如何在SwiftUI中检查按钮点击(而不是使用NavigationLink)的TextField验证？

import SwiftUI

struct ContentView: View {
    @State private var username: String = ""
    @State private var password: String = ""
    @State private var isUsernameValid: Bool = true
    @State private var isPasswordValid: Bool = true
    
    var body: some View {
        VStack {
            TextField("Username", text: $username)
                .textFieldStyle(RoundedBorderTextFieldStyle())
                .padding()
                .background(isUsernameValid ? Color.white : Color.red)
            
            SecureField("Password", text: $password)
                .textFieldStyle(RoundedBorderTextFieldStyle())
                .padding()
                .background(isPasswordValid ? Color.white : Color.red)
            
            Button(action: {
                // 检查用户名和密码的有效性
                isUsernameValid = !username.isEmpty
                isPasswordValid = !password.isEmpty
                
                if isUsernameValid && isPasswordValid {
                    // 执行登录操作
                    // 这里可以调用登录API或执行其他操作
                    print("登录成功")
                } else {
                    // 显示错误消息或执行其他操作
                    print("用户名或密码无效")
                }
            }) {
                Text("登录")
                    .foregroundColor(.white)
                    .padding()
                    .background(Color.blue)
                    .cornerRadius(10)
            }
            .padding()
        }
    }
}

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