Saturday, February 13, 2010

WPF EventArgs = EventObject in Swing -- a message DTO

In every GUI toolkit there's a class hierarchy of of event "value objects". Such an object is passed into event handlers or callbacks as a function argument. In Swing it is known simply an "event" object. I feel this meaning of "event" is traditional, natural and intuitive -- a data transfer object carrying a message about an occurrence.

WPF refers to such a DTO object as "EventArgs", and user "Event" for something else. WPF turned the English word "Event" on its head and gave it a twist. The new meaning is "invitation". When a WPF view model (or model) object fires an event, it sends out an invitation to a (or a inv-list of) registered listener, inviting them to "query me for my current state". The listener is typically a data-bound visual component in the view

Therefore in the WPF context there are 2 slightly different meanings to the English word "event" --
- "An event pseudo field" -- a list of listeners. If 2 visuals bind to a single model object, then list of 2 listeners.
- "firing event" -- a single invitation sent to the listeners. You can rapid-fire 500 events in a minute[1] using the same event-pseudo-field. Each invitation will hit all the listeners.


[1] Incidentally, Such rapid-firing rate is not uncommon, because firing is often programmatic and driven by a collection.

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http://www.linkedin.com/in/tanbin