Session 17
Building a Mobile Web Application
You will view and practice how to use HTML5 tags in a mobile Web application.
Working with Viewport and Media Queries in a Mobile Web Application
Working with jQuery Mobile API .
Introduction
Canvas is one of the most interesting features added in HTML5. The <canvas> element supports advanced graphics and interactions in many forms. In earlier versions of HTML, you could achieve this by using plug-ins. Using the <canvas> element eliminates the need for such plug-ins and makes working with graphics easier and more efficient. The <canvas> element is a drawing area where the user can draw graphics, use images, add animations, and also add text for enhancing the user experience on Web pages.
17.2 Canvas Element
The <canvas> element in HTML5 can be used to draw much more than just rectangles on Web sites - it can be used to dynamically draw graphics using JavaScript. This improves the overall performance of Web sites and avoids the requirement to download images from the sites. The <canvas> element is represented like a rectangle on a page and allows the user to draw arcs, text, shapes, gradients, and patterns. By using <canvas>, the user can draw many complex shapes and also apply various effects and transformations.
The <canvas> in HTML5 is like the <div>, <table>, or <a> tag except that the content used in it is rendered through JavaScript.
The <canvas> element is simple and easy to use with JavaScript. The <canvas> element does not contain any drawing abilities, instead, the drawing is done using a JavaScript code. To make use of the <canvas> element, a user has to add the <canvas> tag on the HTML page.
Code Snippet 1 demonstrates the use of <canvas> element.
Code Snippet 1:
<!DOCTYPE HTML>
<html>
<head>
<title> Canvas </title>
<style>
canvas{border: medium double red; margin: 4px}
</style>
</head>
<body>
<canvas width=”278” height=”200”></canvas>
</body>
</html>
In the code, the <style> element is used to display the border of the <canvas> element. The height and width attributes specify the size of the <canvas> element on the page.
To draw a <canvas> element, the user can use a context object. The context object contains the drawing functions for a specific style of graphics. Two-Dimensional (2d) context is used to work with 2d operations.
The <canvas> element in DOM exposes the HTMLCanvasElement interface. This interface provides the methods and properties for changing the presentation and layout of canvas elements. The HTMLCanvasElement has a getContext(context) method that returns the drawing context for the canvas.
Code Snippet 2 demonstrates the 2d context object for the canvas .
<!DOCTYPE HTML>
<html>
<head>
<title> Canvas </title>
<script>
window.onload = function() {
var canvas = document.getElementById(‘mCanvas’);
var ctext = canvas.getContext(‘2d’);
ctext.beginPath();
ctext.rect(18, 50, 200, 100);
ctext.fillStyle = ”DarkBlue”;
ctext.fill();
};
</script>
</head>
<body>
<canvas id=”mCanvas” width=”578” height=”200”></canvas>
</body>
</html>
In the code, the height and width attributes define the height and width of the canvas element respectively. In the initializer function, the DOM object is accessed through the id attribute and gets a 2d context by using the getContext() method. Here, the rectangle is created by using the rect(18, 50, 200, 100) method with x, y, height, and width parameters and is positioned at left corner of the page .
Drawing a Line in Canvas
You can create lines in a canvas using the stroke(), beginPath(), lineTo(), and moveTo() methods.
The following is the syntax to create a line in canvas:
Syntax:
ctext.beginPath();
ctext.moveTo(x,y);
ctext.lineTo(x,y);
ctext.stroke()
where,
ctext - specifies a context object
beginPath() - Specifies a new drawing path
moveTo() - Specifies the creation of new sub path to the given position
lineTo() - Specifies the drawing of a line from the context position to the given position
stroke() - Specifies how to assign a color to the line and display it
Code Snippet 3 demonstrates creating a line in HTML5 canvas.
