Posts tagged ‘html’

February 21, 2012

Creating an HTML 5 Timer, part 1

This week, we will be creating a functional timer using HTML 5 timer elements. For visualization, we will use the canvas element to visualize time elapsed as well as an overlay that displays a message when the time has expired.

As a first step, we will craft the HTML for the interface. At the top of the page, we will display elapsed time followed by a canvas element the represents the elapsed time.

<input id=”elapsedTime” value=”0:00:00″ readonly=”readonly” style=”border:none; vertical-align:middle text-align:center; width:120px; font-size:32px; font-weight:bold” >


<div id=”container”>

<canvas id=”canvas” width=”100″ height=”100″>

Your browser does not support the canvas tag


<div id=”timesUp” style=”position:absolute; top:95px; left:12px; color:red”></div>



You will notice that the canvas and has been wrapped in a container that includes the another div. This extra div will be used for an overlay message to be displayed when the time has expired.

Next, we’ll add some input boxes so users can specify hours, minutes, and seconds as well as some buttons for starting, stopping, and resetting the timer.


<input id=”hours” type=”text” value=”” style=”width:26px” onkeypress=”validate(event)” />

<input id=”minutes” type=”text” value=”” style=”width:26px” onkeypress=”validate(event)” />

<input id=”seconds” type=”text” value=”” style=”width:26px” onkeypress=”validate(event)” />



<input name=”starter” type=button value=”Start” style=”width:97px; font-weight:bold” onClick=”startClock()” >



<input name=”pauser” type=button value=”Pause” style=”width:97px; font-weight:bold” onClick=”pauseClock()” >



<input name=”resetter” type=button value=”Reset” style=”width:97px; font-weight:bold” onClick=”resetClock()” >


In our input boxes for hours, minutes, and seconds, we want to limit the input to integers. To do this, we’ll use the validate event and regular expressions.

function validate(evt) {

var theEvent = evt || window.event;

var key = theEvent.keyCode || theEvent.which;

key = String.fromCharCode( key );

var regex = /[0-9]|\./;

if( !regex.test(key) ) {

theEvent.returnValue = false;

if(theEvent.preventDefault) theEvent.preventDefault();



Now we will need to create the script that drives the timer. When the page is loaded, we will need to initialize some variables and render our canvas element.

var interval = 1000;

var limit = 0;

var canvas, ctx, size, startAngle, wedgeSize, fillClr, elapsed, intervalId, timerElement;

var hh=0, mm=0, ss=0;

var hInput, mInput, sInput, overlay;

$(document).ready(function() {

canvas = $(“#canvas”)[0];

ctx = canvas.getContext(‘2d’);

size = Math.min(canvas.width, canvas.height);

startAngle = -Math.PI/2;

timerElement = document.getElementById(‘elapsedTime’);

hInput = document.getElementById(‘hours’);

mInput = document.getElementById(‘minutes’);

sInput = document.getElementById(‘seconds’);

overlay = document.getElementById(‘timesUp’);



Notice the initClock function that is called. We don’t have that yet, so let’s create it. This function will be responsible for resetting variables as well as drawing the initial canvas element.

function initClock() {

timerElement.value = “0:00:00”;

overlay.innerHTML = “”;

elapsed = 0;

wedgeSize = (interval / limit) * Math.PI * 2;

fillClr = “#fff8dc”;

var bgClr = ctx.createLinearGradient( 0, 0, canvas.width, canvas.height );

bgClr.addColorStop( 0, “#000000” );

bgClr.addColorStop( 1, “#999999” );

fillClr = ctx.createLinearGradient( 0, 0, canvas.width, canvas.height );

fillClr.addColorStop( 0, “#AAAAAA” );

fillClr.addColorStop( 1, “#EEEEEE” );

var drawX = drawY = radius = size / 2;

var circle = canvas.getContext(“2d”);

circle.globalAlpha = 1;


circle.arc(drawX, drawY, radius, 0, Math.PI * 2, true);

circle.fillStyle = bgClr;


circle.lineWidth = 0.25;

circle.strokeStyle = “#000000”;



At this point, all that is left to do is to plug in the buttons that we defined earlier. So let’s start with getting the timer running. This function will parse the user input for hours, minutes, and seconds and kick off the timer using HTML5’s setInterval function. The setInterval function will repeatedly be called until it is cleared, so we can take advantage of this to advance our visual representations.

function startClock() {

limit = (parseInt(hInput.value == “” ? “0” : hInput.value) * 3600)

+ (parseInt(mInput.value == “” ? “0” : mInput.value) * 60)

