CSS: Class Selectors - learn CSS for free and fun

Class Selectors

Class selectors are used for defining different styles for the same type of HTML element.

If you want to add two different styles (say, two different font styles) to a particular type of
HTML element (say, ‘paragraphs’ appearing on a web page), you can do it with the class
selector.

The general syntax for a class selector is
tag.class attribute {Property:Value;}

Example: Consider the instructional website as given above. If you want to define two
different font colors for the alternate paragraphs, the style sheet will be written as:

p.question {color: #CC0000;}
p.answer {color: #339999;}

Class selectors apply to all the elements with the particular class attribute given in the
statement. With class selectors you can also add more than one class to a particular element.
Sometimes you will need to add different styles to a type of HTML element that appears in a
web page. With class selectors we can do it easily.

Class selectors are used for defining different styles for the same type of HTML element.
If you want to add two different styles (say, two different font styles) to a particular type of
HTML element (say, ‘paragraphs’ appearing on a web page), we can do it with the class
selector.

In an instructional website you may have paragraphs displayed in a webpage, with “Definitions”
and “Examples” in alternate paragraphs. With class selectors we have two different styles for
the alternate paragraphs.

Let us consider the example of the instructional website as given above. If we want to define
two different font colors for the alternate paragraphs, the style sheet will be written as shown
here.

Class selectors apply to all the elements with the particular class attribute given in the
statement.
The following things must be noted about class selectors.
The class attribute for which the style is defined in the style sheet must be included in the
HTML document.

If the tag name is not given in the selector, the style will be applied to all the HTML elements
with the particular class.

For example, the following style will apply to all the HTML elements with the “text” attribute.
If you need to add more than one style to an element, we can still do it with class selectors.
With class selectors you can also add more than one class to a particular element.

CSS: ID Selector - Learn CSS for Free and Fun

ID Selectors

ID selectors are also used to define styles for an object with a unique ID. It is mostly used with
layers as layers are always defined by a unique ID. An ID selector is defined as a #. The general
syntax for an ID selector is #IDSelector{Property:Value;}. ID selectors apply to exactly one
element.

There are TWO types of ID selectors:

Solitary ID selectors 

It can apply to any element with an ID matching the ID of the selector. Any element with #idname syntax will have Solitary ID selectors applied over it.

The following ID selector will apply to an element that has an id attribute with a value of "red":

#red {color: red;}

ID selectors

This ID selector is associated with a particular type of element in a page. This ID selector will have the syntax elementname# idname and will apply only to the
element with this ID.

The following ID selector will apply to a p element that has an id attribute with a value of “para1":

p#para1{
text-align: left;
color: blue;
}


ID selectors are also used for defining styles for HTML elements. These selectors are used to
define a style for an object with a unique ID. The ID selector is mostly used with layers as
layers are always defined by a unique ID.

An ID selector is defined as a # and the general syntax for an ID selector is shown here.
Hash followed by the ID selector and then the property and its value within braces. ID selectors
apply to exactly one element.

There are TWO types of ID selectors: Solitary ID selectors and ID selectors.

Solitary ID selector is a general ID selector that can apply to any element with an ID matching
the ID of the selector. Any element with #idname syntax will have Solitary ID selectors applied
over it.

The solitary ID selector in our example will apply to an element that has an id attribute with a
value of "red":

ID selector is associated with a particular type of element in a page. This ID selector will have
the syntax element-name#idname and will apply only to the element with this ID. It must be
noted here that an ID comprises only alpha numeric characters and hyphens and does not
include any other characters, underscores or spaces, nor can an ID start with a numeral.
The example shows the ID selector that will apply to a p element that has an id attribute with
a value of "para1":

CSS: Tag Selector - Learn CSS for Free and Fun ..........

What are Tag Selectors

 By using Tag Selectors we can define styling for existing HTML tags
 Often used to set the basic styles that will appear throughout a Web site

How Tag Selectors are used

HTML Selector {Property:Value;}
Eg : b {font-family:Arial, Helvetica, sans-serif; font-size:14px; color: red;}

This means Arial font of size 14 and red in color is stored within the tag b.
CSS with HTML tag selector, example:

body {
font-family: Arial;
font-size: 14px;
color: red;
text-align: left;
}

The meaning of the CSS code above is: every text inside the body tag will be appeared in Arial
font face with size 14 px, red color and left alignment. As the tag selector is defined within the
body tag only the text appearing in the body will change. However the paragraph text, header
text and others will remain unaffected.

CSS: Advantages and Disadvantages - Learn CSS for Free and Fun

Advantages of CSS

Using CSS we can keep the content of an html file and the style and layout of the file separate
from each other. This gives several advantages to CSS over the traditional HTML.

 CSS keeps the file size smaller

Unlike HTML where we need to write the code to specify the font size, color, style etc. for
every single page of a document that consists of more than one page, In CSS we need to
specify the attributes only once for each element and the specified style will automatically
be applied whenever the element occurs. Since the style and layout elements are
separated from the HTML this allows the document to be much smaller in size.

