Mutable Binding in SwiftUI Live Preview

If you need a value that can be changed in the live preview, I like to use this helper class:

struct BindingProvider<StateT, Content: View>: View {

    @State private var state: StateT
    private var content: (_ binding: Binding<StateT>) -> Content

    init(_ initialState: StateT, @ViewBuilder content: @escaping (_ binding: Binding<StateT>) -> Content) {
        self.content = content
        self._state = State(initialValue: initialState)
    }

    var body: some View {
        self.content($state)
    }
}

Use it like so:

struct YourView_Previews: PreviewProvider {

    static var previews: some View {
        BindingProvider(false) { binding in
            YourView(yourBindingVar: binding)
        }
    }

}

This allows you to test changing the binding in the live preview.

Updates to a @State variable in a PreviewProvider appear to not update the the read-only computed property previews directly. The solution is to wrap the @State variable in a test holder view. Then use this test view inside the previews property so the Live Preview refreshes correctly. Tested and working in Xcode 11.2.1.

struct ChildView: View {
    @Binding var itemName: String

    var body: some View {
        VStack {
            Text("Name: \(itemName)")
            Button(action: {
                self.itemName = "different value"
            }) {
                Text("Change")
            }
        }
    }
}

struct ChildView_Previews: PreviewProvider {

    struct BindingTestHolder: View {
        @State var testItem: String = "Initial"
        var body: some View {
            ChildView(itemName: $testItem)
        }
    }

    static var previews: some View {
        BindingTestHolder()
    }
}