Method resolution priority between categories and class extensions

In the Objective-C runtime, method resolution is determined by how objc_msgSend traverses the class hierarchy. When a conflict occurs between categories and class extensions (anonymous categories), the priority depends on where the methods are defined relative to the class receiving the message.

Direct Resolution

Does the class extension take precedence over a category method defined in the same class?
Yes. If a category and a class extension define the same selector on the same class, the class extension typically takes precedence. This is because class extensions are compiled into the class's method table at compile time, whereas category methods are prepended to the method list at load time.

How does the runtime prioritize a category method in a superclass versus a class extension in a subclass?
The class extension in the subclass takes priority. The runtime begins its search at the class of the receiver and moves up the inheritance chain. Since the extension is part of the subclass's method table, the runtime finds that implementation before it ever reaches the superclass's category definitions.

Resolution Logic Breakdown

To understand why this happens, consider the order of operations the runtime performs during dispatch:

• Receiver Class Table: The runtime checks the method table of the specific class. This includes methods defined in class extensions.
• Category Lookup: If not found, the runtime checks the methods prepended by categories for that specific class.
• Hierarchy Traverse: If still not found, the runtime moves to the superclass and repeats the process (Table then Category).

Collision Scenarios and Risks

While the hierarchy is predictable, collisions within the same class are a common source of bugs:

Scenario Winner Reason Same class: Extension vs. Category Extension Extensions are part of the compiled class structure; categories are prepended at runtime. Subclass Extension vs. Superclass Category Subclass Extension The search starts at the subclass and finds the method first. Same class: Category A vs. Category B Category B The last category loaded into memory prepends its methods over the previous ones.

If you need to ensure a specific implementation is used regardless of load order, use method_replaceImplementations to manually swap or fix selectors at runtime during initialization.