How to understand NSObject, it is both protocol and interface.

Question

You can see following in NSObject.h file

// 1. here is a protocol

@protocol NSObject

// 2. here is an interface, conforming to the above protocol

@interface NSObject <NSObject> {
    ...

// 3. what is the meaning of "( )" below? NSCoderMethods is a protocol

@interface NSObject (NSCoderMethods)

NSObject is so important that I need to understand, why is it designed like this?

Answer 1

1.

It is a class (interface) because all concrete classes in ObjC nowadays inherit implementations from NSObject, to support features such as reference-counting and run-time type checking.

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2.

Now for the protocol... it exists because NSObject is not the only root class that supports the -retain and -performSelector: etc methods.

One important class is NSProxy, which acts as a proxy to forward messages to some actual objects.

The targets of NSProxy are usually NSObjects, so the interface should also support -retain and -performSelector: etc methods. But NSProxy cannot inherit from NSObject because there is no is-a relationship between the two.

To express the sibling relationship, both are made to adopt the same protocol, which, unfortunately, also called NSObject.

Making NSObject a protocol also has an advantage that, user-defined protocols can request adopters to support all usual NSObject operations by

@protocol Foo <NSObject>
...

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3.

It's a category, which adds extra method implementations to NSObject.