领域事件

领域事件（Domain Event）是领域驱动设计中捕获业务事实的重要模式。它用于记录领域中发生的重要业务事件，实现聚合之间的松耦合通信。

什么是领域事件

领域事件表示在领域中已经发生的事情，具有以下特征：

业务意义：反映真实的业务事实
过去时态：事件名称使用过去时（如 OrderPlaced，而非 PlaceOrder）
不可变性：事件一旦创建就不能修改
异步处理：事件处理器异步响应事件

定义领域事件

基本事件

using MiCake.DDD.Domain;

// 订单已提交事件
public class OrderSubmittedEvent : IDomainEvent
{
    public int OrderId { get; }
    public int CustomerId { get; }
    public decimal TotalAmount { get; }
    public DateTime SubmittedAt { get; }

    public OrderSubmittedEvent(int orderId, int customerId, decimal totalAmount)
    {
        OrderId = orderId;
        CustomerId = customerId;
        TotalAmount = totalAmount;
        SubmittedAt = DateTime.UtcNow;
    }
}

// 用户注册事件
public class UserRegisteredEvent : IDomainEvent
{
    public int UserId { get; }
    public string Email { get; }
    public DateTime RegisteredAt { get; }

    public UserRegisteredEvent(int userId, string email)
    {
        UserId = userId;
        Email = email;
        RegisteredAt = DateTime.UtcNow;
    }
}

Record 事件（推荐）

使用 C# record 可以更简洁地定义事件：

// 使用 record 定义事件
public record ProductCreatedEvent(int ProductId, string Name, decimal Price) : IDomainEvent;

public record PriceChangedEvent(int ProductId, decimal OldPrice, decimal NewPrice) : IDomainEvent;

public record OrderCancelledEvent(int OrderId, string Reason) : IDomainEvent;

触发领域事件

在聚合根中触发

public class Order : AggregateRoot<int>
{
    private List<OrderItem> _items = new();

    public int CustomerId { get; private set; }
    public OrderStatus Status { get; private set; }

    public void Submit()
    {
        if (Status != OrderStatus.Draft)
            throw new DomainException("Only draft orders can be submitted");

        if (!_items.Any())
            throw new DomainException("Cannot submit empty order");

        // 修改状态
        Status = OrderStatus.Submitted;

        // 触发领域事件
        RaiseDomainEvent(new OrderSubmittedEvent(Id, CustomerId, TotalAmount));
    }

    public void Cancel(string reason)
    {
        if (Status == OrderStatus.Shipped)
            throw new DomainException("Cannot cancel shipped order");

        Status = OrderStatus.Cancelled;
        RaiseDomainEvent(new OrderCancelledEvent(Id, reason));
    }

    public void AddItem(int productId, int quantity, decimal price)
    {
        var item = new OrderItem(productId, quantity, price);
        _items.Add(item);

        // 添加商品也可以触发事件
        RaiseDomainEvent(new OrderItemAddedEvent(Id, productId, quantity));
    }
}

处理领域事件

创建事件处理器

using MiCake.DDD.Domain;
using System.Threading;
using System.Threading.Tasks;

// 订单提交事件处理器
public class OrderSubmittedEventHandler : IDomainEventHandler<OrderSubmittedEvent>
{
    private readonly IEmailService _emailService;
    private readonly ILogger<OrderSubmittedEventHandler> _logger;

    public OrderSubmittedEventHandler(
        IEmailService emailService,
        ILogger<OrderSubmittedEventHandler> logger)
    {
        _emailService = emailService;
        _logger = logger;
    }

    public async Task HandleAysnc(OrderSubmittedEvent domainEvent, CancellationToken cancellationToken = default)
    {
        _logger.LogInformation($"Order {domainEvent.OrderId} submitted by customer {domainEvent.CustomerId}");

        // 发送订单确认邮件
        await _emailService.SendOrderConfirmationAsync(
            domainEvent.CustomerId,
            domainEvent.OrderId,
            domainEvent.TotalAmount
        );

