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+<com:TContent ID="body" >

+<h1 id="6701">DOM Events and Javascript</h1>

+

+<h2 id="6702">Basic event handling</h2>

+

+<p id="840735" class="block-content">The syntax for working with events looks like the code below.</p>

+

+<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_840254">

+Event.observe(element, name, observer, [useCapture]);

+</com:TTextHighlighter>

+

+<p id="840736" class="block-content">Assuming for a moment that we want to observe when a link was clicked,

+we could do the following:</p>

+

+<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_840255">

+// &lt;a id="clicker" href="http://foo.com"&gt;Click me&lt;/a&gt;

+Event.observe('clicker', 'click', function(event)

+{

+    alert('clicked!');

+});

+</com:TTextHighlighter>

+

+<p id="840737" class="block-content">If we wanted to get the element that fired the event, we'd do this:</p>

+

+<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_840256">

+Event.observe('clicker', 'click', function(event)

+{

+    alert(Event.element(event));

+});

+</com:TTextHighlighter>

+

+<h2 id="6703">Observing keystrokes</h2>

+

+<p id="840738" class="block-content">If we wanted to observe keystrokes for the entire document, we could do the following:</p>

+

+<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_840257">

+Event.observe(document, 'keypress', function(event)

+{

+    if(Event.keyCode(event) == Event.KEY_TAB)

+        alert('Tab Pressed');

+});

+</com:TTextHighlighter>

+

+<p id="840739" class="block-content">And lets say we wanted to keep track of what has been typed :</p>

+

+<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_840258">

+Event.observe('search', 'keypress', function(event)

+{

+    Element.update('search-results', $F(Event.element(event)));

+});

+</com:TTextHighlighter>

+

+<p id="840740" class="block-content">Prototype defines properties inside the event object for some

+of the more common keys, so feel free to dig around in Prototype to

+see which ones those are.</p>

+

+<p id="840741" class="block-content">A final note on keypress events; If you'd like to detect a

+left click you can use <tt>Event.isLeftClick(event)</tt>.</p>

+

+<h2 id="6704">Getting the coordinates of the mouse pointer</h2>

+

+<p id="840742" class="block-content">Drag and drop, dynamic element resizing, games, and

+much more all require the ability to track the X and Y location of

+the mouse. Prototype makes this fairly simple. The code below tracks

+the X and Y position of the mouse and spits out those values into

+an input box named mouse.</p>

+

+<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_840259">

+Event.observe(document, 'mousemove', function(event)

+{

+    $('mouse').value = "X: " + Event.pointerX(event) +

+                       "px Y: " + Event.pointerY(event) + "px";

+});

+</com:TTextHighlighter>

+

+<p id="840743" class="block-content">If we wanted to observe the mouse location when it was

+hovering over a certain element, we'd just change the document argument to

+the id or element that was relevant.</p>

+

+<h2 id="6705">Stopping Propagation</h2>

+

+<p id="840744" class="block-content"><tt>Event.stop(event)</tt> will stop the propagation of an event .</p>

+

+<h2 id="6706">Events, Binding, and Objects</h2>

+

+<p id="840745" class="block-content">Everything has been fairly straight forward so far, but things

+start getting a little trickier when you need to work with events in

+and object-oriented environment. You have to deal with binding and funky

+looking syntax that might take a moment to get your head around.</p>

+

+<p id="840746" class="block-content">Lets look at some code so you can get a better understanding of what I'm talking about.</p>

+<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_840260">

+EventDispenser = Class.create();

+EventDispenser.prototype =

+{

+  initialize: function(list)

+  {

+    this.list = list;

+

+    // Observe clicks on our list items

+    $$(this.list + " li").each(function(item)

+    {

+        Event.observe(item, 'click', this.showTagName.bindEvent(this));

+    }.bind(this));

+

+    // Observe when a key on the keyboard is pressed.

+    // In the observer, we check for

+    // the tab key and alert a message if it is pressed.

+    Event.observe(document, 'keypress', this.onKeyPress.bindEvent(this));

+

+    // Observe our fake live search box.  When a user types

+    // something into the box, the observer will take that

+    // value(-1) and update our search-results div with it.

+    Event.observe('search', 'keypress', this.onSearch.bindEvent(this));

+

+    Event.observe(document, 'mousemove', this.onMouseMove.bindEvent(this));

+  },

+

+  // Arbitrary functions to respond to events

+  showTagName: function(event)

+  {

+    alert(Event.element(event).tagName);

+  },

+

+  onKeyPress: function(event)

+  {

+    var code = event.keyCode;

+    if(code == Event.KEY_TAB)

+        alert('Tab key was pressed');

+  },

+

+  onSearch: function(event)

+  {

+    Element.update('search-results', $F(Event.element(event)));

+  },

+

+  onMouseMove: function(event)

+  {

+    $('mouse').value = "X: " + Event.pointerX(event) +

+                "px Y: " + Event.pointerY(event) + "px";

+  }

+}

+</com:TTextHighlighter>

+<p id="840747" class="block-content">Whoa! What's going on here? Well, we've defined our a

+custom class <tt>EventDispenser</tt>. We're going to be using this class

+to setup events for our document. Most of this code is a

+rewrite of the code we looked at earlier except this time, we

+are working from inside an object.</p>

+

+<p id="840748" class="block-content">Looking at the <tt>initialize</tt> method, we can really see how

+things are different now. Take a look at the code below:</p>

+<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_840261">

+// Observe clicks on our list items

+$$(this.list + " li").each(function(item)

+{

+    Event.observe(item, 'click', this.showTagName.bindEvent(this));

+}.bind(this));

+</com:TTextHighlighter>

+

+<p id="840749" class="block-content">We've got iterators, binding and all sorts of stuff going on.

