Web Programming Step by Step, 2nd Edition

Lecture 17: More Events; Timers

Reading: 9.1.3; 11.1–11.2

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

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() {
	document.getElementById("textbox").onmouseout = booyah;
	document.getElementById("submit").onclick = 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

Fixing redundant code with this

<input id="huey"  type="radio" name="ducks" value="Huey"  /> Huey
<input id="dewey" type="radio" name="ducks" value="Dewey" /> Dewey
<input id="louie" type="radio" name="ducks" value="Louie" /> Louie

function processDucks() {
	if (document.getElementById("huey").checked) {
		alert("Huey is checked!");
	} else if (document.getElementById("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

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:

Mouse events

Mouse event objects

The event passed to a mouse handler has these properties:

Mouse event example

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

window.onload = function() {
	var target = document.getElementById("target");
	target.onmousemove = target.onmousedown = showCoords;
};

function showCoords(event) {
	document.getElementById("target").innerHTML = 
		+ "screen : (" + event.screenX + ", " + event.screenY + ")\n"
		+ "client : (" + event.clientX + ", " + event.clientY + ")\n"
		+ "button : "  + event.button;
}

Move the mouse over me!

Keyboard/text events

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, document, an outer element, etc.

Key event example

document.getElementById("textbox").onkeydown = textKeyDown;
...
function textKeyDown(event) {
	var key = String.fromCharCode(event.keyCode);
	if (key == 's' && event.altKey) {
		alert("Save the document!");
		this.value = this.value.split("").join("-");
	}
}

• each time you push down any key, even a modifier such as Alt or Ctrl, the keydown event fires
• if you hold down the key, the keydown event fires repeatedly
• keypress event is a bit flakier and inconsistent across browsers

Some useful key codes

Page/window events

• The above can be handled on the window object

Form events

Stopping an event

• you can also return false; from your event handler to stop an event

Stopping an event, example

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

window.onload = function() {
	var form = document.getElementById("exampleform");
	form.onsubmit = checkData;
};

function checkData(event) {
	if (document.getElementById("state").length != 2) {
		alert("Error, invalid city/state.");  // show error message
		event.preventDefault();
		return false;              // stop form submission
	}
}

Multiple listeners to the same event

element.addEventListener("event", function);

var button = document.getElementById("mybutton");
button.addEventListener("click", func1);   // button.onclick = func1;
button.addEventListener("click", func2);   // button.onclick = func2;

• if you assign onclick twice, the second one replaces the first
• addEventListener allows multiple listeners to be called for the same event
• (note that you do not include "on" in the event name!)

Multiple window.onload listeners

window.onload = function;
window.addEventListener("load", function);

• it is considered bad form to directly assign to window.onload
• multiple .js files could be linked to the same page, and if they all need to run code when the page loads, their window.onload statements will override each other
• by calling window.addEventListener instead, all of them can run their code when the page is loaded

Setting a timer

• 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 id="clickme">Click me!</button>
<span id="output"></span>

window.onload = function() {
	document.getElementById("clickme").onclick = delayedMessage;
};

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

function sayBooyah() {   // 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 IE; 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!