WPF 的 MVVM 模式
WPF MVVM 模式全面复习与查缺补漏
1. MVVM 基础概念回顾
1.1 什么是 MVVM？
MVVM（Model-View-ViewModel）是一种软件架构模式，专门为 WPF 和 Silverlight 等 XAML-based 应用程序设计。
csharp
// MVVM 三大组件关系
Model ←→ ViewModel ←→ View
1.2 各层职责
Model（模型）
- 表示业务数据和业务逻辑
- 不包含任何 UI 相关代码
- 通常包含数据验证逻辑
View（视图）
- 用户界面定义
- 仅包含展示逻辑
- 通过数据绑定与 ViewModel 交互
ViewModel（视图模型）
- View 的抽象表示
- 包含命令和可绑定属性
- 协调 Model 和 View 之间的交互
2. ViewModel 实现详解
2.1 INotifyPropertyChanged 接口
csharp
using System.ComponentModel;
using System.Runtime.CompilerServices;
public abstract class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
protected bool SetProperty<T>(ref T field, T value, [CallerMemberName] string propertyName = null)
{
if (EqualityComparer<T>.Default.Equals(field, value))
return false;
field = value;
OnPropertyChanged(propertyName);
return true;
}
}
2.2 完整的 ViewModel 示例
csharp
public class UserViewModel : ViewModelBase
{
private string _name;
private string _email;
private ObservableCollection<User> _users;
public string Name
{
get => _name;
set => SetProperty(ref _name, value);
}
public string Email
{
get => _email;
set => SetProperty(ref _email, value);
}
public ObservableCollection<User> Users
{
get => _users;
set => SetProperty(ref _users, value);
}
public ICommand AddUserCommand { get; }
public ICommand DeleteUserCommand { get; }
public UserViewModel()
{
Users = new ObservableCollection<User>();
AddUserCommand = new RelayCommand(AddUser, CanAddUser);
DeleteUserCommand = new RelayCommand<User>(DeleteUser);
}
private void AddUser()
{
var user = new User { Name = Name, Email = Email };
Users.Add(user);
Name = string.Empty;
Email = string.Empty;
}
private bool CanAddUser()
{
return !string.IsNullOrWhiteSpace(Name) &&
!string.IsNullOrWhiteSpace(Email);
}
private void DeleteUser(User user)
{
if (user != null)
Users.Remove(user);
}
}
3. 命令绑定 (ICommand)
3.1 RelayCommand 实现
csharp
public class RelayCommand : ICommand
{
private readonly Action _execute;
private readonly Func<bool> _canExecute;
public RelayCommand(Action execute, Func<bool> canExecute = null)
{
_execute = execute ?? throw new ArgumentNullException(nameof(execute));
_canExecute = canExecute;
}
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
public bool CanExecute(object parameter) => _canExecute?.Invoke() ?? true;
public void Execute(object parameter) => _execute();
}
// 泛型版本
public class RelayCommand<T> : ICommand
{
private readonly Action<T> _execute;
private readonly Func<T, bool> _canExecute;
public RelayCommand(Action<T> execute, Func<T, bool> canExecute = null)
{
_execute = execute ?? throw new ArgumentNullException(nameof(execute));
_canExecute = canExecute;
}
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
public bool CanExecute(object parameter) => _canExecute?.Invoke((T)parameter) ?? true;
public void Execute(object parameter) => _execute((T)parameter);
}
4. 数据绑定深度解析
4.1 绑定模式
xml
<!-- 各种绑定模式示例 -->
<TextBox Text="{Binding Name, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
<TextBlock Text="{Binding Email, Mode=OneWay}" />
<ListBox ItemsSource="{Binding Users, Mode=OneWay}" />
<ContentControl Content="{Binding SelectedItem, Mode=OneWayToSource}" />
4.2 值转换器 (IValueConverter)
csharp
public class BooleanToVisibilityConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return (value is bool boolValue && boolValue) ?
