axaptapedia: Event broker

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Источник: http://www.axaptapedia.com/Event_broker
==============

Summary: /* Warning */

==Purpose==
* To allow [http://en.wikipedia.org/wiki/Event-driven_programming event driven programming]
* To have [http://en.wikipedia.org/wiki/Loose_coupling loose coupling] between classes
* A (partial) implementation of the [http://en.wikipedia.org/wiki/Observer_pattern Observer pattern]

==Download==
[http://www.axaptapedia.com/images/2/...ventBroker.xpo SysEventBroker.xpo (11 KB)]

==Implementaion==
Three classes are provided:

# ''SysEventListener'' is the interface class for a "listener"
#* ''gotEvent'': every listener must implement
# ''SysEventInfo'' is an implementation for a listener, which simply put received information on the [[Infolog]]
#* ''main'': contains a simple test case
# ''SysEventBroker'', which client classes use to register and notify listeners, it implements the following methods:
#* ''addListener'': to register a listener
#* ''removeListener'': to unregister a listener
#* ''fireEvent'': to fire an event and to notify listeners via the ''gotEvent'' method

==Example==


static void SysEventTest(Args _args)
{
SysEventBroker broker = SysEventBroker::construct();
SysEventInfo info1 = new SysEventInfo("Info-1"); // catch all;
SysEventInfo info2 = new SysEventInfo("Info-2"); // catch specific class
SysEventInfo info3 = new SysEventInfo("Info-3"); // catch specific type
;
// Setup
broker.addListener(info1, classNum(Object));
broker.addListener(info2, classNum(Runbase));
broker.addListener(info3, classNum(Object), "Fire 2"); // catch Fire 2
broker.addListener(info3, classNum(Object), "Fire 4"); // and Fire 4

broker.fireEvent(classNum(CustAutoCreate), "Fire 1", "This is my life");
broker.removeListener(info2, classNum(Runbase));
broker.fireEvent(classNum(CustAutoCreate), "Fire 2", "this is my time");
broker.fireEvent(classNum(CustAutoCreate), "Fire 3", "just show me the light");
broker.removeListener(info1);
broker.fireEvent(classNum(CustAutoCreate), "Fire 4", "and I go there");
}

Generated info:

Info-2 CustAutoCreate(Fire 1): This is my life
Info-1 CustAutoCreate(Fire 1): This is my life
Info-3 CustAutoCreate(Fire 2): this is my time
Info-1 CustAutoCreate(Fire 2): this is my time
Info-1 CustAutoCreate(Fire 3): just show me the light
Info-3 CustAutoCreate(Fire 4): and I go there


==Explanation==
An event broker and 3 event listeners are created.
The event listeners are then registered with ''broker.addListener'' which takes 3 arguments:

# ''Listener object'': identifying the listener object
# ''Class id'': identifying the sender class which the listener is interested in
# ''Event type'': an optional string identifying the event to listen to (all events if not given)

Then events are fired with ''broker.fireEvent'' which as parameter takes 3 arguments:

# ''Class id'': identifying the sender class
# ''Event type'': a string identifying the event
# ''data'': any additional information which can have any type

In the first ''fireEvent'' the class id is ''SalesAutoCreate'' (it is a lie), the event type is "Fire 1" and the data is the first line of a song.
As ''SalesAutoCreate'' has parent class ''RunBase'' and ''Object'' both ''info1'' and ''info2'' listeners receive the ''gotEvent'' call, and the first two lines of info is generated. The ''info3'' listener does not get the event, it is interested in "Fire 2" and "Fire 4" events only.

It is unusual to have ''addListener'' and ''fireEvent'' in the same method. A more interesting way would be for a top level class to implement ''gotEvent'', call ''addListener(this, classNum(SomeClass))'', then deep down the call stack ''SomeClass'' will call ''fireEvent''.

==Global broker==
Client classes need to share a SysEventBroker instance.
A possible solution is to use the global [[Infolog]] object.

In the ''Info.classDeclaration'' add:

SysEventBroker eventBroker;


Add a new method:

public SysEventBroker eventBroker()
{
if (!eventBroker)
eventBroker = SysEventBroker::construct();
return eventBroker;
}


Beware, ''Infolog'' lives at the client, all notified infomation is sent to the client.

You could make the same implementation in the ''Application'' class which lives at the server.
But you must then remember to use the same broker.

==Warning==
This is no silver bullet, do not abuse!
* Do not not use when standard method calls and argument passing is the easy solution.
* Do not forget to unregister the listener, even if an exception is thrown. Use try/catch!
* Registered listeners live as long as the broker, if it is ''Infolog.eventBroker()'' for the entire user session!

==See also==
Another solution on [http://blogs.msdn.com/x/archive/2007...-eventing.aspx Issues concerning X++]

[[Category:Development tools]]


Источник: http://www.axaptapedia.com/Event_broker