1. DOM Events

2. 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

3. 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.

4. 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. CSE 33454

5. 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. CSE 33455

6. Event Example Example 1 Say hello Example 2 var link = documents.getElementsByTagName(“a”)[0]; link.addEventListener(“click”, function(event) { alert(“Hello”); }, false); CSE 33456

7. 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. Say hello CSE 33457

8. 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 CSE 33458

9. Next step in event evolution Apply events in blocks of javascript code. Code can now be cached and is reusable. document.getElementById("my-link").onclick = waveToAudience; function waveToAudience() { alert("Waving like I've never waved before!"); } My link CSE 33459

10. 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 CSE 334510

11. Later step in event evolution Introduced in DOM 2 Event spec Created addEventListener() and removeEventListener() Allows developers to register and remove listeners on elements CSE 334511

12. addEventListeners() Has three parameters 1.type 2.listener 3.useCapture document.body.addEvent(type, listener, useCapture); CSE 334512

13. addEventListener() example document.body.addEventListener('click', function (event) { console.log("hello"); }, false); document.body.addEventListener('click', function(event) { console.log("world");}, false); CSE 334513 type listener useCapture

14. 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. CSE 334514

15. Type values click dblclick mousedown mousemove mouseover mouseout mouseup keydown keypress keyup abort error load resize scroll unload blur change focus reset select submit CSE 334515

16. removeEventListener() example Has 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. CSE 334516

17. 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. CSE 334517

18. Window load event example 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); CSE 334518

19. Getting information from an event The Event interface provides basic contextual information about an event to all registered event handlers.event handlers Supply a parameter to your event handler or use the arguments method to access the Event interface. CSE 334519

20. Getting information from an event function clickHandler(event) { if (event !== undefined) { console.log(event.timeStamp); //1348409181352 console.log(event.type); //click console.log(event.target); // } document.body.addEventListener('click', clickHandler, false); CSE 334520 Event interface

21. CSE 334521 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; };

22. 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. CSE 334522

23. 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

24. CSE 334524 W3C Event Model

25. 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. CSE 334525

26. Event Order Demo http://jsfiddle.net/blinkmacalahan/jLb6d/5/ CSE 334526

27. Default Events Handler Some elements have default event handlers: – navigates to specified href/anchor – submits a form for you – give focus to the corresponding input element CSE 334527

28. 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. CSE 334528

29. How to stop the default action Using the Event interface object, call the preventDefault() method. CSE 334529

30. Stop the link var 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(); }; } CSE 334530

31. 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

32. 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. CSE 334532

33. 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 CSE 334533

34. 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.

35. Keyboard event example document.body.onkeydown = function(e) { console.log("key pressed on body is " + String.fromCharCode(e.which)); } CSE 334535

36. Mouse Events 1.mousedown 2.mouseup 3.click 4.dblclick 5.mousemove 6.mouseover 7.mouseout CSE 334536

37. 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. CSE 334537

38. 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. CSE 334538

39. 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. CSE 334539

40. 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 CSE 334540

41. 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) CSE 334541

42. Event References Handling events with javascript QuirksMode DOM Events W3C Capture vs. Bubble Mode CSE 334542