如何使用户仅在 SwiftUI 文本字段中输入数字与货币，同时保留 $ 和 .？

现在，我有以下内容：

private var currencyFormatter: NumberFormatter = {
    let f = NumberFormatter()
    // allow no currency symbol, extra digits, etc
    f.isLenient = true
    f.numberStyle = .currency
    return f
}()

TextField("Total", value: $totalInput, formatter: currencyFormatter)
    .font(.largeTitle)
    .padding()
    .background(Color.white)
    .foregroundColor(Color.black)
    .multilineTextAlignment(.center)

我希望文本字段以 0.00 美元作为占位符开始，但是当用户开始输入时，前两个输入将以美分填充...因此 5055 将逐渐显示为：

第 1 步（用户点击 5）：0.05 美元
第 2 步（用户点击 0）：0.50 美元
第 3 步（用户点击 5）：5.05 美元
第 4 步（用户点击 5）：50.55 美元

如果金额大于 $999，则将插入逗号。

如何实现这一目标呢？现在我的totalInput 类型是Double?。

要创建一个允许用户从右到左输入金额的货币字段，您需要一个可观察对象（绑定管理器）、一个货币数字格式化程序，并使用 onChange 方法观察每次值变化：

import SwiftUI

struct ContentView: View {
    @ObservedObject private var currencyManager = CurrencyManager(amount: 0)
    @ObservedObject private var currencyManagerUS = CurrencyManager(
        amount: 0,
        locale: .init(identifier: "en_US")
    )
    @ObservedObject private var currencyManagerUK = CurrencyManager(
        amount: 0,
        locale: .init(identifier: "en_UK")
    )
    @ObservedObject private var currencyManagerFR =  CurrencyManager(
        amount: 0,
        locale: .init(identifier: "fr_FR")
    )
    @ObservedObject private var currencyManagerBR =  CurrencyManager(
        amount: 100,
        maximum: 100,
        locale: .init(identifier: "pt_BR")
    )
    var body: some View {
        VStack(alignment: .trailing, spacing: 0) {
            Spacer()
            Group {
                Text("Locale currency")
                TextField(currencyManager.string, text: $currencyManager.string)
                    .keyboardType(.numberPad)
                    .multilineTextAlignment(.trailing)
                    .onChange(of: currencyManager.string, perform: currencyManager.valueChanged)
                Spacer()
            }
            Group {
                Text("American currency")
                TextField(currencyManagerUS.string, text: $currencyManagerUS.string)
                    .keyboardType(.numberPad)
                    .multilineTextAlignment(.trailing)
                    .onChange(of: currencyManagerUS.string, perform: currencyManagerUS.valueChanged)
                Spacer()
            }
            Group {
                Text("British currency")
                TextField(currencyManagerUK.string, text: $currencyManagerUK.string)
                    .keyboardType(.numberPad)
                    .multilineTextAlignment(.trailing)
                    .onChange(of: currencyManagerUK.string, perform: currencyManagerUK.valueChanged)
                Spacer()
            }
            Group {
                Text("French currency")
                TextField(currencyManagerFR.string, text: $currencyManagerFR.string)
                    .keyboardType(.numberPad)
                    .multilineTextAlignment(.trailing)
                    .onChange(of: currencyManagerFR.string, perform: currencyManagerFR.valueChanged)
                Spacer()
            }
            Group {
                Text("Brazilian currency")
                TextField(currencyManagerBR.string, text: $currencyManagerBR.string)
                    .keyboardType(.numberPad)
                    .multilineTextAlignment(.trailing)
                    .onChange(of: currencyManagerBR.string, perform: currencyManagerBR.valueChanged)
                
            }
            Spacer()
        }.padding(.trailing, 25)
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

class CurrencyManager: ObservableObject {
    
    @Published var string: String = ""
    private var amount: Decimal = .zero
    private let formatter = NumberFormatter(numberStyle: .currency)
    private var maximum: Decimal = 999_999_999.99
    private var lastValue: String = ""
    
    init(amount: Decimal, maximum: Decimal = 999_999_999.99, locale: Locale = .current) {
        formatter.locale = locale
        self.string = formatter.string(for: amount) ?? "$0.00"
        self.lastValue = string
        self.amount = amount
        self.maximum = maximum
    }
    
    func valueChanged(_ value: String) {
        let newValue = (value.decimal ?? .zero) / pow(10, formatter.maximumFractionDigits)
        if newValue > maximum {
            string = lastValue
        } else {
            string = formatter.string(for: newValue) ?? "$0.00"
            lastValue = string
        }
    }
}

extension NumberFormatter {
    
    convenience init(numberStyle: Style, locale: Locale = .current) {
        self.init()
        self.locale = locale
        self.numberStyle = numberStyle
    }
}

extension Character {
    
    var isDigit: Bool { "0"..."9" ~= self }
}

extension LosslessStringConvertible {
    
    var string: String { .init(self) }
}

extension StringProtocol where Self: RangeReplaceableCollection {
    
    var digits: Self { filter (\.isDigit) }
    
    var decimal: Decimal? { Decimal(string: digits.string) }
}

这是相当于自定义的 SwiftUI货币字段我已经为 UIKit 实现了。