+Lets break down what this chunk of code is doing.</p>

+

+<p id="840750" class="block-content">First we are hunting for a collection of elements based on

+it's CSS selector. This uses the Prototype selector function <tt>$$()</tt>.

+After we've found the list items we are dealing with we send

+those into an each iteration where we will add our observers.</p>

+

+<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_840262">

+Event.observe(item, 'click', this.showTagName.bindEvent(this));

+</com:TTextHighlighter>

+

+<p id="840751" class="block-content">Now looking at the code above, you'll notice the <tt>bindEvent</tt> function.

+This takes the method before it <tt>showTagName</tt> and treats it as the

+method that will be triggered when, in this case,

+someone clicks one of our list items.</p>

+

+<p id="840752" class="block-content">You'll also notice we pass this as an argument to the <tt>bindEvent</tt> function.

+This simply allows us to reference the object in context <tt>EventDispenser</tt>

+inside our function <tt>showTagName(event)</tt>. If the <tt>showTagName</tt> function

+requires additional parameters, you can attach them to the later parameters of <tt>bindEvent</tt>. For example</p>

+<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_840263">

+this.showTagName.bindEvent(this, param1, param2);

+

+//where the showTagName function is defined as

+showTime : function (event, param1, param2) { ... }

+</com:TTextHighlighter>

+

+<p id="840753" class="block-content">Moving on, you'll see <tt>bind(this)</tt> attached to our iterator function.

+This really has nothing to do with events, it is only here to allow me to

+use <tt>this</tt> inside the iterator. If we did not use <tt>bind(this)</tt>, I could not

+reference the method <tt>showTagName</tt> inside the iterator.</p>

+

+<p id="840754" class="block-content">Ok, so we'll move on to looking at our methods that actually get

+called when an event occurs. Since we've been dealing with <tt>showTagName</tt>, lets look at it.</p>

+

+<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_840264">

+showTagName: function(event)

+{

+    alert(Event.element(event).tagName);

+}

+</com:TTextHighlighter>

+

+<p id="840755" class="block-content">As you can see, this function accepts one argument--the event.

+In order for us to get the element which fired the event we need to

+pass that argument to <tt>Event.element</tt>. Now we can manipulate it at will.</p>

+

+<p id="840756" class="block-content">This covers the most confusing parts of our code. The text above is also

+relevant to the remaining parts of our code. If there is anything about

+this you don't understand, feel free to ask questions in the forum.</p>

+

+<h2 id="6707">Removing Event Listeners</h2>

+

+<p id="840757" class="block-content">This one threw me for a loop the first time I tried to use it.

+I tried something similar to what I did in the <tt>Event.observe</tt>

+call with the exception of using <tt>stopObserving</tt>, but nothing seemed

+to change. In other words, the code below does <b>NOT</b> work.</p>

+

+<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_840265">

+$$(this.list + " li").each(function(item)

+{

+    Event.stopObserving(item, 'click', this.showTagName);

+}.bind(this));

+</com:TTextHighlighter>

+

+<p id="840758" class="block-content">What's the deal here? The reason this does not work is because there

+is no pointer to the observer. This means that when we passed <tt>this.showTagName</tt>

+in the <tt>Event.observe</tt> method before hand, we passed it as an

+anonymous function. We can't reference an anonymous function

+because it simply does not have a pointer.</p>

+

+<p id="840759" class="block-content">So how do we get the job done? All we need to do is give the

+observing function a pointer, or the jargon free version: Set a variable

+that points to <tt>this.showTagName</tt>. Ok, lets change our code a bit.</p>

+

+<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_840266">

+this.showTagObserver = this.showTagName.bindEvent(this);

+

+// Observe clicks on our list items

+$$(this.list + " li").each(function(item)

+{

+    Event.observe(item, 'click', this.showTagObserver);

+}.bind(this));

+</com:TTextHighlighter>

+

+<p id="840760" class="block-content">Now we can remove the event listeners from our list like this:</p>

+<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_840267">

+$$(this.list + " li").each(function(item)

+{

+    Event.stopObserving(item, 'click', this.showTagObserver);

+}.bind(this));

+</com:TTextHighlighter>

+

+<div class="last-modified">$Id: Scripts2.page 1650 2007-01-24 06:55:32Z wei $</div></com:TContent>
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