org.symbian.wrttools.doc.WRTKit/html/WRTKit_Handling_events-GUID-a1a86c8a-6e66-4dc8-8967-b5c9c7bc6563.html
--- a/org.symbian.wrttools.doc.WRTKit/html/WRTKit_Handling_events-GUID-a1a86c8a-6e66-4dc8-8967-b5c9c7bc6563.html Fri Mar 05 19:11:15 2010 -0800
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-<title>
-Handling events</title>
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-
-
-
- <h1 class="topictitle1">
-Handling events</h1>
-
- <div>
-
- <p>
-
- 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.
- </p>
-
- <p>
-
- 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.
- </p>
-
- <p>
-
- 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.
- </p>
-
- <p>
-
- The code below shows a typical event listener function:
- </p>
-
-<pre>
-
-// Callback for event notifications.
-function handleEvent(event) {
- // handle event here
-}
-</pre>
-
- <p>
-
- 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.
- </p>
-
- <p>
-
- 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.
- </p>
-
-<pre>
-
-// 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);
-</pre>
-
- </div>
-
-<div>
-<div class="familylinks">
-<div class="parentlink"><strong>Parent topic:</strong> <a href="WRTKit_Common_WRTKit_tasks-GUID-24870895-4449-4307-9a54-7c90f7b3905e.html">Common WRTKit tasks</a></div>
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