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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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 listener

var 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); //1348409181352

console.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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Event Order Demo

• http://jsfiddle.net/blinkmacalahan/jLb6d/5/

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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 keyup.

• 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.

• 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 a 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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Event References

• Handling events with javascript

• QuirksMode DOM Events

• W3C

• Capture vs. Bubble Mode