带代表的ASP.Net中的观察者

Question

嗯，我在一个asp.net 3.5网站上工作。

我像这样设置了一个观察者：

public delegate void ActionNotification();

protected Dictionary<string, List<ActionNotification>> Observers
{
    get
    {
        Dictionary<string, List<ActionNotification>> _observers = Session["Observers"] as Dictionary<string, List<ActionNotification>>;
        if (_observers == null)
        {
            _observers = new Dictionary<string, List<ActionNotification>>();
            Observers = _observers;
        }
        return _observers;
    }
    set
    {
        Session["Observers"] = value;
    }
}

public void Attach(string actionName, ActionNotification observer)
{
    if (!Observers.ContainsKey(actionName))
    {
        Observers.Add(actionName, new List<ActionNotification>());
    }
    Observers[actionName].Add(observer);
}

public void Detach(string actionName, ActionNotification observer)
{
    if (Observers.ContainsKey(actionName))
    {
        Observers[actionName].Remove(observer);
    }

}
public void DetachAll(string actionName)
{
    if (Observers.ContainsKey(actionName))
    {
        Observers.Remove(actionName);
    }
}

public void Notify(string action)
{
    if (Observers.ContainsKey(action))
    {
        foreach (ActionNotification o in Observers[action])
        {
            o.Invoke();
        }
    }
}

我像这样使用观察者：

//Esta es llamada al notify con cierto action       
protected void btnNext_Click(object sender, ImageClickEventArgs e)          
{           
    Notify("Next");         
} 
//Y este es el register del Listener            
Attach("Next", new ActionNotification(NextButton_Click)); 

如果在o.Invoke()之前；例如，我将页面标题更改为"Hello“。在"NextButton_Click“中，我将其设置为”再见“，在NextButton_Click结束后，标题又回到”Hello“……

知道为什么吗？

Answer 1

我认为问题在于NextButton_Click事件中的“页面”与您将标题设置为"Hello“的页面不是同一个页面。因为在引发事件时，您正在传递会话中的事件，所以acts所在的对象不再在作用域中。您可以使用以下代码(使用EventHandlers，但它们基本上与您在代码中概述的内容相同)重新创建它。

protected void Page_Load(object sender, EventArgs e)
{
    this.Page.Title = "test";

    //Store it in your session...seems like a weird thing to do given how your page should be stateless, so I would think about what you are
    //trying to do a bit more carefully.  You don't want to call an event handler such as test below from another page in your asp.net app.
    Dictionary<string, EventHandler> myEvents = null;
    if (Session["Invokers"] == null)
    {
        myEvents = new Dictionary<string, EventHandler>();
        Session["Invokers"] = myEvents;
    }
    else
    {
        myEvents = Session["Invokers"] as Dictionary<string, EventHandler>;
    }
    //If the event handler key is not in there then add it
    if (myEvents.ContainsKey("buttonClickOnPageDefault") == false)
    {
        //Subscribe to event (i.e. add your method to the invokation list
        this.TestEvent += new EventHandler(test);
        myEvents.Add("buttonClickOnPageDefault", this.TestEvent);
    }
    else
    {
        //if it does contain this key then you may already be subscribed to event, so unsubscribe in case and then resubscribe...you could
        //probably do this more elegantly by looking at the vales in the GetInvokationList method on the eventHandler
        //Wire up the event
        this.TestEvent -= new EventHandler(test);
        this.TestEvent += new EventHandler(test);
    }
    //Resave the dictionary.
    Session["Invokers"] = myEvents;
}

void test(object o, EventArgs e)
{
    this.Page.Title = "testEvent";
}

public event EventHandler TestEvent;

protected void Button1_Click(object sender, EventArgs e)
{
    if (Session["Invokers"] != null)
    {
        Dictionary<string, EventHandler> myEvents = (Dictionary<string, EventHandler>)Session["Invokers"];
        if (myEvents.ContainsKey("buttonClickOnPageDefault"))
        {
            EventHandler ev = myEvents["buttonClickOnPageDefault"];
            ev(null, EventArgs.Empty);
        }
    }
}

如果您将上述代码放在asp.net页面中，它将永远不会更改页面标题，但是如果您在Test方法中放置断点，您将看到它被击中。原因是它在不同的页面中被命中(该页面超出范围，可能不会被垃圾回收，因为您的事件仍然有对它的引用，所以这可能会导致内存leak...be小心处理它！)。实际上，您可能不应该以这种方式使用事件(至少不应作用于page...maybe，它有一些域对象实用程序)。请注意，以下代码将起作用(因为它在同一页面上运行)

protected void Page_Load(object sender, EventArgs e)
{
    this.Page.Title = "test";

    //Store it in your session...seems like a weird thing to do given how your page should be stateless, so I would think about what you are
    //trying to do a bit more carefully.  You don't want to call an event handler such as test below from another page in your asp.net app.
    this.TestEvent += new EventHandler(test);
    Session["Invoker"] = this.TestEvent;
}

void test(object o, EventArgs e)
{
    this.Page.Title = "testEvent";
}

public event EventHandler TestEvent;

protected void Button1_Click(object sender, EventArgs e)
{
    if (Session["Invoker"] != null)
    {
        EventHandler ev = (EventHandler)Session["Invoker"];
        ev(null, EventArgs.Empty);
    }
}

希望这能给你一些提示，指出你的问题所在。