 The pages of the website loads faster

Since the HTML pages have less code after the style and layout is separated from it, the
pages will take less time to load.

 Lower bandwidth requirement

When the pages of a website take lesser time to load, the bandwidth requirement of the
server is also reduced. This also makes CSS a cost effective option.

 CSS saves time

With CSS, changing the style of a document becomes very easy and it also saves much time.
In HTML if we want to make even a small change in the entire website (such as changing
the font style), we need to open every single page and alter the font style manually, which
makes the process to time consuming. However, with We just need to open the CSS file
where the layout of the site is stored and change the font style, which in turn, will change
the font style of the entire website.

 CSS is easier to manage

As to make a change throughout a website that uses CSS, one need to make an edit only in
a single file, it is easier to manage CSS. This greatly helps when the website has several
pages.

Disadvantage of CSS

One major disadvantage of CSS is its incompatibility to different browsers. While some
browsers show partial compatibility to Style Sheets and support only some of the features
of style sheets, very few browsers such as the early versions of Internet Explorer and
Netscape show almost complete incompatibility to CSS.
However, the latest browsers versions show more standards-compliant and have largely
overcome the compatibility problem.

CSS: How to insert three types of Style Sheets - learn CSS for Free and Fun .......

There are THREE ways in which style sheets can be inserted to an HTML document.

Inline Styles within a single HTML element

When a style is intended for a single place or item on the entire website, a single tag CSS is
used. As the term suggests, a single tag CSS affects only a single tag in a HTML document
for which it is defined. Inline styles are used for this purpose. In inline style the style
attribute must be used in the relevant tag that renders the style. The style attribute may
have any CSS property.

The following example shows how a change in the color and the right margin of a paragraph
is brought which is different from the rest of the page.

<p style="color: teal; margin-right: 40px";>
This paragraph has different style
</p>

Internal Style Sheet inside the <head> element of an HTML page

When a style is intended to appear at several places on a single web page, a CSS style is
used which is defined once for the entire page. Internal style sheets (or embedded style
sheets) are used for this purpose. Internal style sheets are also called embedded style
sheets.

The following example shows how an internal style sheet is inserted.

<head>
<style type="text/css">
<!--body {font-family: verdana, Helvetica, sans-serif; color: blue;}--!>
</style>
</head>

Linking an External Style Sheet

The third and the most powerful method of inserting a style sheet is to link the HTMl
document to an external style sheet. When a style is intended to appear at more than one
page in the website, a CSS style is used which is described in an external CSS file. External
style sheets (or linked style sheets) are used for this purpose. External style sheets are also
called linked style sheets.

Inserting an external style sheet provides complete ease to the designer and also save lot
of time whereby the designer can change the look of several pages, or of the entire
website by making the necessary change in only one file. The external style sheet is linked
to each of the pages with the <link> tag which goes inside the head section.

The following example shows how an external file is linked to a document:

<head>
<link rel="stylesheet" type="text/css" href="abcd.css" />
</head>

The browser will read the style definitions as given in file abcd.css and display the page
accordingly.

An external style sheet can be written in any text editor and saved with .css extension.
There should not be any HTML tags in the file.

Following is an example of a style sheet file:

body {background-color: pink;}
h1 {font-size: 30pt;}
h2 {color: black;}
p {margin-left: 20px;}

The Hierarchy

There are three types of Cascading Style Sheets

 Inline
 Embedded and
 Linked/(or) External
The above 3 styles follow a hierarchy

The “cascading” refers to the hierarchy of control

 Inline style takes preference
 Then Embedded styles rules follow
 The Linked external style sheet instructions will be used if Inline or Embedded
instructions are not present

Whenever any conflict arises as to which style rule should be used first, a style rule with higher
preference will get preference.

The following chart explains this principle. The chart is only a brief reference to resolve style
conflicts.

If multiple style rules are in conflict for a given selector, the scale shown in the chart will help
you in determining the order of style rules to be used.

A style rule with higher importance will be preferred than an identical style rule with lower
importance.

CSS: CSS Syntax - Learn CSS for Free and Fun ......

CSS Syntax

A Cascading Style Sheet, whether embedded in the head element of an HTML document or
contained in a separate .css file is comprised of one or a series statements.

A Statement is basically an instruction that does TWO things:
 Identifies the elements in the HTML document which the statement will be affecting
 Instructs the browser how to display the elements

Each statement in the style sheet must conform to the CSS syntax.

The CSS syntax is comprised of THREE parts: a selector, a property and a value.

Selector: The HTML element or tag intended toe defined.
Property: The attribute intended to be changed.
Value: It specifies the property to aid in rendering it to specific media.

A simple CSS syntax is defined as: Selector {property: value;}

selector {property: value;}
The Selector is followed by the first left curly brace ("{"); the braces have the property and value inside them and the property and value are separated by a colon.

The following example shows a selector
body {color: black;}

If the value is defined by multiple words, they are written within quotes.
p {font-family: "courier new";}

If there is more than one property, they must be separated by a semicolon.