        // 其他业务逻辑...
    }
}

// 用户注册事件处理器
public class UserRegisteredEventHandler : IDomainEventHandler<UserRegisteredEvent>
{
    private readonly IEmailService _emailService;
    private readonly IRepository<UserProfile, int> _profileRepository;

    public async Task HandleAysnc(UserRegisteredEvent domainEvent, CancellationToken cancellationToken = default)
    {
        // 1. 发送欢迎邮件
        await _emailService.SendWelcomeEmailAsync(domainEvent.Email);

        // 2. 创建用户档案
        var profile = UserProfile.Create(domainEvent.UserId);
        await _profileRepository.AddAsync(profile, cancellationToken);
        await _profileRepository.SaveChangesAsync(cancellationToken);

        // 3. 记录日志
        Console.WriteLine($"User {domainEvent.UserId} registered at {domainEvent.RegisteredAt}");
    }
}

一个事件多个处理器

一个事件可以有多个处理器：

// 处理器 1：发送邮件
public class OrderSubmittedEmailHandler : IDomainEventHandler<OrderSubmittedEvent>
{
    public async Task HandleAysnc(OrderSubmittedEvent domainEvent, CancellationToken cancellationToken)
    {
        // 发送邮件
    }
}

// 处理器 2：更新库存
public class OrderSubmittedInventoryHandler : IDomainEventHandler<OrderSubmittedEvent>
{
    public async Task HandleAysnc(OrderSubmittedEvent domainEvent, CancellationToken cancellationToken)
    {
        // 扣减库存
    }
}

// 处理器 3：记录日志
public class OrderSubmittedLoggingHandler : IDomainEventHandler<OrderSubmittedEvent>
{
    public async Task HandleAysnc(OrderSubmittedEvent domainEvent, CancellationToken cancellationToken)
    {
        // 记录日志
    }
}

// 这三个处理器会依次执行

事件的自动派发

MiCake 在调用 SaveChangesAsync() 时自动派发领域事件：

public class OrderService
{
    private readonly IRepository<Order, int> _orderRepository;

    public async Task SubmitOrder(int orderId)
    {
        // 1. 加载聚合根
        var order = await _orderRepository.FindAsync(orderId);

        // 2. 调用业务方法（触发事件，但不立即派发）
        order.Submit();  // 内部：RaiseDomainEvent(new OrderSubmittedEvent(...))

        // 3. 更新聚合根
        await _orderRepository.UpdateAsync(order);

        // 4. 保存更改 - 此时自动派发所有事件
        await _orderRepository.SaveChangesAsync();
        // SaveChangesAsync 内部流程：
        // a. 收集聚合根上的所有待处理事件
        // b. 持久化数据到数据库
        // c. 按顺序派发事件到对应的处理器
        // d. 清除已派发的事件
    }
}

事件派发流程

1. 业务方法调用
   order.Submit()
      ↓
2. 触发领域事件
   RaiseDomainEvent(new OrderSubmittedEvent(...))
      ↓
3. 事件暂存在聚合根
   _domainEvents.Add(event)
      ↓
4. 保存更改
   await repository.SaveChangesAsync()
      ↓
5. 收集所有事件
   events = aggregateRoot.DomainEvents
      ↓
6. 持久化数据
   dbContext.SaveChanges()
      ↓
7. 派发事件
   foreach (event in events)
       foreach (handler in GetHandlers(event))
           await handler.HandleAsync(event)
      ↓
8. 清除事件
   aggregateRoot.ClearDomainEvents()

使用场景

1. 跨聚合通信

// 订单聚合
public class Order : AggregateRoot<int>
{
    public void Submit()
    {
        Status = OrderStatus.Submitted;

        // 触发事件，通知其他聚合
        RaiseDomainEvent(new OrderSubmittedEvent(Id, Items));
    }
}

