DOM events

DOM (Document Object Model) events allow event-driven programming languages like JavaScript, JScript, ECMAScript, VBScript and Java to register various event handlers/listeners on the element nodes inside a DOM tree, e.g. HTML, XHTML, XUL and SVG documents.

Historically, like DOM, the event models used by various web browsers had some significant differences. This caused compatibility problems. To combat this, the event model was standardized by the W3C in DOM Level 2.

Events

HTML events

Common/W3C events

There is a huge collection of events that can be generated by most element nodes:

• Mouse events
• Keyboard events
• HTML frame/object events
• HTML form events
• User interface events
• Mutation events (notification of any changes to the structure of a document)

Note that the event classification above is not exactly the same as W3C's classification.

Note that the events whose names start with “DOM” are currently not well supported. Mozilla and Opera support DOMAttrModified, DOMNodeInserted, DOMNodeRemoved and DOMCharacterDataModified. Chrome and Safari also support these events, with the exception of DOMAttrModified.

Microsoft-specific events

Two major types of events are added by Microsoft, and in some cases can only be used in Internet Explorer. Others have been implemented as de-facto standards by other browsers.

• Clipboard events
• Data binding events

Note that Mozilla, Safari and Opera also support readystatechange event for the XMLHttpRequest object. Mozilla also supports the beforeunload event using traditional event registration method (DOM Level 0). Mozilla and Safari also support contextmenu, but Internet Explorer for the Mac does not.

Note that Firefox 6 and later support beforeprint and afterprint events.

Touch events

Web browsers running on modern, touch-enabled devices have events, such as "touchstart", "touchend", "touchenter", "touchleave", "touchmove", and "touchcancel". Apple's iOS and Google's Android are two examples of mobile operating systems with support for these events in their web browsers. Apple didn't join this working group, but according to an Opera employee,^[1][2] has filed last minute patents, for the third time, in what seems like attempts to block forming of open standards.^[3]

When you put a finger down on the screen of the touch enabled devices, it kicks off the lifecycle of touch events and the following events get triggered.

• touchstart: When a finger is placed on the touch surface/screen.
• touchend: When a finger is removed from the touch surface/screen.
• touchmove: When a finger already placed on the screen is moved across the screen.
• touchenter: When a touch point moves onto the interactive area defined by a DOM element.
• touchleave: When a touch point moves off the interactive area defined by a DOM element.
• touchcancel: A user agent must dispatch this event type to indicate when a TouchPoint has been disrupted in an implementation-specific manner, such as by moving outside the bounds of the UA window. A user agent may also dispatch this event type when the user places more touch points (The coordinate point at which a pointer (e.g. finger or stylus) intersects the target surface of an interface) on the touch surface than the device or implementation is configured to store, in which case the earliest TouchPoint object in the TouchList should be removed.^[4]

The TouchEvent, TouchPoint and TouchList interfaces are defined by w3.org in the Touch Events Specification^[5]

• Interface TouchPoint

This interface defines an individual point of contact for a touch event.

   interface TouchPoint {
   readonly attribute long         identifier;
   readonly attribute EventTarget  target;
   readonly attribute DOMTimeStamp timestamp;
   readonly attribute long         screenX;
   readonly attribute long         screenY;
   readonly attribute long         clientX;
   readonly attribute long         clientY;
   readonly attribute long         pageX;
   readonly attribute long         pageY;
   readonly attribute long         radiusX;
   readonly attribute long         radiusY;
   readonly attribute float        rotationAngle;
   readonly attribute float        force;
   readonly attribute boolean      altKey;
   readonly attribute boolean      metaKey;
   readonly attribute boolean      ctrlKey;
   readonly attribute boolean      shiftKey;
   };[6]

Attributes

1. altKey: true if the alt (Alternate) key modifier is activated; otherwise false.
2. clientX: x-coordinate of point relative to the viewport, excluding any scroll offset.
3. clientY: y-coordinate of point relative to the viewport, excluding any scroll offset.
4. ctrlKey: true if the ctrl (Control) key modifier is activated; otherwise false.
5. force: a relative value of pressure applied, in the range 0 to 1, where 0 is no pressure, and 1 is the highest level of pressure the touch device is capable of sensing; 0 if no value is known. This attribute may not be available on all user agents or platforms. In environments where force is available, the absolute pressure represented by the force attribute, and the sensitivity in levels of pressure, may vary.
6. identifier: An identification number for each touch point, unique to that touch point per session.
7. metaKey: true if the meta (Meta) key modifier is activated; otherwise false. On some platforms this attribute may map to a differently-named key modifier.
8. pageX: x-coordinate of point relative to the viewport, including any scroll offset.
9. pageY: y-coordinate of point relative to the viewport, including any scroll offset.
10. radiusX: The radius of the ellipse which most closely circumscribes the touching area (e.g. finger, stylus) along the x-axis, in pixels of the same scale as screenX; 1 if no value is known. This attribute may not be available on all user agents or platforms.
11. radiusY: The radius of the ellipse which most closely circumscribes the touching area (e.g. finger, stylus) along the y-axis, in pixels of the same scale as screenY; 1 if no value is known. This attribute may not be available on all user agents or platforms.
12. rotationAngle: The angle (in degrees) that the ellipse described by radiusX and radiusY is rotated clockwise about its center; 0 if no value is known. The value must be in the range [0, 90). If the ellipse described by radiusX and radiusY is circular, then rotationAngle must be 0.
13. screenX: x-coordinate of point relative to the screen.
14. screenY: y-coordinate of point relative to the screen.
15. shiftKey: true if the shift (Shift) key modifier is activated; otherwise false.
16. target: the original proximal event target for this touch point. The target must be an Element.
17. timestamp: The timestamp attribute represents the time when the TouchPoint was initiated and is represented as a DOMTimeStamp.

