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+<com:TContent ID="body" >

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

+

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

+

+<p>The syntax for working with events looks like the code below.</p>

+

+<com:TTextHighlighter Language="javascript" CssClass="source">

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

+</com:TTextHighlighter>

+

+<p>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">

+// &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>If we wanted to get the element that fired the event, we'd do this:</p>

+

+<com:TTextHighlighter Language="javascript" CssClass="source">

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

+{ 

+    alert(Event.element(event));

+});

+</com:TTextHighlighter>

+

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

+

+<p>If we wanted to observe keystrokes for the entire document, we could do the following:</p>

+

+<com:TTextHighlighter Language="javascript" CssClass="source">

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

+{ 

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

+        alert('Tab Pressed');

+});

+</com:TTextHighlighter>

+

+<p>And lets say we wanted to keep track of what has been typed :</p>

+

+<com:TTextHighlighter Language="javascript" CssClass="source">

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

+{ 

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

+});

+</com:TTextHighlighter>

+

+<p>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>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>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">

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

+{

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

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

+});

+</com:TTextHighlighter>

+

+<p>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><tt>Event.stop(event)</tt> will stop the propagation of an event .</p>

+

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

+

+<p>Everything has been fairly straight forward so far, but things 

+start getting a little tricker 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>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">

+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>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>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">

+// 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>We've got iterators, binding and all sorts of stuff going on. 

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

+

+<p>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">

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

+</com:TTextHighlighter>

+

+<p>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>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">

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

+

+//where the showTagName function is defined as

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

+</com:TTextHighlighter>

+

+<p>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 didn't use <tt>bind(this)</tt>, I couldn't 

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

+

+<p>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">

+showTagName: function(event) 

+{

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

+}

+</com:TTextHighlighter>

+

+<p>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>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>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">

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

+{

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

+}.bind(this));

+</com:TTextHighlighter>

+

+<p>What's the deal here? The reason this doesn't 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 doesn't have a pointer.</p>

+

+<p>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">

+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>Now we can remove the event listeners from our list like this:</p>

+<com:TTextHighlighter Language="javascript" CssClass="source">

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

+{

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

+}.bind(this));

+</com:TTextHighlighter>

+

+</com:TContent>
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