// 库存聚合在事件处理器中响应
public class OrderSubmittedInventoryHandler : IDomainEventHandler<OrderSubmittedEvent>
{
    private readonly IRepository<Product, int> _productRepository;

    public async Task HandleAysnc(OrderSubmittedEvent domainEvent, CancellationToken cancellationToken)
    {
        // 扣减库存
        foreach (var item in domainEvent.Items)
        {
            var product = await _productRepository.FindAsync(item.ProductId);
            product.DecreaseStock(item.Quantity);
            await _productRepository.UpdateAsync(product);
        }

        await _productRepository.SaveChangesAsync(cancellationToken);
    }
}

2. 业务流程协调

// 用户注册流程
public class User : AggregateRoot<int>
{
    public void Register(string email, string password)
    {
        // 注册逻辑
        Email = email;
        SetPassword(password);
        Status = UserStatus.Pending;

        // 触发注册事件
        RaiseDomainEvent(new UserRegisteredEvent(Id, email));
    }
}

// 多个处理器协调完成注册流程
public class SendVerificationEmailHandler : IDomainEventHandler<UserRegisteredEvent>
{
    public async Task HandleAysnc(UserRegisteredEvent domainEvent, CancellationToken cancellationToken)
    {
        // 发送验证邮件
    }
}

public class CreateUserProfileHandler : IDomainEventHandler<UserRegisteredEvent>
{
    public async Task HandleAysnc(UserRegisteredEvent domainEvent, CancellationToken cancellationToken)
    {
        // 创建用户档案
    }
}

public class InitializeUserSettingsHandler : IDomainEventHandler<UserRegisteredEvent>
{
    public async Task HandleAysnc(UserRegisteredEvent domainEvent, CancellationToken cancellationToken)
    {
        // 初始化用户设置
    }
}

3. 审计和日志

public class OrderStatusChangedEvent : IDomainEvent
{
    public int OrderId { get; }
    public OrderStatus OldStatus { get; }
    public OrderStatus NewStatus { get; }
    public DateTime ChangedAt { get; }
}

public class OrderAuditEventHandler : IDomainEventHandler<OrderStatusChangedEvent>
{
    private readonly IAuditLogRepository _auditRepository;

    public async Task HandleAysnc(OrderStatusChangedEvent domainEvent, CancellationToken cancellationToken)
    {
        var auditLog = new AuditLog
        {
            EntityType = nameof(Order),
            EntityId = domainEvent.OrderId,
            Action = "StatusChanged",
            OldValue = domainEvent.OldStatus.ToString(),
            NewValue = domainEvent.NewStatus.ToString(),
            Timestamp = domainEvent.ChangedAt
        };

        await _auditRepository.AddAsync(auditLog);
        await _auditRepository.SaveChangesAsync(cancellationToken);
    }
}

4. 发送通知

public class OrderShippedEvent : IDomainEvent
{
    public int OrderId { get; }
    public int CustomerId { get; }
    public string TrackingNumber { get; }
}

public class OrderShippedNotificationHandler : IDomainEventHandler<OrderShippedEvent>
{
    private readonly INotificationService _notificationService;

    public async Task HandleAysnc(OrderShippedEvent domainEvent, CancellationToken cancellationToken)
    {
        // 发送邮件通知
        await _notificationService.SendEmailAsync(
            domainEvent.CustomerId,
            "Order Shipped",
            $"Your order has been shipped. Tracking number: {domainEvent.TrackingNumber}"
        );

        // 发送短信通知
        await _notificationService.SendSmsAsync(
            domainEvent.CustomerId,
            $"Order shipped. Track: {domainEvent.TrackingNumber}"
        );

        // 推送通知
        await _notificationService.SendPushNotificationAsync(
            domainEvent.CustomerId,
            "Order Shipped",
            "Your order is on the way!"
        );
    }
}

最佳实践

1. 事件命名使用过去时

// ✅ 正确 - 使用过去时
public class OrderPlacedEvent : IDomainEvent { }
public class PaymentCompletedEvent : IDomainEvent { }
public class UserRegisteredEvent : IDomainEvent { }

