用WPF MVVM实现一个登录DEMO

好的，以下是一个简单的WPF MVVM登录DEMO的代码示例：

XAML代码：

<Window x:Class="LoginDemo.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Login Demo" Height="250" Width="400">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <Label Grid.Row="0" Grid.Column="0" Content="Username:" Margin="10"/>
        <TextBox Grid.Row="0" Grid.Column="1" Text="{Binding Username}" Margin="10"/>
        <Label Grid.Row="1" Grid.Column="0" Content="Password:" Margin="10"/>
        <PasswordBox Grid.Row="1" Grid.Column="1" Password="{Binding Password}" Margin="10"/>
        <Button Grid.Row="2" Grid.Column="1" Content="Login" Command="{Binding LoginCommand}" Margin="10"/>
        <TextBlock Grid.Row="3" Grid.Column="1" Text="{Binding ErrorMessage}" Foreground="Red" Margin="10"/>
        <CheckBox Grid.Row="4" Grid.Column="1" Content="Remember me" IsChecked="{Binding RememberMe}" Margin="10"/>
    </Grid>
</Window>

ViewModel代码：

using System;
using System.ComponentModel;
using System.Windows;
using System.Windows.Input;

namespace LoginDemo
{
    public class LoginViewModel : INotifyPropertyChanged
    {
        private string _username;
        private string _password;
        private bool _rememberMe;
        private string _errorMessage;

        public string Username
        {
            get { return _username; }
            set
            {
                _username = value;
                OnPropertyChanged("Username");
            }
        }

        public string Password
        {
            get { return _password; }
            set
            {
                _password = value;
                OnPropertyChanged("Password");
            }
        }

        public bool RememberMe
        {
            get { return _rememberMe; }
            set
            {
                _rememberMe = value;
                OnPropertyChanged("RememberMe");
            }
        }

        public string ErrorMessage
        {
            get { return _errorMessage; }
            set
            {
                _errorMessage = value;
                OnPropertyChanged("ErrorMessage");
            }
        }

        public ICommand LoginCommand { get; set; }

        public LoginViewModel()
        {
            LoginCommand = new RelayCommand(Login);
        }

        private void Login(object parameter)
        {
            if (string.IsNullOrEmpty(Username) || string.IsNullOrEmpty(Password))
            {
                ErrorMessage = "Please enter both username and password.";
            }
            else if (Username == "admin" && Password == "password")
            {
                ErrorMessage = "";
                MessageBox.Show("Login successful!");
            }
            else
            {
                ErrorMessage = "Invalid username or password.";
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged(string propertyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

其中，RelayCommand是一个自定义的ICommand实现，用于绑定按钮的Command属性，代码如下：

using System;
using System.Windows.Input;

namespace LoginDemo
{
    public class RelayCommand : ICommand
    {
        private readonly Action<object> _execute;
        private readonly Predicate<object> _canExecute;

        public RelayCommand(Action<object> execute, Predicate<object> canExecute = null)
        {
            _execute = execute ?? throw new ArgumentNullException(nameof(execute));
            _canExecute = canExecute;
        }

        public bool CanExecute(object parameter)
        {
            return _canExecute?.Invoke(parameter) ?? true;
        }

        public void Execute(object parameter)
        {
            _execute(parameter);
        }

        public event EventHandler CanExecuteChanged
        {
            add { CommandManager.RequerySuggested += value; }
            remove { CommandManager.RequerySuggested -= value; }
        }
    }
}

在MainWindow的构造函数中，将ViewModel与View绑定：

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        DataContext = new LoginViewModel();
    }
}

这样，就完成了一个简单的WPF MVVM登录DEMO。