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DOM Events
DOM Events
JS Events Outline• About Events
– Introduction– Registering events– Getting information from events
• Event order/phases– Preventing default action– Preventing bubble
• Events in detail– Click– Keyboard– Mouse
DOM Events
• An event system that allows registration of event listeners
• Describes event flow through a tree structure
• Provide a common subset of current event systems used in existing browsers.
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What are events?
• Events occur when some type of interaction takes place in a web page. (Mouse click, mouse move, key press, etc)
• Events are also things that happen to the web browser. (Loading a page, window resize, scroll window)
• With javascript, developers can detect when certain events happen, and cause things to happen in response.
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How events work• Events can be registered to HTML elements, window, and
document objects.
• When an event happens on an element, the web page checks to see if the element has any event handlers attached.
• If it does, the webpage will call registered event handlers with references and info for each event that occurred.
• Event handlers then act upon the event with whatever code is specified by the developer.
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Event ExampleExample 1 <a href="http://www.opera.com/" onclick="alert('Hello')">Say hello</a>
Example 2
<script type="text/javascript">var link = documents.getElementsByTagName(“a”)[0];link.addEventListener(“click”, function(event) {alert(“Hello”);}, false);
</script>
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Evolution of Events
• In the early days, events use to be handled directly by HTML elements.
• Uses inline javascript which is very messy and didn’t allow reusability.
<a href="http://www.opera.com/" onclick="alert('Hello')">Say hello</a>
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Inline Javascript events• onclick• ondblclick• onmousedown• onmousemove• onmouseover• onmouseout• onmouseup• onkeydown• onkeypress• onkeyup• onabort• onerror• onload• onresize• onscroll• onunload• onblur• onchange• onfocus• onreset• onselect• onsubmit
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Next step in event evolution
• Apply events in blocks of javascript code.
• Code can now be cached and is reusable.
<script type="text/javascript"> document.getElementById("my-link").onclick = waveToAudience; function waveToAudience() { alert("Waving like I've never waved before!"); }</script>
<a id="my-link" href="http://www.opera.com/">My link</a>
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Inline Javascript events
• You can get/set events using DOM
• Each event has an associated node property
• Properties get overwritten when new values are given.
document.body.onclick = function(event) {return 1;}document.body.onclick = function(event) {return 2;} //overidding any previously set onclick values
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Later step in event evolution
• Introduced in DOM 2 Event spec
• Created addEventListener() and removeEventListener()
• Allows developers to register and remove listeners on elements
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addEventListeners()
Has three parameters 1. type 2. listener 3. useCapture
document.body.addEvent(type, listener, useCapture);
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addEventListener() example
document.body.addEventListener('click', function (event) { console.log("hello");},
false);
document.body.addEventListener('click', function(event) { console.log("world");}, false);
type
listener
useCapture
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addEventListener()
• For the type parameter, you don’t need to prepend the “on” for each event type.
• Using addEventListener(), event handlers AREN’T overwritten.– That means you can have multiple listeners on the
same element.
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Type values• click• dblclick• mousedown• mousemove• mouseover• mouseout• mouseup• keydown• keypress• keyup• abort• error• load• resize• scroll• unload• blur• change• focus• reset• select• submit
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removeEventListener() exampleHas three parameters 1. type 2. listener 3. useCapture
document.body.removeEventListener('click', function(event) { console.log("hello");
}, false);
document.body.removeEventListener('click', function(event) { console.log("world");},
false);
Calling removeEventListener() with arguments which do not identify any currently registered event handlers on a target has no effect.
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Your turn!
• Add a click listener to an element so that each time the element is clicked, its top increases by 10px.
document.body.style.top = "0px";document.body.style.top;// "0px";
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Window load Event
• Javascript has a window object that represents an open window in a browser.
• The window’s load event is triggered when the complete document (DOM) is loaded.– This includes images and elements
• We should put all logic that deals with accessing the DOM in the window’s load event listener.
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Window load event example<script type="text/javascript"> window.addEventListener('load', function(event) {
//You should put all javascript logic that access nodes in the DOM inside//this event listenervar element = document.getElementById("content");element.style.color = "rgb(255,255,255);
}, false);</script>
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Getting information from an event
• The Event interface provides basic contextual information about an event to all registered event handlers.
• Supply a parameter to your event handler or use the arguments method to access the Event interface.
