NumberOnlyTextField.swift

import SwiftUI
//import Combine


struct NumbersOnlyViewModifier: ViewModifier {
    @Binding var text: String
    var includeDecimal: Bool
    
    func body(content: Content) -> some View {
        content
            .keyboardType(includeDecimal ? .decimalPad : .numberPad)
            .onChange(of: text) { newValue in
                if includeDecimal {
                    do {
                        let fixup: String
                        if let lastChar = newValue.last, !lastChar.isNumber {   // we have a decimal point at the end?
                            fixup = String(lastChar)
                        } else {
                            fixup = ""
                        }
                        let parsedResult = try Double(newValue, format: .number).formatted(.number.grouping(.never)) + fixup
                        if newValue != parsedResult {
                            text = String(newValue.dropLast())
                        }
                    } catch {
                        text = String(newValue.dropLast())
                    }
                } else {
                    do {
                        let sanitizedNewValue = String(newValue.dropFirst(newValue.prefix(while: { $0 == "0" }).count))
                        let parsedResult = try Int(sanitizedNewValue, format: .number).formatted(.number.grouping(.never))
                        if sanitizedNewValue != parsedResult {
                            text = String(newValue.dropLast())
                        } else if newValue != sanitizedNewValue {
                            text = sanitizedNewValue
                        }
                    } catch {
                        text = String(newValue.dropLast())
                    }
                }
            }
    }
}

extension View {
    func numberOnly(_ text: Binding<String>, includeDecimal: Bool = false) -> some View {
        modifier(NumbersOnlyViewModifier(text: text, includeDecimal: includeDecimal))
    }
}


struct NumericTextFieldDemo: View {
    @State private var anInteger = 0
    @State private var aDecimal = 0.0

    
    @State private var textInt = "0"
    @State private var textDec = "0."
    
    var body: some View {
        VStack {
            // this allows for only Int values, but the problem is you can enter anything, not number only
            // how to restrict this to number only?
            // with a String value TextField, you can monitor the input and not allow non-number
            // but here we have nothing to monitor.
            TextField("Enter a integer", value: $anInteger, format: .number)
                .textFieldStyle(.roundedBorder)
                .keyboardType(.numberPad)
            TextField("Enter a integer", value: $aDecimal, format: .number)
                .textFieldStyle(.roundedBorder)
                .keyboardType(.decimalPad)
            
            TextField("Number only!", text: $textInt)
                .numberOnly($textInt)

            TextField("Number and decimal only!", text: $textDec)
                .numberOnly($textDec, includeDecimal: true)
        }
    }
}

struct NumericTextFieldDemo_Previews: PreviewProvider {
    static var previews: some View {
        NumericTextFieldDemo()
    }
}