Question

The necessity for protocols are to abstract the methods of classes which are not hierarchically related.

The similar things also can be done with the help a class (interface) which encompasses all those methods and subclass them ? (This is not really possible due to the Multiple inheritance problem since a class has to be derived already from NSObject.ignore the NSProxy case)

What special things that protocols can do than a class?

Are protocols trying to solve only the multiple inheritance problem?

Answer 1

Protocols main advantage is, that they describe what a object should be able to do, without enforcing subclassing. In languages that dont have multiple inheritance such a mechanism is needed, if you want others programmers be able to use your classes. (see delegation)

For an instance Java has something similar, called interfaces.

This means a huge advantage, as it is very easy to build dynamic systems, as I can allow other developers to enhance my classes via a clearly defined protocol.

A practical example:
I am just designing a REST API and I am providing a Objective-C client library.
As my api requires information about the user, I add a protocol

@protocol VSAPIClientUser <NSObject>
-(NSString *)lastName;
-(NSString *)firstName;
-(NSString *)uuid;
@end

Anywhere I need this user information, I will have an basic id-object, that must conform to this protocol

-(void)addUserWithAttributes:(id<VSAPIClientUser>)user;

You can read this line as: “I don’t care, what kind of object you provide here, as long as it knows about lastName, firstName and uuid“. So I have no idea, how the rest of that object looks like — and I don’t care.

As the library author I can use this safely:

 NSDictionary *userAttributes = @{@"last_name" : [user lastName],
                                  @"first_name": [user firstName],
                                        @"uuid": [user uuid]};

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BTW: I wouldn’t call the absence of multi-inheritance a problem. It is just another design.

“[…] If I revisited that decision today, I might even go so far as to remove single inheritance as well. Inheritance just isn’t all that important. Encapsulation is OOP’s lasting contribution.” — Brad Cox was asked, why Objective-C doesn’t have multiple inheritance. (Masterminds of Programming: Conversations with the Creators of Major Programming Languages, p. 259)