// ❌ 错误 - 使用现在时或命令式
public class PlaceOrderEvent : IDomainEvent { }
public class CompletePaymentEvent : IDomainEvent { }
public class RegisterUserEvent : IDomainEvent { }

2. 事件应该是不可变的

// ✅ 正确 - 所有属性只读
public class OrderCreatedEvent : IDomainEvent
{
    public int OrderId { get; }  // 只读
    public DateTime CreatedAt { get; }

    public OrderCreatedEvent(int orderId)
    {
        OrderId = orderId;
        CreatedAt = DateTime.UtcNow;
    }
}

// ❌ 错误 - 属性可修改
public class OrderCreatedEvent : IDomainEvent
{
    public int OrderId { get; set; }  // 可修改
    public DateTime CreatedAt { get; set; }
}

3. 事件处理器保持幂等性

public class OrderCreatedEmailHandler : IDomainEventHandler<OrderCreatedEvent>
{
    private readonly IEmailService _emailService;
    private readonly IEmailLogRepository _emailLogRepository;

    public async Task HandleAysnc(OrderCreatedEvent domainEvent, CancellationToken cancellationToken)
    {
        // 检查是否已发送（幂等性）
        var alreadySent = await _emailLogRepository.ExistsAsync(
            l => l.OrderId == domainEvent.OrderId && l.Type == "OrderCreated"
        );

        if (alreadySent)
            return;  // 已发送，跳过

        // 发送邮件
        await _emailService.SendOrderConfirmationAsync(domainEvent.OrderId);

        // 记录日志
        await _emailLogRepository.AddAsync(new EmailLog
        {
            OrderId = domainEvent.OrderId,
            Type = "OrderCreated",
            SentAt = DateTime.UtcNow
        });

        await _emailLogRepository.SaveChangesAsync(cancellationToken);
    }
}

4. 事件包含足够的信息

// ✅ 好的做法 - 包含必要信息
public class OrderSubmittedEvent : IDomainEvent
{
    public int OrderId { get; }
    public int CustomerId { get; }
    public decimal TotalAmount { get; }
    public List<OrderItemDto> Items { get; }  // 包含详细信息
    public DateTime SubmittedAt { get; }

    // 事件处理器不需要再查询订单详情
}

// ❌ 不好的做法 - 信息不足
public class OrderSubmittedEvent : IDomainEvent
{
    public int OrderId { get; }  // 只有 ID

    // 事件处理器需要查询数据库获取详情
}

5. 避免在事件处理器中执行长时间操作

// ❌ 避免 - 同步执行耗时操作
public class OrderPlacedHandler : IDomainEventHandler<OrderPlacedEvent>
{
    public async Task HandleAysnc(OrderPlacedEvent domainEvent, CancellationToken cancellationToken)
    {
        // 这会阻塞事务
        await SendEmailAsync();  // 可能很慢
        await CallExternalApiAsync();  // 可能失败
        await GeneratePdfAsync();  // 很耗时
    }
}

// ✅ 推荐 - 发布到消息队列异步处理
public class OrderPlacedHandler : IDomainEventHandler<OrderPlacedEvent>
{
    private readonly IMessageQueue _messageQueue;

    public async Task HandleAysnc(OrderPlacedEvent domainEvent, CancellationToken cancellationToken)
    {
        // 快速发布到队列
        await _messageQueue.PublishAsync(new SendOrderEmailCommand(domainEvent.OrderId));
        await _messageQueue.PublishAsync(new GenerateInvoiceCommand(domainEvent.OrderId));
    }
}

小结

MiCake 的领域事件机制：

实现 IDomainEvent 接口定义事件
在聚合根中通过 RaiseDomainEvent 触发事件
实现 IDomainEventHandler<TEvent> 处理事件
在 SaveChangesAsync 时自动派发事件
用于实现聚合间松耦合通信
支持一个事件多个处理器

下一步：

学习领域服务了解服务设计
阅读工作单元理解事务管理
查看聚合根回顾聚合设计