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干净地处理事件

events java

nkr1pt · 技术社区 · 16 年前

我在处理事件总线类触发的事件的所有观察者类中都有类似的代码。正如您所看到的,有很多instanceof检查来选择适当处理事件所需的操作路径,我想知道是否可以更干净地完成这项工作,消除instanceof测试?

@Override
public void handleEvent(Event event) {
    if (event instanceof DownloadStartedEvent) {
        DownloadStartedEvent dsEvent = (DownloadStartedEvent)event;
        dsEvent.getDownloadCandidateItem().setState(new BusyDownloadingState());
    } else if (event instanceof DownloadCompletedEvent) {
        DownloadCompletedEvent dcEvent = (DownloadCompletedEvent)event;
        dcEvent.getDownloadCandidateItem().setState(new FinishedDownloadingState());
        DownloadCandidate downloadCandidate = dcEvent.getDownloadCandidateItem(). getDownloadCandidate();
        if (downloadCandidate.isComplete()) {
            // start extracting
        }
    } else if (event instanceof DownloadFailedEvent) {
        DownloadFailedEvent dfEvent = (DownloadFailedEvent)event;
        dfEvent.getDownloadCandidateItem().setState(new FailedDownloadingState());
    }
}

6 回复 | 直到 16 年前

Ryan 16 年前

您可以通过依次为每个事件添加侦听器来消除这些事件。例如考虑

public void handleStartedEvent(DownloadStartedEvent ev) { ... }
public void handleCompletedEvent(DownloadCompletedEvent ev) { ... }
public void handleFailedEvent(DownloadFailedEvent ev) { ... }

您还可以考虑将完成/失败合并为单个事件,因为看起来您已经有了一个完成的方法。你可以处理完完整的事件并检查是否成功。如果成功,您可以开始提取,否则您可以设置失败状态。

我的另一个想法是,为什么要设置一个新对象作为状态指示器。使用常量/枚举值可以更好地服务于此吗?

jqno 16 年前

是的,假设你控制了 Event 类和/或其子类。你可以加个摘要 handle 你的方法 事件 类,并将每个子类的特定代码移到该方法中。

abstract class Event {
  //...
  public abstract void handle();
}

class DownloadStartedEvent extends Event {
  //...
  @Override
  public void handle() {
    getDownloadCandidateItem().setState(new BusyDownloadingState());
  }
}

// The same for the other classes

在调用代码中,只需编写:

@Override
public void handleEvent(Event event) {
  event.handle();
}

Behrang Saeedzadeh 16 年前

@EventHandler 和 @HandlesEvent

@EventHandler
public class MyEventHandler {

   @HandlesEvent(DownloadStartedEvent.class)
   public void handleDownloadStartedEvent(DownloadStartedEvent dse) {
      dsEvent.getDownloadCandidateItem().setState(new BusyDownloadingState());
   }

   @HandlesEvent(DownloadCompletedEvent.class)
   public void handleDownloadCompletedEvent(DownloadComletedEvent dse) {
      dcEvent.getDownloadCandidateItem().setState(new FinishedDownloadingState());
      DownloadCandidate downloadCandidate = dcEvent.getDownloadCandidateItem(). getDownloadCandidate();
      if (downloadCandidate.isComplete()) {
            // start extracting
      }
   }


   // etc.

}

Jack 16 年前

如果你需要做的只是实例化“状态”,比如 BusyDownloadedState 或 FinishedDownloadedState Event 类一个属性,表示必须根据事件类型设置的状态。

如果您需要同时拥有简单事件和复杂事件,您可以同时拥有两件事:

class Event
{
   State state;
   boolean isSimple;
}

public void handleEvent(Event event)
{
  if (event.isSimple)
    setState(event.state)
  else
  {
     if (event instanceof WhateverEvent)
        // special handling
  }
}

Dustin 16 年前

改进了Ryan的答案,您可以使用泛型,而不是为每个事件使用单独的方法。然后你可以写:

public void registerListeners() {

eventSource.addListener(DownloadStartedEvent.class, new EventHandler<DownloadStartedEvent>() {
    public void handleEvent(DownloadStartedEvent event) {
        dsEvent.getDownloadCandidateItem().setState(new BusyDownloadingState());
    }
});

eventSource.addListener(DownloadCompletedEvent.class, new EventHandler<DownloadCompletedEvent>() {
    public void handleEvent(DownloadCompletedEvent event) {
        dcEvent.getDownloadCandidateItem().setState(new FinishedDownloadingState());
        DownloadCandidate downloadCandidate = dcEvent.getDownloadCandidateItem(). getDownloadCandidate();
        if (downloadCandidate.isComplete()) {
            // start extracting
        }
    }
});

eventSource.addListener(DownloadFailedEvent.class, new EventHandler<DownloadFailedEvent>() {
    public void handleEvent(DownloadFailedEvent event) {
        dfEvent.getDownloadCandidateItem().setState(new FailedDownloadingState());
    }
});
}

我将addListener()的实现留给读者作为练习。

Skip Head 16 年前

怎么样:

public void handleEvent(Event e) {
   // Not interested
}


public void handleEvent(DownloadStartedEvent dsEvent) {
    dsEvent.getDownloadCandidateItem().setState(new BusyDownloadingState());
}


public void handleEvent(DownloadCompletedEvent dcEvent) {
    dcEvent.getDownloadCandidateItem().setState(new FinishedDownloadingState());
    DownloadCandidate downloadCandidate = dcEvent.getDownloadCandidateItem().getDownloadCandidate();
    if (downloadCandidate.isComplete()) {
        // start extracting
    }
}

public void handleEvent(DownloadFailedEvent dfEvent) {
    dfEvent.getDownloadCandidateItem().setState(new FailedDownloadingState());
}