<!DOCTYPE HTML>
<html>
<head>
<title>Line</title>
<style>
body {
margin: 0px;
padding: 0px;
}
#mCanvas {
border: 1px solid red;
}
</style>
<script>
window.onload = function() {
var canvas = document.getElementById(“mCanvas”);
var ctext = canvas.getContext(“2d”);
ctext.beginPath();
ctext.moveTo(100, 150);
ctext.lineTo(250, 50);
ctext.lineWidth = 5;
ctext.strokeStyle = “blue”;
ctext.stroke();
};
</script>
</head>
<body>
<canvas id=”mCanvas” width=”360” height=”200”></canvas>
</body>
</html>
In the code, the height and width attributes are defined. The initializer function has the DOM object which is accessed through the id attribute and gets a 2d context by using the getContext() method. The beginPath() method is called through the context object to draw the path of the line. The moveTo(100, 150) method is called that creates a new path for the given point to place the drawing cursor. This method moves the position of the window to the upper-left corner by giving the x and y coordinates. The lineTo(250, 50) method is called to draw the line from the context point to given point. The lineWidth property is specified as 5 to define the width of the line on the canvas. The strokeStyle property sets the color of the line to blue. The stroke() method assigns the color to the line.
Working with Drawing Objects in Canvas
HTML5 canvas allows the user to work with different types of drawing objects. Following objects can be drawn on a canvas element:
Rectangle
With HTML5 canvas, the user can create a rectangle using the rect() method. The HTML5 canvas is placed by using the x and y parameters and appropriately sized through height and width properties. There is a collection of methods and properties that are used to draw different types of shapes. lists the common properties and methods of various shapes.
Properties and Methods Description
fillStyle The values can be gradient, pattern, or a CSS color. The default property style is solid black, but the user can set the color according to the requirements.
filRect(x,y,width, height) Enables the user to draw a rectangle with the existing fill style.
strokeStyle() The values can be gradient, pattern, or a CSS color.
Properties and Methods Description
s t r o k e R e c t ( x , y , width, height) Enables the user to draw a rectangle with the existing stroke style. This property is used to draw the edges of the rectangle.
clearRect(x, y, width, height)
<!DOCTYPE HTML>
<html>
<head>
<style>
#mCanvas {
border: 1px solid green;
}
body {
margin: 0px;
padding: 0px;
}
</style>
<script>
window.onload = function() {
var canvas = document.getElementById(‘mCanvas’);
var ctext = canvas.getContext(‘2d’);
ctext.beginPath();
ctext.rect(30, 50, 150, 100);
ctext.fillStyle = “Magenta”;
ctext.fill();
ctext.lineWidth = 5;
ctext.strokeStyle = ‘black’;
ctext.stroke();
};
</script>
</head>
<body>
<canvas id=”mCanvas” width=”278” height=”200”></canvas>
</body>
</html>
In the code, the height and width attributes are defined. The initializer function has the DOM object which is accessed through the id attribute and gets a 2d context by using the getContext() method. The beginPath() method is called through the context object to draw the rectangle. The rect(30, 50, 150, 100) method takes x, y, height, and width as the parameters. The fillStyle property fills the rectangle with magenta color. The fill() method is used to paint the rectangle. The lineWidth property is specified as 5 to define the width of line on the canvas. The strokeStyle property sets the stroke style of the rectangle to black. The stroke() method assigns the color to the rectangle.
Arcs
With HTML5 canvas, the user can create an arc by using the arc() method. Arcs are represented using a start angle, an end angle, a radius, a center point, and the drawing direction (anticlockwise or clockwise).
Canvas and JavaScript V 1.1 © zetutorials Limited The syntax to draw an arc in HTML5 is as follows:
Syntax:
arc(x, y, radius, startAngle, endAngle, anticlockwise)
where,
x, y - Specifies the coordinates of the center of an arc
radius - Specifies the distance from the center to any point on the circle
startAngle, endAngle - Specifies the start and end points in the arc
anticlockwise - Draws the arc clockwise or anticlockwise and accepts a boolean value
Code Snippet 5 demonstrates how to create an arc in HTML5 canvas.
