random(in: 0...1),
            green: .random(in: 0...1),
            blue: .random(in: 0...1)
        )
    }
}

And now go ahead and use it with background() whenever you’re curious what’s happening:

struct ContentView: View {
    var body: some View {
        Text("Hello, world!")
            .background(.random)
    }
}

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