C#: Triggering an Event when an object is added to

Question

I need to be able to trigger a event whenever an object is added to a Queue<Delegate>.

I created a new class that extends Queue:

public delegate void ChangedEventHandler(object sender, EventArgs e);

public class QueueWithChange<Delegate> : Queue<Delegate>
{
    public event ChangedEventHandler Changed;

    protected virtual void OnChanged(EventArgs e) {
        if (Changed != null)
        {
            Changed(this, e);
        }
    }
}

And then attached the event from another class, like such:

QueueWithChange<TimerDelegate> eventQueue = new QueueWithChange<TimerDelegate>();

//

eventQueue.Changed += new ChangedEventHandler(delegate(object s, EventArgs ex) {
    //This event is not being triggered, so this code is unreachable atm...and that is my problem

    if (eventQueue.Count > 0)
    {
        eventQueue.Dequeue().Invoke(new DispatcherTimer() { Interval = TimeSpan.FromMilliseconds(5) });
        actionTimer.Stop();
    }
});

But whenever I enqueue an object (eventQueue.Enqueue(something)), the attached event is not being fired.

What am I missing here?

Answer 1

If you mean the non-generic Queue class, then you can just override Enqueue:

public override void Enqueue(object obj)
{
    base.Enqueue(obj);
    OnChanged(EventArgs.Empty);
}

However, if you mean the generic Queue<T> class, then note that there is no suitable virtual method to override. You might do better to encapsulate the queue with your own class:

(** important edit: removed base-class!!! **)

class Foo<T>
{
    private readonly Queue<T> queue = new Queue<T>();
    public event EventHandler Changed;
    protected virtual void OnChanged()
    {
        if (Changed != null) Changed(this, EventArgs.Empty);
    }
    public virtual void Enqueue(T item)
    {
        queue.Enqueue(item);
        OnChanged();
    }
    public int Count { get { return queue.Count; } }

    public virtual T Dequeue()
    {
        T item = queue.Dequeue();
        OnChanged();
        return item;        
    }
}

However, looking at your code, it seems possible that you are using multiple threads here. If that is the case, consider a threaded queue instead.

Answer 2

You have to override Enqueue, to call OnChanged.

Answer 3

I just did write up on what I call a TriggeredQueue. It's inspired the answer by Marc Gravell.

You can find my post here: http://joesauve.com/triggeredqueuet

And the Gist here: http://gist.github.com/jsauve/b2e8496172fdabd370c4

It has four events:

• WillEnqueue
• WillDequeue
• DidEnqueue
• DidDequeue

You can hook into any of these like so:

YourQueue.WillEnqueue += (sender, e) => {
    // kick off some process
};
YourQueue.DidEnqueue += (sender, e) => {
    // kick off some process
    // e.Item provides access to the enqueued item, if you like
};
YourQueue.WillDequeue += (sender, e) => {
    // kick off some process
};
YourQueue.DidDequeue += (sender, e) => {
    // kick off some process
    // e.Item provides access to the dequeued item, if you like
};

One neat trick is that you can use the DidDequeue method to kick off some process to ensure that the queue is full by making a web request or loading some data from a filesystem, etc. I use this class in Xamarin mobile apps to ensure that data and images are pre-cached in order to provide a smooth user experience, instead of loading images AFTER they scroll onto the screen (like you might see in Facebook and countless other apps).

Answer 4

try

public new void Enqueue(Delegate d)
{
    base.Enqueue(d);
    OnChanged(EventArgs.Empty);
}