<!DOCTYPE HTML>
<html>
<head>
<style>
body {
margin: 0px;
padding: 0px;
}
#mCanvas {
border: 1px solid black;
}
</style>
<script>
window.onload = function() {
var canvas = document.getElementById(“mCanvas”);
var ctext = canvas.getContext(“2d”);
var x = canvas.width / 2;
var radius = 75;
var startAngle = 1.1 * Math.PI;
var endAngle = 1.9 * Math.PI;
var ctrClockwise = false;
ctext.beginPath();
ctext.arc(x, y, radius, startAngle, endAngle, ctrClockwise);
ctext.lineWidth = 25;
// line color
ctext.strokeStyle = “DarkGreen”;
ctext.stroke();
};
</script>
</head>
<body>
<canvas id=”mCanvas” width=”278” height=”250”></canvas>
</body>
</html>
In the code, the beginPath() method is called through the context object to draw an arc by using the arc() method which has x, y, and radius as the parameters. The x and y are the coordinates of the circle, radius is the distance from the center to draw the arc on the canvas. The startAngle and the endAngle are the start and end points of the arc respectively. The anticlockwise specifies the direction of the arc between the two start and end points.
Circle
In HTML5, you can draw a circle using the arc() method. You have to set the start angle with 0 and the end angle is specified as 2 * PI.
The following is the syntax to draw a circle in HTML5 is as follows:
Syntax:
arc(x, y, radius, startAngle, endAngle, anticlockwise)
where,
x, y - Specifies the coordinates of the center of a circle
radius - Specifies the distance from the center to any point on the circle
startAngle, endAngle - Specifies the start and end points in the circle
anticlockwise - Draws the circle clockwise or anticlockwise and accepts a boolean value
Code Snippet 6 demonstrates how to create a circle using HTML5.
<!DOCTYPE HTML>
<html>
<head>
<style>
body {
margin: 0px;
padding: 0px;
}
#mCanvas {
border: 1px solid blue;
}
</style>
<script>
window.onload = function() {
var canvas = document.getElementById(“mCanvas”);
var ctext = canvas.getContext(“2d”);
var ctrX = canvas.width / 2;
var ctrY = canvas.height / 2;
var radius = 70;
ctext.beginPath();
ctext.arc(ctrX, ctrY, radius, 0, 2 * Math.PI, false);
ctext.fillStyle = “DarkOrchid”;
ctext.fill();
ctext.lineWidth = 4;
ctext.strokeStyle = “black”;
ctext.stroke();
};
</script>
</head>
<body>
<canvas id=”mCanvas” width=”356” height=”150”></canvas>
</body>
</html>
In the code, a circle is defined by using the arc() method which has ctrX, ctrY, and radius as the parameters. To define the arc with the points the startAngle is set to 0 and the endAngle is specified as 2*PI. The anticlockwise defines the direction of the path of an arc between the two start and end points.
Bezier Curves
Using HTML5 canvas, you can create a Bezier curve using the bezierCurveTo() method. Bezier curves are represented with the two control points, context points, and an end point.
Code Snippet 7 demonstrates how to create a Bezier curve using HTML5.
<!DOCTYPE HTML>
<html>
<head>
<style>
body {
margin: 0px;
padding: 0px;
}
#mCanvas {
border: 1px solid maroon;
}
</style>
<script>
window.onload = function() {
var canvas = document.getElementById(“mCanvas”);
var ctext = canvas.getContext(“2d”);
ctext.beginPath();
ctext.moveTo(188, 130);
ctext.bezierCurveTo(140, 10, 388, 10, 288, 100);
ctext.lineWidth = 15;
// line color
ctext.strokeStyle = “purple”;
ctext.stroke();
};
</script>
</head>
<body>
<canvas id=”mCanvas” width=”378” height=”200”></canvas>
</body>
</html>
In the code, the Bezier curve uses the bezierCurveTo() method. This method defines the current context point, two control points, and an end point. The context point uses the moveTo() method. The first portion of the curve is tangential to the imaginary line defined in the context point and first control point. The second portion of the curve is tangential to the imaginary line which is defined by the second control point and the ending point.
Quadratic Curves
HTML5 canvas allows the user to create quadratic curves using the quadraticCurveTo() method. Quadratic curves are represented through the context point, an end point, and a control point.
Code Snippet 8 demonstrates how to create a quadratic curve using HTML5.
