How to use #State if #Binding not provided in the initializer - ios

Question

Imagine a view with some #Binding variables:
init(isEditing: Binding<Bool>, text: Binding<Bool>)
How can we have the selection working with an internal #State if it is not provided in the initializer?
init(text: Binding<Bool>)
This is how to make TextField become first responder in SwiftUI
Note that I know we can pass a constant like:
init(isEditing: Binding<Bool> = .constant(false), text: Binding<Bool>)
But!
This will kill the dynamicity of the variable and it won't work as desire. Imagine re-inventing the isFirstResponder of the UITextField.
It can't be .constant(false). The keyboard will be gone on each view update.
It can't be .constant(true). The view will take the keyboard on each view update.
Maybe! Apple is doing it somehow with TabView.

Answer 1

One solution is to pass an optional binding and use a local state variable if the binding is left nil. This code uses a toggle as an example (simpler to explain) and results in two interactive toggles: one being given a binding and the other using local state.
import SwiftUI
struct ContentView: View {
#State private var isOn: Bool = true
var body: some View {
VStack {
Text("Special toggle:")
SpecialToggle(isOn: $isOn)
.padding()
SpecialToggle()
.padding()
}
}
}
struct SpecialToggle: View {
/// The binding being passed from the parent
var isOn: Binding<Bool>?
/// The fallback state if the binding is left `nil`.
#State private var defaultIsOn: Bool = true
/// A quick wrapper for accessing the current toggle state.
var toggleIsOn: Bool {
return isOn?.wrappedValue ?? defaultIsOn
}
init(isOn: Binding<Bool>? = nil) {
if let isOn = isOn {
self.isOn = isOn
}
}
var body: some View {
Toggle(isOn: isOn ?? $defaultIsOn) {
Text("Dynamic label: \(toggleIsOn.description)")
}
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}

Answer 2

You may create separate #State and #Binding properties and sync them using onChange or onReceive:
struct TestView: View {
#State private var selectionInternal: Bool
#Binding private var selectionExternal: Bool
init() {
_selectionInternal = .init(initialValue: false)
_selectionExternal = .constant(false)
}
init(selection: Binding<Bool>) {
_selectionInternal = .init(initialValue: selection.wrappedValue)
_selectionExternal = selection
}
var body: some View {
if #available(iOS 14.0, *) {
Toggle("Selection", isOn: $selectionInternal)
.onChange(of: selectionInternal) {
selectionExternal = $0
}
} else {
Toggle("Selection", isOn: $selectionInternal)
.onReceive(Just(selectionInternal)) {
selectionExternal = $0
}
}
}
}
struct ContentView: View {
#State var selection = false
var body: some View {
VStack {
Text("Selection: \(String(selection))")
TestView(selection: $selection)
TestView()
}
}
}