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How to run some code when state changes using onChange()

Paul Hudson    @twostraws   

Updated for Xcode 13.0 beta 2

SwiftUI lets us attach an onChange() modifier to any view, which will run code of our choosing when some state changes in our program. This is important, because we can’t always use property observers like didSet with something like @State.

For example, this will print name changes as they are typed:

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

    var body: some View {
        TextField("Enter your name:", text: $name)
            .textFieldStyle(.roundedBorder)
            .onChange(of: name) { newValue in
                print("Name changed to \(name)!")
            }
    }
}

Download this as an Xcode project

Important: If you’re using Xcode 12 you need to use RoundedBorderTextFieldStyle() rather than .roundedBorder.

Personally, I prefer to add a custom extension to Binding so that I attach observing code directly to the binding rather than to the view – it lets me place the observer next to the thing it’s observing, rather than having lots of onChange() modifiers attached elsewhere in my view.

So, I would use code like this:

extension Binding {
    func onChange(_ handler: @escaping (Value) -> Void) -> Binding<Value> {
        Binding(
            get: { self.wrappedValue },
            set: { newValue in
                self.wrappedValue = newValue
                handler(newValue)
            }
        )
    }
}

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

    var body: some View {
        TextField("Enter your name:", text: $name.onChange(nameChanged))
            .textFieldStyle(.roundedBorder)
    }

    func nameChanged(to value: String) {
        print("Name changed to \(name)!")
    }
}
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