+ parseInt(sInput.value == “” ? “0” : sInput.value);

limit *= 1000;

intervalId = setInterval(function () {

elapsed = elapsed + interval;

wedgeSize = (elapsed / limit) * Math.PI * 2;


var drawX = drawY = radius = size / 2;

var endAngle = startAngle + wedgeSize;

var wedge = canvas.getContext(“2d”);


wedge.moveTo(drawX, drawY);

wedge.arc(drawX, drawY, radius, startAngle, endAngle, false);


wedge.fillStyle = fillClr;


wedge.lineWidth = 0.25;

wedge.strokeStyle = “#eeeeee”;


if (elapsed >= limit) {




}, interval);


You should notice a call to an updateTime() function. We’ll need to create this function as it is responsible for the text display of elapsed time.

function updateTime() {

var s,m;

ss += 1;

s = ss < 10 ? ‘0’ + ss : ss;

if (ss > 59) {

ss = 0;

s = ’00’;

mm += 1;


m = mm < 10 ? ‘0’ + mm : mm;

if (mm > 59) {

mm = 0;

m = ’00’;

hh += 1;


timerElement.value = hh + ‘:’ + m + ‘:’ + s;


Once the elapsed time has reached the limit, clearInterval () is called as well as a finish().  The finish() function is something we will need to create. Its purpose for now is to display some text in the overlay.

function finish(){

overlay.innerHTML = “Time is up!”;


Since we want our users to be able to pause the timer, we can attach the clearInterval() function to the Pause button.

function pauseClock() {



Finally, we will need to create a function for the Reset button. This function will stop the clock, zero out the elapsed time, and re-initialize the canvas.

function resetClock() {


elapsed = hh = mm = ss = 0;



You can see the demo in action here. Next week we will add a bit a polish to the interface as well as add some audio indicators for interval updates and an alarm when the time limit is hit.

February 14, 2012

Multiple View Models with Knockout

In a recent project, I was required to create a form for user input in a web page. Each section of the form is distinct and serves a unique purpose, so I chose to use a tabbed layout . The order in which the forms are completed was not a strict requirement, so some visual indication of which sections were completed was also a requirement.

I decided to use Knockout js for the data binding to take advantage its easy data bindings as well as the ko.computed method to determine which sections are complete.  While creating a design for this, it became apparent that each section (tab) should be its own view model. The problem was that there really isn’t any documentation on how to create multiple view models on a single page. There is an obscure reference deep in the Knockout js documentation that does not clearly explain how to do it.

After a bit of googling, I could not find any clear instructions on how to solve the problem; only some single line answers. After getting small hints from several google search results and some trial and error, I was able to come up with a clean solution.

The solution is actually quite simple, but I thought I would share it with since having a clear example would have saved me a lot of time.

The first thing to do is to create your sections on the page with a unique id.

<section class="tab" id="firstSection">
	<h1>First Section</h1>
	<p>First Name: <strong data-bind="text: name"></strong></p>
	<p>First name: <input data-bind="value: name" /></p>


<section class="tab" id="secondSection">
	<h1>Second Section</h1>
	<p>Second Name: <strong data-bind="text: name"></strong></p>
	<p>Second name: <input data-bind="value: name" /></p>	

One thing you may notice about this example is that it is using the same variable name called “name” in each section. This is one of the benefits of using multiple view models since those variables are only accessible in the scope of the view model.

Second, you will need to define your view models. There is nothing special about how these are defined for this particular solution.

function firstViewModel() {
	var self = this; = ko.observable("Enter Name");			

function secondViewModel() {
	var self = this; = ko.observable("Enter Name");	

Lastly, you will need to apply the bindings for the view models and assign them to the previously defined sections.

ko.applyBindings(new firstViewModel, $("#firstSection")[0]);
ko.applyBindings(new secondViewModel, $("#secondSection")[0]);

Take note of the second parameter passed to the applyBindings method as it does the magic of isolating the view model to a specific element on the page.  I am not sure why this isn’t clearly documented anywhere, but hopefully this post proves useful to anyone that is looking to add support for multiple view models on a single page.

A working demo can be found here

January 31, 2012

Creating a simple pie chart with HTML5 Canvas Part 3

Over the last two weeks, we have created a simple pie chart and then added some functionality. This week we will focus on some design aspects to improve the presentation of the chart.

While contrasting colors for each slice of the pie can be used for a clear distinction between slices, using a border for each one generally looks nicer. This is easily accomplished using the .stroke method.


ctx.lineWidth = 1;

ctx.strokeStyle = “#fff”;



Another simple way to improve the appearance of the chart is to add some color gradient for each slice. Using color gradients will give the chart a more modern look and are extremely simple to add. The first step is to define the gradient as follows.


var gradient = ctx.createLinearGradient( 0, 0, canvas.width, canvas.height );

gradient.addColorStop( 0, “#ddd” );

gradient.addColorStop( 1, colors[i] );


Once the gradient is defined, we can set it as the fillStyle for the context and call the fill method.


ctx.fillStyle = gradient;



Last week we added rendering of the pie slice values at the bottom of the screen. While that does the job of giving the user information about the slice, it forces them to look away from the chart. To improve on that, we will have the value of the slice be rendered at the position of the mouse. The effect will be that the value follows the user as they hover over the slices.


ctx.fillStyle = ‘#fff’;

ctx.font = ‘bold 10px verdana’;

ctx.fillText(pieData[slice][‘value’], x, y, 20);


While adding these improvements may seem trivial, they are worth mentioning due to the fact that they transform what was a simple and dull chart into one that is more functional, intuitive, and professional.

