| author | Eugene Ostroukhov <eugeneo@symbian.org> |
| Wed, 14 Apr 2010 14:54:13 -0700 | |
| changeset 305 | be8783adb3a8 |
| parent 230 | 7848c135d915 |
| permissions | -rw-r--r-- |
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<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> |
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<html lang="en" xml:lang="en"> |
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<head> |
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<meta content="text/html; charset=utf-8" http-equiv="Content-Type" /> |
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<meta name="copyright" content="(C) Copyright 2005" /> |
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<meta name="DC.rights.owner" content="(C) Copyright 2005" /> |
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<meta content="concept" name="DC.Type" /> |
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<meta name="DC.Title" content="Handling events" /> |
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<meta scheme="URI" name="DC.Relation" content="WRTKit_Common_WRTKit_tasks-GUID-24870895-4449-4307-9a54-7c90f7b3905e.html" /> |
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<meta content="XHTML" name="DC.Format" /> |
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<meta content="GUID-A1A86C8A-6E66-4DC8-8967-B5C9C7BC6563" name="DC.Identifier" /> |
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<meta content="en" name="DC.Language" /> |
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<link href="commonltr.css" type="text/css" rel="stylesheet" /> |
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<title> |
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Handling events</title> |
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</head> |
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<body id="GUID-A1A86C8A-6E66-4DC8-8967-B5C9C7BC6563"><a name="GUID-A1A86C8A-6E66-4DC8-8967-B5C9C7BC6563"><!-- --></a> |
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<h1 class="topictitle1"> |
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Handling events</h1> |
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<div> |
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<p> |
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Event handling in the WRTKit is based on the "observer pattern", |
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meaning that events are reported from the event source to observers |
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that are said to be "listening" to event notifications. An event |
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listener is simply a JavaScript function that takes a single |
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argument: the event message. In fact even that single argument is |
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optional and if the function is not interested in the event message |
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then it can simply ignore it. |
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</p> |
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<p> |
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All views and controls inherit from a common base class called |
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"UIElement" that defines the mechanics for event observation and |
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notification. From the point of view of event listeners, the most |
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important methods are addEventListener() and removeEventListener(). |
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These two methods are used to register and unregister listener |
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functions from a view or control. |
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</p> |
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<p> |
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There are different types of events though, and event listeners are |
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typically not interested in receiving notifications of all events. |
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For example an event listener that wants to know when a button has |
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been pressed doesn't usually care if the pointer is currently |
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hovering above the button or not. Filtering of event notifications |
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works based on event type names. E.g. in this case the event |
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listener would have been added so that it should be called only for |
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events of the "ActionPerformed" type. The event type is given to the |
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addEventListener() function when a listener is registered. If a |
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listener function really wants to be notified of all event types |
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then null can be specified as the event type. Note that the event |
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type must also be specified when an event listener is unregistered. |
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</p> |
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<p> |
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The code below shows a typical event listener function: |
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</p> |
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<pre> |
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// Callback for event notifications. |
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function handleEvent(event) {
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// handle event here |
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} |
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</pre> |
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<p> |
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The event message is passed to the first argument (called event in |
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this case) of the event handler function. The event message is a |
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JavaScript object with three properties: type, source and value. The |
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type property specifies the event type name and is useful if a |
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listener function is listening to several types of events. The |
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source argument is a reference to the source view or control that |
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sent out the event notification. If a listener function is listening |
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to events from many different controls then this is useful to figure |
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out in which of the controls the event occurred. Here the unique |
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identifier of views and controls can come in handy to identify the |
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source without needing to retain references to all the source |
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controls. Finally the value property is a event-type specific |
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property that contains some information about the event. For example |
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if the event type is "TextChanged" from a text entry control then |
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the value would be the new text value that the user has typed into |
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the control. |
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</p> |
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<p> |
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The code below demonstrates how to add and remove an event listener |
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to/from a control. The example assumes that the control has already |
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been created and that the ctrl variable refers to it. |
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</p> |
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<pre> |
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// add listener to ctrl |
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// function to add is handleEvent() for event type "ActionPerformed" |
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ctrl.addEventListener("ActionPerformed", handleEvent);
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// remove listener from ctrl |
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// function to remove is handleEvent() for event type "ActionPerformed" |
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ctrl.removeEventListener("ActionPerformed", handleEvent);
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</pre> |
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</div> |
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<div> |
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<div class="familylinks"> |
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<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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</div> |
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</div> |
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</body> |
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</html> |