<!DOCTYPE HTML>
<html>
<head>
<style>
body {
margin: 0px;
padding: 0px;
}
#mCanvas {
border: 1px solid #9C9898;
}
window.onload = function() {
var canvas = document.getElementById(“mCanvas”);
var ctext = canvas.getContext(“2d”);
ctext.beginPath();
ctext.moveTo(178, 150);
ctext.quadraticCurveTo(220, 0, 320, 150);
ctext.lineWidth = 15;
// line color
ctext.strokeStyle = “Fuchsia”;
ctext.stroke();
};
</script>
</head>
<body>
<canvas id=”mCanvas” width=”378” height=”200”></canvas>
</body>
</html>
In the code, the control point defines the curve of the quadratic by two tangential lines that are connected to both the context point and the end point. The context point is represented using the moveTo() method. This method moves the control point from the context point and the end point to create a sharper curve. It also moves the control point close to the context point and end point to create broad curves.
Working with Images
In HTML5, the user can draw image objects on canvas using the drawImage() method. The drawImage() method can also draw parts of an image and increase or reduce the size of the image. This method accepts nine parameters, depending on editing that is required on the image. The image object can be a video, an image, or another canvas element.
Code Snippet 9 demonstrates how to create an image using HTML5.
<!DOCTYPE HTML>
<html>
<head>
<style>
body {
margin: 0px;
padding: 0px;
}
#mCanvas {
border: 1px solid #9C9898;
}
</style>
<script>
window.onload = function() {
var canvas = document.getElementById(“mCanvas”);
var ctext = canvas.getContext(“2d”);
var imgObj = new Image();
imgObj.onload = function() {
ctext.drawImage(imgObj, 69, 50);
};
imgObj.src = “bird.jpg”;
};
</script>
</head
<body>
<canvas id=”mCanvas” width=”368” height=”300”></canvas>
</body>
</html>
Code Snippet 10 demonstrates how to resize images with height and width attributes
<!DOCTYPE HTML>
<html>
<head>
<style>
body {
margin: 0px;
padding: 0px;
}
#mCanvas {
border: 1px solid black;
}
</style>
<script>
window.onload = function() {
var canvas = document.getElementById(“mCanvas”);
var ctext = canvas.getContext(“2d”);
var x = 69;
var y = 50;
var w = 150;
var h = 137;
var imgObj = new Image();
imgObj.onload = function() {
ctext.drawImage(imgObj, x, y, w, h);
};
imgObj.src = “bird.jpg”;
};
</script>
</head>
<body>
Working with Text
HTML5 canvas enables you to set the font, style, and size of text by using the font properties. The font style can be italic, normal, or bold. For setting the text color, you can use the fillStyle property of the canvas.
Code Snippet 11 demonstrates how to set the font, size, style, and color of the text on a HTML5 canvas .
<!DOCTYPE HTML>
<html>
<head>
<style>
body {
margin: 0px;
padding: 0px;
}
#mCanvas {
border: 1px solid blue;
}
</style>
<script>
window.onload = function() {
var canvas = document.getElementById(“mCanvas”);
var ctext = canvas.getContext(“2d”);
ctext.font = “italic 30pt Calibri”;
ctext.fillStyle = “MediumVioletRed”;
ctext.fillText(“Welcome to HTML5!”, 40, 100);
};
</script>
</head>
<body>
<canvas id=”mCanvas” width=”380” height=”170”></canvas>
</body>
</html>
In the code, the font text is specified as Calibri, style as italic, and size is set to 30pt. The fillStyle property specifies the text color and the fillText property is used to set the text on the canvas.
In HTML5 canvas, the user can set the stroke color by using the strokeText() method and strokeStyle property of the canvas context.
Code Snippet 12 demonstrates the use of stroke text in HTML5 canvas.
<!DOCTYPE HTML>
<html>
<head>
<style>
body {
margin: 0px;
padding: 0px;
}
#mCanvas {
border: 1px solid black;
}
</style>
<script>
window.onload = function() {
var canvas = document.getElementById(“mCanvas”);
var ctext = canvas.getContext(“2d”);
var x = 80;
var y = 110;
ctext.font = “40pt Calibri”;
ctext.lineWidth = 2;
// stroke color
ctext.strokeStyle = “Brown”;
ctext.strokeText(“HTML5”, x, y);
};
</script>
</head>
<body>
<canvas id=”mCanvas” width=”360” height=”200”></canvas>
</body>
</html>
In the code, the stroke color is set by using the strokeStyle property and the strokeText() method.
