在Scala中重载通用事件处理程序

Question

如果我定义以下通用事件处理程序

trait Handles[E <: Event] {
  def handle(event: E)
}

事件类型是这样的

trait Event {

}
class InventoryItemDeactivated(val id: UUID) extends Event;

class InventoryItemCreated(val id: UUID, val name: String) extends Event;

那么我如何创建一个为每个事件实现事件处理程序的类？我试过了:

class InventoryListView extends Handles[InventoryItemCreated] with Handles[InventoryItemDeactivated] {
    def handle(event: InventoryItemCreated) = {

    }

    def handle(event: InventoryItemDeactivated) = {

    }
  }

但斯卡拉抱怨说,一个特质不能被遗传两次.

我发现这个答案暗示了一个解决方案,但它接缝需要多个类(每个处理程序一个).这是否真的是唯一的方法,或者是否有其他一些Scala构造我可以用来使单个类实现多个通用事件处理程序(即使用案例类,清单或其他一些奇特的构造)？

Answer 1

我不知道在一个类中做到这一点的方法(除了通过制作EventADT和定义句柄来接受类型的参数Event.但这会带走你似乎正在寻找的类型安全).

我建议使用类型模式.

trait Handles[-A, -E <: Event] {
  def handle(a: A, event: E)
}

trait Event {
  ...
}
class InventoryItemDeactivation(val id: UUID) extends Event
class InventoryItemCreation(val id: UUID, val name: String) extends Event

class InventoryListView {
  ...
}

implicit object InventoryListViewHandlesItemCreation extends 
    Handles[InventoryListView, InventoryItemCreation] = {
  def handle(v: InventoryListView, e: InventoryItemCreation) = {
    ...
  }
}

implicit object InventoryListViewHandlesItemDeactivation extends 
    Handles[InventoryListView, InventoryItemDeactivation] = {
  def handle(v: InventoryListView, e: InventoryItemDeactivation) = {
    ...
  }
}

def someMethod[A, E <: Event](a: A, e: E)
              (implicit ev: InventoryListView Handles InventoryItemCreation) = {
  ev.handle(a, e)
  ...
}