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[The following is a continuation of our series of excerpts from chapter 5 of the O'Reilly title, Creating Applications with Mozilla.]
Once you are comfortable with how JavaScript works in the context of the user interface layer and are familiar with some of the primary DOM methods used to manipulate the various elements and attributes, you can add your own scripts to your application. Though you can use other techniques to get scripts into the UI, one of the most common methods is to use Mozilla's event model, which is described in the next few sections.
Events are input messages that pass information from the user interface to the application code. Capturing this information, or event handling, is how you usually tell scripts when to start and stop.
When the user clicks a XUL button, for instance, the button "listens" for the click event, and may also handle that event. If the button itself does not handle the event (e.g., by supplying executable JavaScript in an event handler attribute), then the event "bubbles," or travels further up into the hierarchy of elements above the button. The event handlers in Example 5-3 use simple inline JavaScript to show that the given event (e.g., the window loading in the first example, the button getting clicked in the second, and so on) was fired and handled.
As in HTML, predefined event handlers are available as attributes on a XUL element. These attributes are entry points where you can hook in your JavaScript code, as these examples show. Note that event handler attributes are technically a shortcut, for which the alternative is to register event listeners explicitly to specified elements. The value of these on[event] event handler attributes is the inline JavaScript that should be executed when that event is triggered. Example 5-5 shows some basic button activation events.
Example 5-5: Basic event handler attributes
<window onload="dump('this window has loaded\n');" />
<button label="onclick-test"
onclick="dump('The event handler onclick has just been used\n');" />
<button label="oncommand-test"
oncommand="dump('The event handler oncommand has just been used\n');" />
<menulist id="custom"
onchange="doMyCustomFunction( );" />While the window and button events in Example 5-5 carry out some inline script, there is a variation with the onchange handler attached to the menulist element. onchange contains a JavaScript function call whose definition may live in the XUL document itself or in an external file that is included by using the src attribute on a script element:
<script type="application/x-javascript" src="chrome://mypackage/content/myfile.js" />A large basic set of event handler attributes is available for use on XUL elements (and HTML elements). Appendix C has a full listing of these events along with explanations. The following subset shows the potential for script interaction when the UI uses event handlers:
onabort
onblur
onerror
onfocus
onchange
onclick
oncontextmenu
ondestroy
onload
onpaint
onkeydown
onkeypress
onkeyup
onunload
onmousemove
onmouseout
onmouseover
onmouseup
onmousedown
onrest
onresize
onscroll
onselect
onsubmitSome of these event handlers work only on particular elements, such as window, which listens for the load event, the paint event, and other special events.
To see all event handler attributes on a particular element, you can execute the short script in Example 5-6, which uses the for in loop in JavaScript to iterate over the members of an object-in this case, a XUL element.
Example 5-6: Getting event handler attributes from an element
<script type="application/x-javascript">
function listElementHandlers(aObj)
{
if(!aObj)
return null;
for(var list in aObj)
if(list.match(/^on/))
dump(list+'\n');
}
</script>
<button label="oncommand" oncommand="listElementHandlers(this);" />The function you added in Example 5-4 is also an example of event handler code in an application's interface.
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Created: September 26, 2002
Revised: September 26, 2002
URL: http://webreference.com/programming/javascript/mozillaapps/chap5/2/