The following example shows how a left aligned paragraph with a blue text color must be
defined.
p {text-align:left;color:blue;}

It is a convention to describe one property in one line followed by another property in another
line, which also makes the style definition more  readable.

P{
text-align: left;
color: blue;
font-family: “courier new”;
}

Introduction to CSS

What is CSS?

 CSS stands for Cascading Style Sheets
 CSS is a set of instructions in a linked external file or within the source code of a page
 The CSS tells a browser how to render page elements
 A CSS file is identified by .css file extension.

Therefore, we can simply define Cascading Style Sheets (CSS) as a mechanism for adding style
(e.g. fonts, colors, spacing) to Web Documents.

What are Style Sheets?

In simple words, style sheets define styles such as fonts, color, margins, spacing, etc. that a
web browser must use while rendering a webpage.

 Style Sheets have Styles stored in them
 Styles define how HTML elements should be displayed in the browser
 Styles were added to HTML 4.0 to eliminate complexity
 External Style Sheets are stored in CSS files
 External Style Sheets can save a lot of time and work

Tutorial: Create and run Android Virtual Device :- Create Android App for free and fun ........

Create AVD

To define an Android Virtual Device (ADV) open the AVD Manager dialog via Windows → Android Virtual Device Manager and press the New button.

Enter the values similar to the following screenshot.

Ensure that the Use Host GPU option is selected. This makes the AVD use the graphical processing unit of your computer and this makes rendering on the AVD much faster.

Afterwards press the OK button. This will create the AVD configuration and display it under the list of available virtual devices.

 Run AVD

To test if your setup is correct, select your your new entry and press the Start button

After some time your AVD starts. Do not interrupt this startup process, as this might corrupt the AVD.

After the AVD started, you can use the AVD via the mouse and via the virtual keyboard of the emulator.

Stopping the emulator

During development you don't stop the AVD, you just re-deploy your application.

Android virtual device - Emulator :- Create android app for free ..........

What is the Android Emulator?

The Android Development Tools (ADT) include an emulator to run an Android system. The emulator behaves like a real Android device (in most cases) and allows you to test your application without having a real device.

You can configure the version of the Android system you would like to run, the size of the SD card, the screen resolution and other relevant settings. You can define several of them with different configurations.

These devices are called Android Virtual Device and you can start several in parallel.

Google vs. Android AVD

During the creation of an AVD you decide if you want an Android device or a Google device.

An AVD created for Android will contain the programs from the Android Open Source Project. An AVD created for the Google API's will also contain several Google applications, most notable the Google Maps application.

If you want to use functionality which is only provided via the Google API's, e.g. Google Maps you must run this application on an AVD with Google API's.

Emulator Shortcuts

The following shortcuts are useful for working with the emulator.

Alt+Enter Maximizes the emulator. Nice for demos.

Ctrl+F11 changes the orientation of the emulator.

F8 Turns network on / off.

Parameter

The graphics of the emulator can use the native GPU of the computer. This makes the rendering in the emulator very fast. To enable this, add the GPU Emulation property to the device configuration and set it to true.

You can also set the Enabled flag for Snapshots. This will save the state of the emulator and will let it start much faster. Unfortunately currently native GPU rendering and Snapshots do not work together.

Android devices do not have to have hardware button. If you want to create such an AVD, add the Hardware Back/Home keys property to the device configuration and set it to false.

Android:- Using Resources

Reference to resources in code

The Resources class allows to access individual resources. An instance of Resources can get access via thegetResources() method of the Context class.

The Resources class is also used by other Android classes, for example the following code shows how to create aBitmap file from a reference ID.

BitmapFactory.decodeResource(getResources(), R.drawable.ic_action_search);

Reference to resources in XML files

In your XML files, for example your layout files, you can refer to other resources via the @ sign.

For example, if you want to refer to a color which is defined in a XML resource, you can refer to it via@color/your_id. Or if you defined a "hello" string in an XML resource, you could access it via @string/hello.

Activities and Layouts

The user interface for activities is defined via layouts. The layout defines the included Views (widgets) and their properties.

A layout can be defined via Java code or via XML. In most cases the layout is defined as an XML file.

XML based layouts are defined via a resource file in the /res/layout folder. This file specifies the ViewGroups,Views, their relationship and their attributes for this specific layout.

If a View needs to be accessed via Java code, you have to give the View a unique ID via the android:id attribute. To assign a new ID to a View use . The following shows an example in which a @+id/yourvalue Button gets the button1ID assigned.

<Button

  android:id="@+id/button1"

  android:layout_width="wrap_content"

  android:layout_height="wrap_content"

  android:text="Show Preferences" >

</Button>

By conversion this will create and assign a new yourvalue ID to the corresponding View. In your Java code you can later access a View via the method findViewById(R.id.yourvalue).

Defining layouts via XML is usually the preferred way as this separates the programming logic from the layout definition. It also allows the definition of different layouts for different devices. You can also mix both approaches.