如何在SwiftUI中自动填充文本字段？

import SwiftUI

struct ContentView: View {
    @State private var username: String = ""
    @State private var password: String = ""
    
    var body: some View {
        VStack {
            TextField("用户名", text: $username)
                .padding()
                .border(Color.gray, width: 1)
                .onAppear {
                    // 在页面加载时自动填充用户名
                    self.username = "John Doe"
                }
            
            SecureField("密码", text: $password)
                .padding()
                .border(Color.gray, width: 1)
        }
        .padding()
    }
}

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