• Interface TouchList

This interface defines a list of individual points of contact for a touch event.

   interface TouchList {
   readonly attribute unsigned long length;
   caller getter TouchPoint item (in unsigned long index);
   caller getter object     identifiedPoint (in long identifier);
   };[7]

Attributes

1. length: returns the number of touchpoints in the list.

Methods

1. identifiedPoint : returns the first touchpoint with long identifier from the list. The value for the identifier should not be nullable and it is not an optional parameter.
2. item : returns the touchpoint with index from the list,which is sorted in order from latest to earliest.The value for the index should not be nullable and it is an not optional parameter.

• Interface TouchEvent

This interface defines the touchstart, touchend, touchmove, touchenter, touchleave, and touchcancel event types.

   interface TouchEvent : UIEvent {
   readonly attribute TouchList touches;
   readonly attribute TouchList targetTouches;
   readonly attribute TouchList changedTouches;
   void initTouchEvent (in DOMString type, in boolean canBubble, in boolean cancelable, in DOMWindow view, in long detail,
   in boolean ctrlKey, in boolean altKey, in boolean shiftKey, in boolean metaKey, in TouchList touches, in TouchList targetTouches,
   in TouchList changedTouches);
   };[8]

Attribute

1. changedTouches: Its a list of TouchPoints for every point of contact which contributed to the event.
2. targetTouches :Its a list of TouchPoints for every point of contact currently touching the surface, which started on the same target.
3. touches:Its a list of TouchPoints for every point of contact currently touching the surface.

Methods

1. initTouchEvent: initializes a TouchEvent created through the DocumentEvent interface.

XUL events

In addition to the common/W3C events, Mozilla defined a set of events that work only with XUL elements.

Other events

For Mozilla and Opera 9, there are also undocumented events known as "DOMContentLoaded" and "DOMFrameContentLoaded" which fire when the DOM content is loaded. These are different from "load" as they fire before the loading of related files (e.g., images). However, DOMContentLoaded has been added to the HTML 5 Draft Specification. The "DOMContentLoaded" event was also implemented in the Webkit rendering engine build 500+.^[9] This correlates to all versions of Google Chrome and Safari 3.1+. DOMContentLoaded will be implemented in Internet Explorer 9.^[10]

Opera 9 also supports the Web Forms 2.0 events "DOMControlValueChanged", "invalid", "forminput" and "formchange".

Event flow

Consider the situation when there are 2 elements nested together. Both have event handlers registered on the same event type, say "click". When the user clicks on the inner element, there are two possible ways to handle it:

• Trigger the elements from outer to inner (event capturing). This model is implemented in Netscape Navigator.
• Trigger the elements from inner to outer (event bubbling). This model is implemented in Internet Explorer and other browsers.

W3C takes a middle position in this struggle. Events are first captured until it reaches the target element, and then bubbled up. During the event flow, an event can be responded to at any element in the path (an observer) in either phase by causing an action, and/or by stopping the event (with method event.stopPropagation() for W3C-conforming browsers and command event.cancelBubble = true for Internet Explorer), and/or by cancelling the default action for the event.

Event object

The Event object provides a lot of information about a particular event, including information about target element, key pressed, mouse button pressed, mouse position, etc. Unfortunately, there are very serious browser incompatibilities in this area. Hence only the W3C Event object is discussed in this article.

Event handling models

DOM Level 0

This event handling model was introduced by Netscape Navigator, and remains the most cross-browser model as of 2005^[update]. There are two model types: inline model and traditional model.

Inline model

In the inline model, event handlers are added as attribute of element. Event handlers can also be removed:

<head>
  <script type="text/javascript">
    function helloWorld( name )
    {
        window.alert( "Hello " + name );
    }
 
    function removeHandler(object)
    {
        object.onclick = null;
    }
  </script>
</head>
<body>
 
 
Hello <a href="http://www.example.com" onclick="helloWorld('Joe'); removeHandler(this);">Joe</a>!
 
