An event is considered to be an external action that occurs during a program's execution. Correctly dealing with events is vital part of developing an EiffelVision 2 application. For example, if a user clicks on a button, you will want to respond to this event by calling a routine that deals with the request. EiffelVision 2 contains action sequences for all kinds of widget events. To view the kind of events available to every widget, click here. The Events cluster contains classes for event handling within EiffelVision 2.
How Do I Connect to an Event?
Every widget and item has an action sequence associated with it that relates to some kind of event. For example an EV_BUTTON has a
select_actions action sequence. This gets fired when the user selects (clicks) the button. To have a procedure called on this event, you need to create an agent based on this procedure, and then add this to the action sequence (via extend). For a more detailed description of agents and their uses click here .
An example of adding an agent to an action_sequence:
my_button.select_actions.extend (agent print ("Button Clicked!%N"))
All EiffelVision 2 action sequences inherit from EV_ACTION_SEQUENCE and when it is called, all of the agents held within are fired, thus calling all of the procedures represented by the agents. The signature of any agent that you place in an action sequence must conform to those of the action sequence's actual generic parameter(s).
When you want an agent to be called from a certain action sequence and the signatures do not match, you may use force_extend . This will call your agent but there are no guarantees as to the arguments passed to your procedure.