Prism 命令详解：从遥控器到智能家居的编程哲学

一、命令是什么？—— 生活中的遥控器类比

想象一下，你有一个电视遥控器：

• 遥控器按钮 = UI元素（按钮、菜单等）

• 按按钮的动作 = 用户交互事件

• 电视的反应 = 后台业务逻辑

在Prism中，命令（Command）就是连接UI动作与后台逻辑的"遥控器机制"。

csharp

// 传统事件处理（直接接线）—— 紧耦合
button.Click += (s, e) => { /* 直接在这里写业务逻辑 */ };

// Prism命令（通过遥控器）—— 松耦合
// View: 按钮按下 -> Command
// ViewModel: 接收命令 -> 执行业务逻辑

二、DelegateCommand：最基本的命令类型

2.1 基础使用：简单的开灯场景

csharp

// ViewModel中
using Prism.Commands;
using Prism.Mvvm;

public class LightViewModel : BindableBase
{
private bool _isLightOn;
public bool IsLightOn
{
get => _isLightOn;
set => SetProperty(ref _isLightOn, value);
}

// 1. 定义命令属性
private DelegateCommand _toggleLightCommand;
public DelegateCommand ToggleLightCommand =>
_toggleLightCommand ?? (_toggleLightCommand = new DelegateCommand(ExecuteToggleLight));

// 2. 执行方法
private void ExecuteToggleLight()
{
IsLightOn = !IsLightOn;
Console.WriteLine($"灯现在是 {(IsLightOn ? "开" : "关")}");
}
}

xml

<!– View中 –>
<Button Content="开关灯"
Command="{Binding ToggleLightCommand}" />

2.2 带参数的命令：调节音量

csharp

public class AudioViewModel : BindableBase
{
private int _volume = 50;
public int Volume
{
get => _volume;
set => SetProperty(ref _volume, value);
}

// 带参数的命令（使用泛型DelegateCommand<T>）
private DelegateCommand<int> _adjustVolumeCommand;
public DelegateCommand<int> AdjustVolumeCommand =>
_adjustVolumeCommand ?? (_adjustVolumeCommand = new DelegateCommand<int>(ExecuteAdjustVolume));

private void ExecuteAdjustVolume(int change)
{
Volume = Math.Max(0, Math.Min(100, Volume + change));
Console.WriteLine($"音量调整到: {Volume}");
}
}

xml

<Button Content="音量+" Command="{Binding AdjustVolumeCommand}"
CommandParameter="10" />
<Button Content="音量-" Command="{Binding AdjustVolumeCommand}"
CommandParameter="-10" />

2.3 带条件判断的命令：需要钥匙才能启动汽车

csharp

public class CarViewModel : BindableBase
{
private bool _hasKey = false;
private bool _isEngineRunning = false;

public bool HasKey
{
get => _hasKey;
set
{
SetProperty(ref _hasKey, value);
// 钥匙状态改变时，重新评估命令可执行性
StartEngineCommand.RaiseCanExecuteChanged();
}
}

// 带CanExecute检查的命令
private DelegateCommand _startEngineCommand;
public DelegateCommand StartEngineCommand =>
_startEngineCommand ?? (_startEngineCommand = new DelegateCommand(
ExecuteStartEngine, // 执行逻辑
CanStartEngine // 条件判断逻辑
));

private void ExecuteStartEngine()
{
_isEngineRunning = true;
Console.WriteLine("引擎启动了!");
}

private bool CanStartEngine()
{
// 只有有钥匙时才能启动
return HasKey;
}
}

三、CompositeCommand：组合命令 – 智能家居场景

想象一个"离家模式"按钮，需要同时：

csharp

// 1. 定义各个独立的命令
public class LightSystemViewModel : BindableBase
{
public DelegateCommand TurnOffAllLightsCommand { get; }

public LightSystemViewModel()
{
TurnOffAllLightsCommand = new DelegateCommand(() =>
{
Console.WriteLine("所有灯已关闭");
});
}
}

public class ACSystemViewModel : BindableBase
{
public DelegateCommand TurnOffACCommand { get; }

public ACSystemViewModel()
{
TurnOffACCommand = new DelegateCommand(() =>
{
Console.WriteLine("空调已关闭");
});
}
}

// 2. 主ViewModel定义组合命令
public class SmartHomeViewModel : BindableBase
{
public CompositeCommand LeaveHomeCommand { get; }

private LightSystemViewModel _lightSystem;
private ACSystemViewModel _acSystem;
private SecuritySystemViewModel _securitySystem;

public SmartHomeViewModel(
LightSystemViewModel lightSystem,
ACSystemViewModel acSystem,
SecuritySystemViewModel securitySystem)
{
_lightSystem = lightSystem;
_acSystem = acSystem;
_securitySystem = securitySystem;

// 创建组合命令
LeaveHomeCommand = new CompositeCommand();

// 注册子命令
LeaveHomeCommand.RegisterCommand(_lightSystem.TurnOffAllLightsCommand);
LeaveHomeCommand.RegisterCommand(_acSystem.TurnOffACCommand);
LeaveHomeCommand.RegisterCommand(_securitySystem.ActivateSecurityCommand);
}
}

xml

<!– 一个按钮触发所有操作 –>
<Button Content="离家模式"
Command="{Binding LeaveHomeCommand}" />

四、最佳实践与场景

4.1 最佳实践原则

1. 使用懒加载初始化命令

csharp

private DelegateCommand _saveCommand;
public DelegateCommand SaveCommand =>
_saveCommand ?? (_saveCommand = new DelegateCommand(ExecuteSave, CanSave))
.ObservesProperty(() => Document) // 自动监听属性变化
.ObservesProperty(() => IsDirty);