For the full source and a working demo, please follow the link below.

January 24, 2012

Creating a simple pie chart with HTML5 Canvas Part 2

Last week we created a pie chart using HTML5’s Canvas element. This week we will utilize jQuery to add some simple functionality.  For this exercise, we want to be able to visualize the exact values of each slice of the pie when the user positions the mouse over them. To do this, we will use the .mousemove event from jQuery to update text on the page.

The first step is to include jQuery.


<script src=””></script&gt;


Next, we will need to refactor the script from last week a bit in order to reference it from the mousemove event. To do this, we will create an array to store the data for each slice.


var pieData = [];

for(var i in data) {

                pieData[i] = [];

                pieData[i][‘value’] = data[i];

                pieData[i][‘startAngle’] = 2 * Math.PI * lastPosition;

                pieData[i][‘endAngle’] = 2 * Math.PI * (lastPosition + (data[i]/total));

                lastPosition += data[i]/total;



Now that the data is stored a bit differently than last week, we will need to update the script that renders the pie chart. Specifically, we will change the ctx.arc call to reference the data we just stored in the pieData array.


for(var i = 0; i < data.length; i++)







                ctx.fillStyle = colors[i];




Next we will add some text to the page that will be used to display the value of the pie chart that the user is currently hovering over with the mouse.



        Hover over a slice



Now that we have a placeholder for the text, we can add the mousemove event that will first evaluate whether the user is within the radius of the chart and then do a bit of math to determine which slice the mouse is over.


$(“#canvas”).mousemove(function(e) {

                var x = Math.floor((e.pageX-$(“#canvas”).offset().left));

                var y = Math.floor((e.pageY-$(“#canvas”).offset().top));

                var fromCenterX = x – center[0];

                var fromCenterY = y – center[1];

                var fromCenter = Math.sqrt(Math.pow(Math.abs(fromCenterX), 2) + Math.pow(Math.abs(fromCenterY), 2 ));


                if (fromCenter <= radius) {

                                var angle = Math.atan2(fromCenterY, fromCenterX);

                                if (angle < 0) angle = 2 * Math.PI + angle; // normalize


                                for (var slice in pieData) {

                                                if (angle >= pieData[slice][‘startAngle’] && angle <= pieData[slice][‘endAngle’]) {

                                                                $(“div.sliceValue”).text(“The value for the slice is ” + pieData[slice][‘value’]);







If all went well, you should now have a working demo. If not, you can view the source of the working demo linked below.

January 17, 2012

Creating a simple pie chart with HTML5 Canvas

This week we take a look at how to use the HTML5 canvas element to create a simple pie chart. To begin, we need to define the element using the canvas tag. Please note the id, height, and width are required.

<canvas id=”canvas” width=”400″ height=”300″></canvas>

In order to interact with this canvas through Java script, we will need to first get the element by Id and then create a context.

<script type="text/javascript">

var canvas = document.getElementById(‘canvas’);

var ctx = canvas.getContext(“2d”);


These two steps are essentially all that is required to start drawing to the canvas. For this example, we will be drawing a simple pie chart, so let’s add some data. We’ll use some hard coded values for data in the interest of simplicity.


var data = [75,68,32,95,20,51];

var colors = [“#7E3817”, “#C35817”, “#EE9A4D”, “#A0C544”, “#348017”, “#307D7E”];

var center = [canvas.width / 2, canvas.height / 2];

var radius = Math.min(canvas.width, canvas.height) / 2;

var lastPosition = 0, total = 0;


Next we will need to get the total value of all the data being represented in the pie chart. This is required in order to scale the size of each slice correctly. To do this, we simply need to loop through the data array and add each value into the ‘total’ variable defined above.


for(var i in data) { total += data[i]; }


And finally, we are ready start using canvas to draw each slice of the pie. To do this, we will loop through the data array again and call the canvas methods to create and fill the shapes.


for (var i = 0; i < data.length; i++) {

ctx.fillStyle = colors[i];






lastPosition += Math.PI*2*(data[i]/total);



Loading the page in an HTML5 enabled browser should result in our chart being rendered as follows.




Next week we will take a look at adding labels to the chart as well as adding some user interaction with animation.