- - Event handling in the WRTKit is based on the "observer pattern", - meaning that events are reported from the event source to observers - that are said to be "listening" to event notifications. An event - listener is simply a JavaScript function that takes a single - argument: the event message. In fact even that single argument is - optional and if the function is not interested in the event message - then it can simply ignore it. -
- -- - All views and controls inherit from a common base class called - "UIElement" that defines the mechanics for event observation and - notification. From the point of view of event listeners, the most - important methods are addEventListener() and removeEventListener(). - These two methods are used to register and unregister listener - functions from a view or control. -
- -- - There are different types of events though, and event listeners are - typically not interested in receiving notifications of all events. - For example an event listener that wants to know when a button has - been pressed doesn't usually care if the pointer is currently - hovering above the button or not. Filtering of event notifications - works based on event type names. E.g. in this case the event - listener would have been added so that it should be called only for - events of the "ActionPerformed" type. The event type is given to the - addEventListener() function when a listener is registered. If a - listener function really wants to be notified of all event types - then null can be specified as the event type. Note that the event - type must also be specified when an event listener is unregistered. -
- -- - The code below shows a typical event listener function: -
- -
-
-// Callback for event notifications.
-function handleEvent(event) {
- // handle event here
-}
-
-
- - - The event message is passed to the first argument (called event in - this case) of the event handler function. The event message is a - JavaScript object with three properties: type, source and value. The - type property specifies the event type name and is useful if a - listener function is listening to several types of events. The - source argument is a reference to the source view or control that - sent out the event notification. If a listener function is listening - to events from many different controls then this is useful to figure - out in which of the controls the event occurred. Here the unique - identifier of views and controls can come in handy to identify the - source without needing to retain references to all the source - controls. Finally the value property is a event-type specific - property that contains some information about the event. For example - if the event type is "TextChanged" from a text entry control then - the value would be the new text value that the user has typed into - the control. -
- -- - The code below demonstrates how to add and remove an event listener - to/from a control. The example assumes that the control has already - been created and that the ctrl variable refers to it. -
- -
-
-// add listener to ctrl
-// function to add is handleEvent() for event type "ActionPerformed"
-ctrl.addEventListener("ActionPerformed", handleEvent);
-
-// remove listener from ctrl
-// function to remove is handleEvent() for event type "ActionPerformed"
-ctrl.removeEventListener("ActionPerformed", handleEvent);
-
-
-