Web Programming Step by Step, 2nd Edition

Chapter 11: Events

Except where otherwise noted, the contents of this document are Copyright 2012 Marty Stepp, Jessica Miller, and Victoria Kirst. All rights reserved. Any redistribution, reproduction, transmission, or storage of part or all of the contents in any form is prohibited without the author's expressed written permission.

JavaScript events

• the click event (onclick) is just one of many events that can be handled
• problem: events are tricky and have incompatibilities across browsers
  • reasons: fuzzy W3C event specs; IE disobeying web standards; etc.
• solution: Prototype includes many event-related features and fixes

Attaching event handlers the Prototype way

element.onevent = function;
element.observe("event", function);

// call the playNewGame function when the Play button is clicked
$("play").observe("click", playNewGame);

• to use Prototype's event features, you must attach the handler using the DOM element object's observe method (added by Prototype)
• pass the event name as a string, and the function name to call
• handlers must be attached this way for Prototype's event features to work

• observe substitutes for addEventListener (not supported by IE)

The event object

function name(event) {
	// an event handler function ...
}

• Event handlers can accept an optional parameter to represent the event that is occurring. Event objects have the following properties / methods:

• *   replaces non-standard srcElement and which properties
• ** replaces non-standard return false;, stopPropagation, etc.

Mouse events

Mouse event objects

The event passed to a mouse handler has these properties:

• *   replaces non-standard properties pageX and pageY
• ** replaces non-standard properties button and which

Mouse event example

<pre id="target">Move the mouse over me!</pre>

window.onload = function() {
	$("target").observe("mousemove", showCoords);
};

function showCoords(event) {
	$("target").innerHTML = 
		  "pointer: (" + event.pointerX() + ", " + event.pointerY() + ")\n"
		+ "screen : (" + event.screenX + ", " + event.screenY + ")\n"
		+ "client : (" + event.clientX + ", " + event.clientY + ")";
}

Move the mouse over me!

The keyword this

this.fieldName                  // access field
this.fieldName = value;          // modify field

this.methodName(parameters);    // call method

• all JavaScript code actually runs inside of an object
• by default, code runs in the global window object (so this === window)
  • all global variables and functions you declare become part of window
• the this keyword refers to the current object

Event handler binding

window.onload = function() {
	$("textbox").observe("mouseout", booyah);   // bound to text box here
	$("submit").observe("click", booyah);       // bound to submit button here
};

function booyah() {           // booyah knows what object it was called on
	this.value = "booyah";
}

• event handlers attached unobtrusively are bound to the element
• inside the handler, that element becomes this (rather than the window)

Fixing redundant code with this

<fieldset>
	<label><input type="radio" name="ducks" value="Huey"  /> Huey</label>
	<label><input type="radio" name="ducks" value="Dewey" /> Dewey</label>
	<label><input type="radio" name="ducks" value="Louie" /> Louie</label>
</fieldset>

function processDucks() {
	if ($("huey").checked) {
		alert("Huey is checked!");
	} else if ($("dewey").checked) {
		alert("Dewey is checked!");
	} else {
		alert("Louie is checked!");
	}
	alert(this.value + " is checked!");
}

• if the same function is assigned to multiple elements, each gets its own bound copy

Page/window events

• The above can be handled on the window object. An alternative to window.onload:

window.onload = function() { ... };
document.observe("dom:loaded", function() {
	// attach event handlers, etc.
});

Keyboard/text events

• focus: the attention of the user's keyboard (given to one element at a time)

Key event objects

• issue: if the event you attach your listener to doesn't have the focus, you won't hear the event
  • possible solution: attach key listener to entire page body, outer element, etc.

Form events

• Prototype adds the following methods to form controls' DOM objects:

Prototype form shortcuts

$F("formID")["name"]

• gets parameter with given name from form with given id

$F("controlID")

• $F function returns the value of a form control with the given id

  if ($F("username").length < 4) {
  	$("username").clear();
  	$("login").disable();
  }
  

Stopping an event

<form id="exampleform" action="http://foo.com/foo.php">...</form>

window.onload = function() {
	$("exampleform").observe("submit", checkData);
};

function checkData(event) {
	if ($F("city") == "" || $F("state").length != 2) {
		alert("Error, invalid city/state.");  // show error message 
		event.stop();
		return false;
	}
}

• to abort a form submit or other event, call Prototype's stop method on the event

Timer events

• both setTimeout and setInterval return an ID representing the timer
  • this ID can be passed to clearTimeout/Interval later to stop the timer

setTimeout example

<button onclick="delayMsg();">Click me!</button>
<span id="output"></span>

function delayMsg() {
	setTimeout(booyah, 5000);
	document.getElementById("output").innerHTML = "Wait for it...";
}

function booyah() {   // called when the timer goes off
	document.getElementById("output").innerHTML = "BOOYAH!";
}

setInterval example

var timer = null;  // stores ID of interval timer

function delayMsg2() {
	if (timer == null) {
		timer = setInterval(rudy, 1000);
	} else {
		clearInterval(timer);
		timer = null;
	}
}

function rudy() {   // called each time the timer goes off
	document.getElementById("output").innerHTML += " Rudy!";
}

Passing parameters to timers

function delayedMultiply() {
	// 6 and 7 are passed to multiply when timer goes off
	setTimeout(multiply, 2000, 6, 7);
}
function multiply(a, b) {
	alert(a * b);
}

• any parameters after the delay are eventually passed to the timer function
  • doesn't work in IE6; must create an intermediate function to pass the parameters
• why not just write this?

  setTimeout(multiply(6 * 7), 2000);
  

Common timer errors

• many students mistakenly write () when passing the function

  setTimeout(booyah(), 2000);
  setTimeout(booyah, 2000);
  
  setTimeout(multiply(num1 * num2), 2000);
  setTimeout(multiply, 2000, num1, num2);
  

  • what does it actually do if you have the () ?
  • it calls the function immediately, rather than waiting the 2000ms!