2. 使用ObservesProperty自动更新命令状态

csharp

// 当SelectedItem变化时，自动重新评估CanExecute
private DelegateCommand _deleteCommand;
public DelegateCommand DeleteCommand =>
_deleteCommand ?? (_deleteCommand = new DelegateCommand(ExecuteDelete, CanDelete)
.ObservesProperty(() => SelectedItem));

3. 异步命令处理

csharp

using Prism.Commands;

private DelegateCommand _loadDataCommand;
public DelegateCommand LoadDataCommand =>
_loadDataCommand ?? (_loadDataCommand = new DelegateCommand(
async () => await ExecuteLoadDataAsync()
));

private async Task ExecuteLoadDataAsync()
{
IsLoading = true;
try
{
await Task.Delay(1000); // 模拟异步操作
Data = await _service.FetchDataAsync();
}
finally
{
IsLoading = false;
}
}

4.2 最佳使用场景

场景1：表单提交

csharp

public class OrderViewModel : BindableBase
{
public DelegateCommand SubmitOrderCommand { get; }

public OrderViewModel()
{
SubmitOrderCommand = new DelegateCommand(
async () => await SubmitOrderAsync(),
CanSubmitOrder
)
.ObservesProperty(() => CustomerName)
.ObservesProperty(() => SelectedProducts)
.ObservesProperty(() => IsValidAddress);
}

private bool CanSubmitOrder()
{
// 所有条件满足才能提交
return !string.IsNullOrEmpty(CustomerName) &&
SelectedProducts?.Any() == true &&
IsValidAddress &&
!IsSubmitting;
}
}

场景2：导航命令

csharp

public class MainViewModel : BindableBase
{
private readonly IRegionManager _regionManager;

public DelegateCommand<string> NavigateCommand { get; }

public MainViewModel(IRegionManager regionManager)
{
_regionManager = regionManager;

NavigateCommand = new DelegateCommand<string>(Navigate);
}

private void Navigate(string viewName)
{
if (CanNavigate(viewName))
{
_regionManager.RequestNavigate("MainRegion", viewName);
}
}

private bool CanNavigate(string viewName)
{
// 检查权限、当前状态等
return _userService.HasAccessToView(viewName) &&
!IsBusy;
}
}

4.3 高级模式：命令工厂

csharp

// 1. 定义命令接口
public interface ICommandFactory
{
DelegateCommand CreateSaveCommand(Action execute, Func<bool> canExecute = null);
DelegateCommand<T> CreateParameterizedCommand<T>(Action<T> execute, Func<T, bool> canExecute = null);
}

// 2. 实现命令工厂
public class CommandFactory : ICommandFactory
{
public DelegateCommand CreateSaveCommand(Action execute, Func<bool> canExecute = null)
{
var command = new DelegateCommand(execute, canExecute);

// 添加公共行为，如日志、异常处理
return command;
}

public DelegateCommand<T> CreateParameterizedCommand<T>(
Action<T> execute, Func<T, bool> canExecute = null)
{
return new DelegateCommand<T>(
param =>
{
try
{
Log($"执行命令，参数: {param}");
execute(param);
}
catch (Exception ex)
{
HandleCommandException(ex);
}
},
canExecute
);
}
}

// 3. 在ViewModel中使用
public class ProductViewModel : BindableBase
{
private readonly ICommandFactory _commandFactory;

public DelegateCommand SaveCommand { get; }

public ProductViewModel(ICommandFactory commandFactory)
{
_commandFactory = commandFactory;

SaveCommand = _commandFactory.CreateSaveCommand(
ExecuteSave,
CanSave
);
}
}

五、常见陷阱与解决方案

陷阱1：内存泄漏

csharp

// ❌ 错误：匿名方法可能阻止垃圾回收
public DelegateCommand MyCommand => new DelegateCommand(() =>
{
// 这里捕获了this，可能导致内存泄漏
this.DoSomething();
});

// ✅ 正确：使用弱引用或标准模式
private DelegateCommand _myCommand;
public DelegateCommand MyCommand =>
_myCommand ?? (_myCommand = new DelegateCommand(ExecuteMyCommand));

private void ExecuteMyCommand()
{
// 明确的方法，没有隐藏的引用
}

陷阱2：异步命令异常处理

csharp

// 创建一个安全的异步命令包装器
public static class AsyncCommandExtensions
{
public static DelegateCommand ToAsyncCommand(
this Func<Task> asyncAction,
Action<Exception> errorHandler = null)
{
return new DelegateCommand(async () =>
{
try
{
await asyncAction();
}
catch (Exception ex)
{
errorHandler?.Invoke(ex);
// 或使用统一的错误处理
HandleException(ex);
}
});
}
}

// 使用
public DelegateCommand LoadDataCommand =>
_loadDataCommand ?? (_loadDataCommand =
(async () => await LoadDataAsync()).ToAsyncCommand());

六、总结

什么时候使用Prism命令？

核心价值

Prism命令就像是UI和业务逻辑之间的"翻译官"，它让两者能够顺畅沟通，但又保持适当的距离，这正是良好架构的关键所在。