Subversion-Projekte lars-tiefland.prado

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<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>

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

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<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>

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// <a id="clicker" href="http://foo.com">Click me</a>

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

{

    alert('clicked!');

});

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

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

{

    alert(Event.element(event));

});

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<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>

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

{

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

        alert('Tab Pressed');

});

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

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

{

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

});

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<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>

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

{

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

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

});

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<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>

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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";

  }

}

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<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>

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// Observe clicks on our list items

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

{

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

}.bind(this));

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<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>

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

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<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>

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

//where the showTagName function is defined as

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

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<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>

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showTagName: function(event)

{

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

}

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<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>

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

{

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

}.bind(this));

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<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>

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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));

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

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

{

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

}.bind(this));

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