SwiftUI:如何阻止文本字段中的某些字符？

import SwiftUI

struct ContentView: View {
    @State private var text: String = ""
    
    var body: some View {
        TextField("输入文本", text: $text)
            .onReceive(Just(text)) { newString in
                let allowedCharacters = CharacterSet(charactersIn: "abcdefghijklmnopqrstuvwxyz")
                let filteredString = newString.filter { allowedCharacters.isSuperset(of: CharacterSet(charactersIn: String($0).lowercased())) }
                
                if filteredString != newString {
                    self.text = filteredString
                }
            }
            .textFieldStyle(RoundedBorderTextFieldStyle())
            .padding()
    }
}

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