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A protocol states a programming interface that a class decides to implement or not. Protocols establishes communication between the classes through inheritance. There are two types of protocols defined in Objective-C - 

• Formal Protocol - A formal protocol declares a list of several methods that a client class is expected to implement. Formal protocols possess their own type-checking syntax, declaration, and adoption. The methods under @required keyword must be implemented in the classes which sends confirmation to the formal protocol. The methods under @optional keyword are optional to implement.
• Informal Protocol - Informal protocols are categories of NSObject, which creates all object adopters of the protocol implicitly. Before invoking a method, the calling object verifies whether the target object is going to implement it or not. Implementation of the informal protocol's methods are optional.

The import keyword is a superset of include and ensures that a file is included only once. #include includes all the functions, and algorithms related to that particular library.

Categories provides the feature of adding a specific functionality to a class making its sub-class. The added methods are derived by the sub-classes and cannot be distinguished from the original methods of the parent class. The developers can also use categories to distribute the classes implementation into separate source files. 

NSArray is the super-class of NSMutableArray which handles ordered collection of objects known as arrays. NSArray is responsible for creating static arrays while NSMutableArray creates dynamic arrays.

Fast Enumeration is the mechanism that helps in enumerating through a collection. There are various types of collections which are responsible for efficiently storing and retrieving objects.

Grand Central Dispatch (GCD) is a robust technique to perform tasks efficiently in Objective-C. GCDs are the dispatch queues which lets freelance Objective-C developers execute arbitrary code blocks either synchronously or asynchronously. GCDs can perform all the tasks that you perform using several threads. 

The advantages of GCDs over NSThread are - 

• GCD lessens the memory consumption that an application requires for storing thread stacks in the app's memory.
• GCD eliminates the need of code required for thread creation and configuration.
• GCD eliminates the requirement of managing and scheduling work on threads.
• Using GCD-type locking is about 80% faster as compared to @synchronized in NSThread. 

Blocks are the additional features provided by Objective-C that allows a freelance Objective-C developer to create distinct code segments which can be passed as values to methods or functions. Blocks in Objective-C have the ability to take values from enclosing scope and make them equivalent to lambda expressions in Java/Python or Closures in Haskell.  Blocks can be added just like NSDictionary and NSArray collections. The syntax to define a block is :

^{

NSLog("@This is an example of block.");

}

The responder chain is a series of events that takes palce once we start interacting with the iOS phones. A responder chain starts with the first responder object and ends with the app object. If the first responder cannot handle the event, it forwards / resends that event to another responder in the chain.

If you are trying to hire a Objective-C Developer. Here is the Objective-C Developer Job Description that you can use in your hiring.