Question

Looking inside the runtime.h, I found the definition of the structure objc_class.

Among various members, We have this :-

struct objc_method_list **methodLists

We definitely need to know what all methods a class has,
But a list of methods should be fine, but why do we have “lists” ?

Why not just one list ?

Also, can anyone specify that, Are methods inherited from superclass part of that list or we get to them via superclass pointer that points to parent class’s structure.

Answer 1

The purpose is explained in objc-class.m, as linked by Georg:

cls->methodLists may be in one of three forms:

1. NULL: The class has no methods.
2. non-NULL, with CLS_NO_METHOD_ARRAY set: cls->methodLists points
   to a single method list, which is the class’s only method list.
3. non-NULL, with CLS_NO_METHOD_ARRAY clear: cls->methodLists points to
   an array of method list pointers. The end of the array’s block
   is set to -1. If the actual number of method lists is smaller
   than that, the rest of the array is NULL.

Attaching categories and adding and removing classes may change
the form of the class list. In addition, individual method lists
may be reallocated when fixed up.

Classes are initially read as #1 or #2. If a category is attached
or other methods added, the class is changed to #3. Once in form #3,
the class is never downgraded to #1 or #2, even if methods are removed.
Classes added with objc_addClass are initially either #1 or #3.

The short answer is therefore “because of categories.” When a category is injected, rather than try to combine its method list with the one existing list, a new entry is simply added to methodLists, and set to the list coming from the category. This probably makes category injection faster, since it avoids (potential) large reallocations and copying.