Using Transparency for Text in Canvas
There are two ways to set the transparency for the text and shapes. The first method is to use the strokeStyle and fillStyle by using the rgb function. The second method is to use globalAlpha drawing state property, which can be applied universally. The globalAlpha property is a value that ranges between 0 (fully transparent) and 1 (fully opaque).
Code Snippet 13 demonstrates the use of globalAlpha property.
<!DOCTYPE HTML>
<html>
<head>
<style>
body {
margin: 0px;
padding: 0px;
}
#mCanvas {
border: 1px solid black;
}
</style>
<script>
window.onload = function() {
var canvas = document.getElementById(“mCanvas”);
var ctext = canvas.getContext(“2d”);
ctext.fillStyle = “Indigo”;
ctext.strokeStyle =”black”;
ctext.lineWidth=2;
ctext.font = “italic 30pt Calibri”;
ctext.fillText(“HTML5”, 40, 100);
ctext.strokeText(“HTML5”, 40, 100);
ctext.fillStyle=”blue”;
ctext.globalAlpha=0.5;
ctext.fillRect(100, 10, 150, 100);
};
</script>
</head>
<body>
<canvas id=”mCanvas” width=”350” height=”170”></canvas>
</body>
</html>
In the code, the fillStyle and strokeStyle is used to color the text. The ‘HTML5’ text lineWidth is specified as 2 and the font-family is set to Calibri with italic style and font-size to 30pt. The fillText property fills the color and strokeText property applies the stroke color to the HTML5 text. The fillStyle is set to blue and globalAlpha property is set to 0.5. The fillRect(100, 10, 150, 100) specifies the x, y, height, and width of the rectangle .
Using Events with jQuery
jQuery also offers different events to deal with common interactions when the user moves the mouse or switch between two actions while clicking.
hover() event
The mouseenter and mouseleave are the two events often used together. For example, when a user moves a mouse over a menu, a tooltip appears and when the user moves the mouse off the menu, the tooltip disappears. Combining these two events is very common, therefore, jQuery provides a hover() function that accepts two parameters. The first parameter executes when the mouse moves over the element and the second function executes when the mouse moves away from the element.
Code Snippet 14 demonstrates the hover event .
<!DOCTYPE html>
<html>
<head>
<script src=”jquery-1.7.2.min.js”></script>
<script>
$(document).ready(function(){
$(“p”).hover(function(){
$(“p”).css(“background-color”,”red”);
},function(){
$(“p”).css(“background-color”,”maroon”);
});
});
</script>
</head>
<body>
<p>Hover the mouse on this line.</p>
</body>
</html>
In the code, the hover() method is used. When the mouse is placed on the text, then the background color changes to red. Similarly, when the user moves the mouse outside the text, the background-color changes to maroon.
The toggle() event
The toggle() event works in a similar manner as that of the hover() event, except that it responds to mouse clicks. The toggle() function accepts more than two functions as arguments. For example, you want to perform some action on the first click, another action on the second click, and one more action on the third click. All the functions passed to the toggle() event will react to its corresponding click action.
<!DOCTYPE html>
<html>
<head>
<script src=”jquery-1.7.2.min.js”></script>
<script>
$(document).ready(function(){
$(“p”).toggle(function(){
$(“body”).css(“background-color”,”blue”);},
function(){
$(“body”).css(“background-color”,”pink”);},
function(){
$(“body”).css(“background-color”,”grey”);}
);
});
</script>
</head>
<body>
<p>Click to change the colors.</p>
</body>
</html>
In the code, the toggle() method is used. When the user clicks the text then the background-color of the document is changed to blue, pink, and grey respectively.
Inclusion of External Content in Web Pages
HTML5 introduces the <eventsource> tag that allows the user to push external content in the Web page. This model is referred to as push model. Since the <eventsource> tag is not supported in many browsers, users make use of the <embed> tag for this purpose. The <embed> tag is a new element in HTML5 and it is represented as a container for an interactive content or an external application. The <embed> tag is often used to add elements such as image, audio, or video on a Web page.
Code Snippet 16 demonstrates the use of the <embed> tag.
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