How to convert a String to generic T, where T is unconstrainted?

Using Swift
1

I need to write generic code to work with some SwiftUI.TextField of this kind:

TextField.init<S, T>(_ title: S, value: Binding, formatter: Formatter, onEditingChanged: @escaping (Bool) -> Void = { _ in }, onCommit: @escaping() -> Void = {}) where S : StringProtocol

basically I need whatever magic the TextField does inside to convert String to T, TextField must be doing this inside to convert the input String value to T and assign to a binding. Something kind of like this:

import SwiftUI

// I need something like this that work for any T
func convert<T>(_ s: String, _ b: Binding<T>) {
    // this is what I need:
    // how to convert s to T? so we can do this:
//    b.wrappedValue = s as! T   // of course this won't work, will crash
    // so for illustration, I'm just going to use `Double`
    b.wrappedValue = Double(s) as! T
}


var d: Double = Double.nan

let binding = Binding<Double>(get: { d }, set: { d = $0 })

// to be use like this: need to work for any Binding<T> where T is unconstraint
convert("123.456", binding)
print(d)

How to make convert work?

this is actually my code, look to the end of this code to see where my problem is:

import SwiftUI
import Introspect


extension View {
    func addDecimalAndDoneKeyboardToolbar<T>(_ value: Binding<T>) -> some View {
        modifier(AddDecimalAndDoneKeyboardToolbar(value: value))
    }
}

/// Use like this to add keyboard toolbar to a TextField:
/// ```
/// @State var value: Double
/// TextField("Enter a number", value: $value, formatter: NumberFormatter())
///     .addDecimalAndDoneKeyboardToolbar($value)
///     .keyboardType(.numberPad)
struct AddDecimalAndDoneKeyboardToolbar<T>: ViewModifier {
    @Binding var value: T

    func body(content: Content) -> some View {
        content
            .introspectTextField(customize: addToolbar)
    }


    // for keeping a strong ref when this created inside `addToolbar(...)`
    @State var buttonHandler: ButtonHadler<T>?

    // add a decimal point button to input a "."
    // add a done button to commit textField input and dismiss keyboard
    func addToolbar(to textField: UITextField) {
        let toolBar = UIToolbar(
            frame: CGRect(
                origin: .zero,
                size: CGSize(width: textField.frame.size.width, height: 44)
            )
        )
        let flexButton = UIBarButtonItem(
            barButtonSystemItem: UIBarButtonItem.SystemItem.flexibleSpace,
            target: nil,
            action: nil
        )

        buttonHandler = ButtonHadler(textField, $value)

        let decimalButton = UIBarButtonItem(
            title: " • ",
            style: .plain,
            target: buttonHandler,     // or self, action is not called :(
            action: #selector(buttonHandler?.decimalPointAction(_:))    // <<<<< why is this not called?
        )


        let doneButton = UIBarButtonItem(
            title: "Done",
            style: .done,
            target: buttonHandler,     // or self, action is not called :(
            action: #selector(buttonHandler?.doneAction(_:))    // <<<<< why is this not called?
        )

        toolBar.setItems([decimalButton, flexButton, doneButton], animated: true)
        textField.inputAccessoryView = toolBar
    }


    final class ButtonHadler<T> {
        let textField: UITextField
        @Binding var field: T

        init(_ t: UITextField, _ f: Binding<T>) {
            textField = t
            _field = f
        }

        @objc func decimalPointAction(_ button: UIBarButtonItem) -> Void {
            textField.text = (textField.text ?? "0") + "."
        }

        @objc func doneAction(_ button: UIBarButtonItem) -> Void {
            // 😳😩👇👇
            // problem here: how to convert a string to T? Don't even know what T is
            // ideally, I want to to be unconstraint, just like TextField
            // what to do here?
            field = (textField.text ?? "0") as! T   // this will crash at runtime! won't work!!!
            textField.resignFirstResponder()
        }
    }

}

How does the TextField do the convert from String to T? It might be using the Formatter, if so, I'm willing to pass the formatter into my code.

2

It uses Formatter.

3

Edit: I got some help here.