What is MVC ?

The Model View Controller pattern is one of architectural and design pattern that emerged from the object oriented design pattern.

The idea behind MVC is separation of concern. This means the component of software system is separated (decomposed ) into 3 category:

1) Those that are concerned with managing structure and storage of data (Model)

2) Those that are concerned with providing user interface and interaction (View)

3) Those that govern the flow of control in the application and traffic of information between view and model (Controller)

 

 

What is the difference between Category and extension in objective c?

The main difference between categories and extensions is that the extension expects you to implement the methods inside your main implementation, whereas with a category, it can be in another implementation. It also seems that people are using extensions mainly for private methods.

What’s the difference between formal and informal protocols in Objective-C?

Formal and Informal Protocols

There are two varieties of protocol, formal and informal:

• An informal protocol is a category on NSObject, which implicitly makes almost all objects adopters of the protocol. (A category is a language feature that enables you to add methods to a class without subclassing it.) Implementation of the methods in an informal protocol is optional. Before invoking a method, the calling object checks to see whether the target object implements it. Until optional protocol methods were introduced in Objective-C 2.0, informal protocols were essential to the way Foundation and AppKit classes implemented delegation.

• A formal protocol declares a list of methods that client classes are expected to implement. Formal protocols have their own declaration, adoption, and type-checking syntax. You can designate methods whose implementation is required or optional with the @required and @optional keywords. Subclasses inherit formal protocols adopted by their ancestors. A formal protocol can also adopt other protocols.

Formal protocols are an extension to the Objective-C language.

How to edit the hosts file in Mac OS X – Leopard

Introduction

The hosts file is a text file that maps hostnames to IP addresses.
Upon typing a url address on the browser, the system is checking if there is a relevant entry on the hosts file and gets the corresponding IP address, else it resolves the IP via the active connection’s DNS servers.

The hosts file can be edited to block certain hostnames (like ad-serving/malicious hosts), or used for web development purposes, i.e. to redirect domains to local addresses.

Editing the hosts file

Editing the hosts file in Mac OS X – Leopard, is a pretty easy task, especially if you are familiar with the terminal.

Step 1 – Open the Terminal.app

Either by start typing Terminal on the Spotlight, or by going into Applications -> Utilities -> Terminal.

Step 2 – Open the hosts file

Open the hosts by typing on the Terminal that you have just opened:

1	$ sudo nano /private/etc/hosts

Type your user password when prompted.

Step 3 – Edit the hosts file

The hosts file contains some comments (lines starting with the # symbol), as well as some default hostname mappings (e.g. 127.0.0.1 – localhost).
Simply append your new mappings underneath the default ones. Or edit one of the default values if you know what you are doing!
You can navigate the file using the arrow keys.

Step 4 – Save the hosts file

When done editing the hosts file, press control-o to save the file.
Press enter on the filename prompt, and control-x to exit the editor.

Step 5 – Flush the DNS cache

On Leopard you can issue a simple Terminal command to flush the DNS cache, and have your host file changes to take immediate effect:

1	$ dscacheutil -flushcache

You can now test your new mapping on the browser!