Visibility.Visible : Visibility.Collapsed;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
return value is Visibility visibility && visibility == Visibility.Visible;
}
}
// 在 XAML 中使用
<Window.Resources>
<local:BooleanToVisibilityConverter x:Key="BoolToVisibilityConverter"/>
</Window.Resources>
<Button Visibility="{Binding IsEnabled, Converter={StaticResource BoolToVisibilityConverter}}"/>
4.3 数据验证
csharp
public class User : IDataErrorInfo
{
public string Name { get; set; }
public string Email { get; set; }
public string this[string columnName]
{
get
{
switch (columnName)
{
case nameof(Name):
if (string.IsNullOrWhiteSpace(Name))
return "姓名不能为空";
break;
case nameof(Email):
if (string.IsNullOrWhiteSpace(Email))
return "邮箱不能为空";
else if (!Regex.IsMatch(Email, @"^[^@\s]+@[^@\s]+\.[^@\s]+$"))
return "邮箱格式不正确";
break;
}
return null;
}
}
public string Error => null;
}
5. 高级 MVVM 模式
5.1 消息传递 (Messenger Pattern)
csharp
public class Messenger
{
private static readonly Dictionary<Type, List<Action<object>>> _actions =
new Dictionary<Type, List<Action<object>>>();
public static void Register<T>(Action<T> action)
{
var type = typeof(T);
if (!_actions.ContainsKey(type))
_actions[type] = new List<Action<object>>();
_actions[type].Add(obj => action((T)obj));
}
public static void Send<T>(T message)
{
var type = typeof(T);
if (_actions.ContainsKey(type))
{
foreach (var action in _actions[type])
{
action(message);
}
}
}
}
// 使用示例
public class UserAddedMessage
{
public User User { get; set; }
}
// 发送消息
Messenger.Send(new UserAddedMessage { User = newUser });
// 接收消息
Messenger.Register<UserAddedMessage>(message =>
{
// 处理新用户添加逻辑
});
5.2 依赖注入在 MVVM 中的应用
csharp
public interface IUserService
{
void AddUser(User user);
IEnumerable<User> GetUsers();
}
public class UserService : IUserService
{
public void AddUser(User user) { /* 实现 */ }
public IEnumerable<User> GetUsers() { /* 实现 */ }
}
public class UserViewModel : ViewModelBase
{
private readonly IUserService _userService;
public UserViewModel(IUserService userService)
{
_userService = userService;
}
}
6. 常见陷阱与最佳实践
6.1 内存泄漏预防
csharp
// 错误示例 - 可能导致内存泄漏
public class LeakyViewModel
{
public event EventHandler SomethingHappened;
// 如果没有正确注销事件，可能导致内存泄漏
}
// 正确做法
public class SafeViewModel : ViewModelBase, IDisposable
{
private bool _disposed = false;
public void Dispose()
{
if (!_disposed)
{
// 清理资源、注销事件
_disposed = true;
}
}
~SafeViewModel()
{
Dispose();
}
}
6.2 异步编程模式
csharp
public class AsyncViewModel : ViewModelBase
{
private readonly IUserService _userService;
private bool _isLoading;
private string _statusMessage;
public bool IsLoading
{
get => _isLoading;
set => SetProperty(ref _isLoading, value);
}
public string StatusMessage
{
get => _statusMessage;
set => SetProperty(ref _statusMessage, value);
}
public IAsyncCommand LoadUsersCommand { get; }
public AsyncViewModel(IUserService userService)
{
_userService = userService;
LoadUsersCommand = new AsyncRelayCommand(LoadUsersAsync);
}
private async Task LoadUsersAsync()
{
try
{
IsLoading = true;
StatusMessage = "正在加载用户...";
var users = await _userService.GetUsersAsync();
// 处理用户数据
}
catch (Exception ex)
{
StatusMessage = $"加载失败: {ex.Message}";
}
finally
{
IsLoading = false;
}
}
}
// 异步命令实现
public class AsyncRelayCommand : ICommand
{
private readonly Func<Task> _execute;
private readonly Func<bool> _canExecute;
private bool _isExecuting;
public AsyncRelayCommand(Func<Task> execute, Func<bool> canExecute = null)
{
_execute = execute;
_canExecute = canExecute;
}
public bool CanExecute(object parameter)
{
return !_isExecuting && (_canExecute?.Invoke() ?? true);
}
public async void Execute(object parameter)
{
_isExecuting = true;
OnCanExecuteChanged();
try
{
await _execute();
}
finally
{
_isExecuting = false;
OnCanExecuteChanged();
}
}
public event EventHandler CanExecuteChanged;
protected virtual void OnCanExecuteChanged()
{
CanExecuteChanged?.Invoke(this, EventArgs.Empty);
}
}
7. 测试 MVVM 应用程序
7.1 ViewModel 单元测试
csharp
[TestFixture]
public class UserViewModelTests
{
[Test]
public void AddUserCommand_WhenDataValid_AddsUserToCollection()
{
// Arrange
var viewModel = new UserViewModel();
viewModel.Name = "Test User";
viewModel.Email = "test@example.com";
// Act
viewModel.AddUserCommand.Execute(null);
// Assert
Assert.AreEqual(1, viewModel.Users.Count);
Assert.AreEqual("Test User", viewModel.Users[0].Name);
}
[Test]
public void CanAddUser_WhenNameEmpty_ReturnsFalse()
{
// Arrange
var viewModel = new UserViewModel();
viewModel.Name = string.Empty;
viewModel.Email = "test@example.com";
// Act & Assert
Assert.IsFalse(viewModel.AddUserCommand.CanExecute(null));
}
}