</body>

In the example above, an alert dialog box with the message "Hello Joe" will appear when the hyperlink is clicked and open URI in href attribute.

The default action can be cancelled by returning false in the event handler:

<head>
  <script type="text/javascript">
    function helloWorld( name )
    {
        window.alert( "Hello " + name );
    }
  </script>
</head>
<body>
 
 
Stay <a href="http://www.example.com" onclick="helloWorld('Joe'); return false;">here</a>!
 
</body>

In the example above, the browser will not go to "example.com" when the hyperlink is clicked.

One common misconception with the inline model is the belief that it allows the registration of event handlers with custom arguments, e.g. name in the helloWorld function. While it may seem like that is the case in the example above, what is really happening is that the JavaScript engine of the browser creates an anonymous function containing the statements in the onclick attribute. The onclick handler of the element would be bound to the following anonymous function:

function()
{
    helloWorld('Joe');
    return false;
}

This limitation of the JavaScript event model is usually overcome by assigning attributes to the function object of the event handler or by using closures.

Traditional model

In the traditional model, event handlers can be added/removed by scripts. Like the inline model, each event can only have one event handler registered. The event is added by assigning the handler name to the event property of the element object. To remove an event handler, simply set the property to null:

<head>
  <script type="text/javascript">
    function helloWorld()
    {
        window.alert( "Hello World" );
    }
 
    // Add an event handler
    window.onload = helloWorld;
 
    // Add another event handler
    document.onclick = helloWorld;
 
    // Remove the event handler just added
    document.onclick = null;
  </script>
</head>
<body>
 
 
Hello World!
 
</body>

To add parameters:

var name='Joe';
document.onclick = (function(name)
{
 return function () {
   alert('Hello '+ name +'!');
 }
})(name);

Inner functions preserve their scope.

DOM Level 2

The W3C designed a more flexible event handling model in DOM Level 2.

Some useful things to know :

• To prevent an event from bubbling, developers must call the "stopPropagation()" method of the event object.
• To prevent the default action of the event to be called, developers must call the "preventDefault" method of the event object.

The main difference from the traditional model is that multiple event handlers can be registered for the same event. The useCapture option can also be used to specify that the handler should be called in the capture phase instead of the bubbling phase. This model is supported by Mozilla, Opera, Safari, Chrome and Konqueror.

A rewrite of the example used in traditional model:

<head>
  <script type="text/javascript">
    function helloWorld()
    {
        window.alert( "Hello World" );
    }
 
    // Add an event handler
    window.addEventListener( "load", helloWorld, false );  // bubbling phase
 
    // Add another event handler
    document.addEventListener( "click", helloWorld, true );  // capture phase
 
    // Remove the event handler just added
    document.removeEventListener( "click", helloWorld, true );
  </script>
</head>
<body>
 
 
Hello World!
 
</body>

Microsoft-specific model

Microsoft does not follow the W3C model up until Internet Explorer 8, as its own model was created prior to the ratification of the W3C standard. Internet Explorer 9 is supposed to follow DOM level 3 events.^[11]

Some useful things to know :

• To prevent an event bubbling, developers must set the event's "cancelBubble" property.
• To prevent the default action of the event to be called, developers must set the event's "returnValue" property.
• The this keyword refers to the global window object.

Again, this model differs from the traditional model in that multiple event handlers can be registered for the same event. However the useCapture option can not be used to specify that the handler should be called in the capture phase. This model is supported by Microsoft Internet Explorer and Trident based browsers (e.g. Maxthon, Avant Browser).

A rewrite of the example used in traditional model:

<head>
  <script type="text/javascript">
    function helloWorld()
    {
        window.alert( "Hello World" );
    }
 
    // Add an event handler
    window.attachEvent( "onload", helloWorld );
 
    // Add another event handler
    document.attachEvent( "onclick", helloWorld );
 
    // Remove the event handler just added
    document.detachEvent( "onclick", helloWorld );
  </script>
</head>
<body>
 
 
Hello World!
 
</body>

See also

• XML Events A syntactic binding to DOM events

References

• Deitel, Harvey. (2002). Internet and World Wide Web: how to program (Second Edition). ISBN 0-13-030897-8
• The Mozilla Organization. (2009). DOM Event Reference. Retrieved August 25, 2009.
• Quirksmode (2008). Event compatibility tables. Retrieved November 27, 2008.
• http://www.sitepen.com/blog/2008/07/10/touching-and-gesturing-on-the-iphone/

External links

• Document Object Model (DOM) Level 2 Events Specification
• Document Object Model (DOM) Level 3 Events Working Draft

References