NumberField.swift

struct NumberField<T: Numeric & LosslessStringConvertible>: View {

    let prompt: String
    @Binding var value: T?

    @StateObject var viewModel = ViewModel()

    class ViewModel: ObservableObject {
        @Published var text = ""
        var ignoreNextChange = false
    }

    var body: some View {
        TextField(prompt, text: text)
            .onAppear(perform: {
                viewModel.text = value.map(String.init) ?? ""
            })
            .onChange(of: self.value, perform: { value in
                if let v = value, !self.viewModel.ignoreNextChange {
                    self.viewModel.text = String(v)
                }
                self.viewModel.ignoreNextChange = false
            })
    }

    var text: Binding<String> {
        Binding {
            viewModel.text
        } set: { (value) in
            let sanitized = sanitize(value: value)
            self.viewModel.text = sanitized

            viewModel.ignoreNextChange = true
            self.value = T(sanitized)
        }
    }

    func sanitize(value: String) -> String {
        return value.trimmingCharacters(in: CharacterSet(charactersIn: "-.0123456789").inverted)
    }
}

// Usage

struct ContentView: View {
    @State var num: Float? = 0
    var body: some View {
        Text("VALUE \(num ?? 0)")
        NumberField(prompt: "Enter number", value: $num)

        Stepper("Thing", value: Binding(get: { num ?? 0 }, set: { num in
            self.num = num
        }))
    }
}