有没有标准的方法来实现"Vetoable"事件？

Question

我正在使用事件作为游戏模型的一部分,并且为了扩展性和代码"地点性"的缘故,我需要能够否决大多数行为.

更清楚的是,几乎每种具有副作用的方法都采用以下形式:

public event TryingToDoSomethingHandler TryingToDoSomething;
public event SomethingHappenedHandler   SomethingHappened;

/*
 * Returning true indicates Something happened successfully.
 */
public bool DoSomething(...)
{
  //Need a way to indicate "veto" here
  TryingToDoSomething(...);

  //Actual do it

  SomethingHappened(...);

  return true;
}

我想要的是TryingToDoSomething(...)能够指示已注册的事件处理程序对象(通过返回false,修改out参数或其他内容).所以代码在道德上等同于:

/*
 * Returning true indicates Something happened successfully.
 */
public bool DoSomethingImproved(...)
{
  //Pretty sure multicast delegates don't work this way, but you get the idea
  if(!TryingToDoSomething(...)) return false;

  //Actual do it

  SomethingHappened(...);

  return true;
}

在C#/ .NET中是否有可接受的或标准的方法来执行此操作？

Answer 1

你在考虑Cancelable事件吗？该框架广泛使用它.

创建一个EventArgs具有Cancel实现get/set 的属性的类.然后,事件处理程序可以将Cancel属性设置为true,您可以在调用返回时进行检查.

public bool TrySomething()
{
    CancelEventArgs e = new CancelEventArgs(); 
    if (Event1 != null) Event1.Invoke(e);
    if (e.Cancel == false)
    {
        if (Event2 != null) Event2.Invoke(e);
    }
}

Answer 2

看一下CancelEventHandler和CancelEventArgs类.它们遵循每个事件处理程序可以取消事件的模式.