C#

delegateQueue.Invoke(new SendOrPostCallback(delegate(object state)
{
    List<Subscriber> subscribers = (List<Subscriber>)events[eventName];

    if(subscribers == null)
    {
        throw new InvalidOperationException("Event does not exist.");
    }
    else
    {
        subscribers.Add(new Subscriber(handler, context));
    }
}), null);

delegateQueue.Invoke(new SendOrPostCallback(delegate(object state)
{
    List<Subscriber> subscribers = (List<Subscriber>)events[eventName];

    if(subscribers == null)
    {
        throw new InvalidOperationException("Event does not exist.");
    }
    else
    {
        subscribers.Add(new Subscriber(handler, context));
    }
}), new object());

bool OnMessage( ISystem sys, IMessage msg );

IMessage m = sys.NewMessage( "music.play" );
// and then send it.
m.Send();

Leslie Sanford wrote:

The System object could have a hash table where it keeps its messages. When a message is created, the name of the message is stored as a key in the hash table and the message object is stored as its value. Next time someone calls the NewMessage method asking for the same message, instead of creating a new message, the original message is retrieved from the hash table. So only one message of each type is created.

Leslie Sanford wrote:

When a message object is created, it can be passed the EventQueue along with the name of the message. When its Send method is called, the Message sends the message using the EventQueue. So the EventQueue is hidden inside the Message class. The plugins don't have to know anything about it.

class System
    {
        Hashtable hashList = new Hashtable();
        EventQueue eventQueue = new EventQueue();  
        
        public Message NewMessage( string sub )
        {
            Message m = null; // = new Message( "music.play" );
            
            if( !hashList.ContainsKey( sub ) )
            {
                m = new Message( "music.play" );
                hashList.Add( sub, m );
                eventQueue.CreateEvent( sub );     
            }
            else
            {
               m = (Message)hashList[sub];
            }
            
            return m;
        }     
    }
    
    class Message
    {
        string subject;
        string data;

        public Message(string s)
        {
            this.subject = s;
        }
        
        public void Send()
        {
            string sub = this.subject;
            string datium = this.data;

            // what should happen here?
        }
    }

public class Message
{
    string subject;
    EventQueue queue;
    string data = string.Empty;

    public Message(string s, EventQueue queue)
    {
        this.subject = s;
        this.queue = queue;
    }

    public Send()
    {
        queue.Send(this.subject, this, data);
    }
}

smesser wrote:

I thought that the eventQueue was part of the system object.

smesser wrote:

Each instance of message would have it's own eventqueue that would get
disposed of when the message was disposed of taking with it any subscriptions
that may have been made to it.

smesser wrote:

Simple overview:

1. System object creates messages and manages the global eventqueue
2. System object creates and sends events from plugins via the Message send method.
This way there is one global event queue.
3. Plugin has no concept of event queue nor cares about it.

Leslie Sanford wrote:

I think it's number 2 that's giving me trouble. Let's run through a scenario to see if I understand:

1. The System sends a message to a plugin via the OnMessage method.
2. The plugin does something with the message.
3. The plugin wants to send a message telling the System and any other plugins listening that something has happened.
4. The plugin asks the System for a message object representing a specific message, e.g. "PlayCompleted".
5. The plugin gets the message back from the System and sends it calling the message's Send method.

Is this close?

smesser wrote:

5. Close, the plugin would get a the message from the System/EventQueue and either ignore it,
respond to it by using the system object to create a new message and then sending that new
message. The plugin's OnMessage function would return true if the message was consumed.
You wouldn't resend a message sent via the OnMessage function but create a new one if a
response it warranted.

public bool OnMessage(ISystem system, IMessage message)
{
    // Do something with message.
    if(message...)
    {
    }

    // Send a message to system and other plugins:
    IMessage msg = system.NewMessage("Completed");

    msg.Send();

    return true;    
}

public bool OnMessage(ISystem system, IMessage message)
{
    // Do something with message.
    if(message...)
    {
    }

    // Send a message to system and other plugins:
    system.Send("Completed", this, someData);

    return true;    
}

Leslie Sanford wrote:

One thing you may want to consider is to use the System class as a wrapper around the EventQueue class. So instead of using message objects, a plugin would call a method on the system object itself. So...

Leslie Sanford wrote:

One thing that I think needs clearing up is the event handlers in the plugins. If a plugin sends an event to the other plugins, would you require those events to be handled in the OnMessage method, or would you have other methods for handling events from other plugins?

I'm trying to find out if a plugin distinquishes between events sent from the System and from other plugins.

smesser wrote:

The only problem for me with this approach is that my message object inherits from a Data item and also can cantain a list of this data items. It is very flexible and can be used to send lists of any type of data.

public bool OnMessage(ISystem system, IMessage message)
{    
    // Do something with message.    
    if(message...)    
    {    
    }    

    // Send a message to system and other plugins:    
    system.Send("Completed", this, new SomeMessage()); 

    return true;    
}