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Getting information from an event
function clickHandler(event) {if (event !== undefined) {
console.log(event.timeStamp); //1348409181352console.log(event.type); //clickconsole.log(event.target); //<html element>
}}document.body.addEventListener('click', clickHandler, false);
Event interface
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interface Event{
// PhaseType const unsigned short NONE = 0; const unsigned short CAPTURING_PHASE = 1; const unsigned short AT_TARGET = 2; const unsigned short BUBBLING_PHASE = 3;
readonly attribute DOMString type; readonly attribute EventTarget? target; readonly attribute EventTarget? currentTarget; readonly attribute unsigned short eventPhase; readonly attribute boolean bubbles; readonly attribute boolean cancelable; readonly attribute DOMTimeStamp timeStamp; void stopPropagation(); void preventDefault(); void initEvent(DOMString eventTypeArg, boolean canBubbleArg, boolean cancelableArg); // Introduced in DOM Level 3: void stopImmediatePropagation(); readonly attribute boolean defaultPrevented; readonly attribute boolean isTrusted;
};
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Important Event Interface variables
• type – the name of the event type (click, focus, blur, etc.)
• target – the object the event happened on
• currentTarget – the object associated with the event handler currently being handled.
JS Events Outline• About Events
– Introduction– Registering events– Getting information from events
• Event order/phases– Preventing default action– Preventing bubble
• Events in detail– Click– Keyboard– Mouse
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W3C Event Model
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Specifying which event order
• When using the addEventListener() method, you can specify the event order with the useCapture parameter.
• useCapture === true will specify the event order to be capture.
• useCapture === false will specify the event order to be bubble.
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Default Events Handler
• Some elements have default event handlers:– <a> navigates to specified href/anchor– <input type=“submit”> submits a form for you– <labels> give focus to the corresponding input
element
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What if I want to stop default handlers?
• It is possible to stop the default action of an element in your event handler.
• This might be useful for doing client side validation before submitting a form, preventing a user from navigating to a certain link, etc.
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How to stop the default action
Using the Event interface object, call the preventDefault() method.
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Stop the linkvar links = document.getElementsByTagName("a");for (var i=0; i<links.length; i++) { links[i].onclick = function (e) {
alert("NOPE! I won't take you there!, I’m calling preventDefault()");e.preventDefault();
};}
JS Events Outline• About Events
– Introduction– Registering events– Getting information from events
• Event order/phases– Preventing default action– Preventing bubble
• Events in detail– Click– Keyboard– Mouse
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Quickfacts about click events
• A click event is fired only if a mousedown AND mouseup event happen on an element.
• That means if you press down on an element and then move your mouse off the element and release no click event will happen.
• If you press down outside an element and then move your mouse onto the element and release no click event will happen.
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Keyboard events
• You can register events for keyboards and detect keydown, keypress, and keydown.
• keydown – fires when the user depresses a key. It repeats while the user keeps the key depressed.
• keypress – fires when an actual character is being inserted ($, %, ^). Repeats while key is depressed.
• keyup – fires when the user releases a key
Keyboard events
• Use the Event property which to determine which key was pressed.
• which returns a char code value, not a string.
• Use String.fromCharCode() to convert the which char code into a string.
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Keyboard event exampledocument.body.onkeydown = function(e) {
console.log("key pressed on body is " + String.fromCharCode(e.which));}
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Mouse Events
1. Mousedown2. mouseup 3. click4. dblclick5. mousemove6. mouseover7. mouseout
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Dblclick
• This event is rarely used
• If you decide to use it, you should never register both a click and dblclick event handler on the same element.
• Using click and dblclick makes it almost impossible to figure out what the user did.
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Mousemove
• mousemove is a great event, but care must be taken when using it.
• It takes system time to process all mousemove events, and this event is fired every time you move one pixel.
• Only registered mousemove when you need it and remove it when you’re done.
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Mouseout and Mouseover
• These events can be confusing
• Events may fire at unexpected times with confusing values for the target property.
• My advice is to use jQuery to handle these events.
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Other Cool Mouse tricks
• Figure out which mouse button was clicked:– Use the button property of an mouse event:• 0 is the left button• 1 is the middle button• 2 is the right button
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Get mouse coordinate
• On the mouse event you can use the following properties:1. clientX – the horizontal coordinate relative to the
viewport (browser window).2. clientY – the vertical coordinate relative to the
viewport (browser window).3. screenX – the horizontal coordinate relative to the
origin of the screen (your monitor).4. screenY – the vertical coordinate relative to the
origin of the screen (your monitor)
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Your turn!
• Create event handlers to allow dragging of an element across the screen.
• Don’t be wasteful and have a handler for mousemove when you don’t need one.
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Event References
• Handling events with javascript
• QuirksMode DOM Events
• W3C
• Capture vs. Bubble Mode
