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How to create constant bindings

Paul Hudson    @twostraws   

Fully updated for Xcode 11 GM

When you’re prototyping some UI, or when you just need to pass in a value to give your SwiftUI preview something meaningful to show, you will find it useful to use constant bindings: hard-coded values that don’t change, but can still be used like regular bindings so your code works.

For example, if you want to create a toggle switch you would normally have to create an @State property to store the Boolean, then send that into the toggle switch when you create it. However, if you’re just prototyping your user interface you can use a constant binding instead, like this:

Toggle(isOn: .constant(true)) {
    Text("Show advanced options")
}

That switch is read-only and always on because that’s what our constant binding says, but it’s enough to get you moving for now – you can come back and replace it with a full @State property later.

These constant bindings come in all sorts of types: Booleans, strings, integers, and more are all available, and Swift will make sure you use the right one for each view type.

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