From Apple documentation:
Although the Objective-C language currently allows you to use a category to override methods the class inherits, or even methods declared in the class interface, you are strongly discouraged from doing so. A category is not a substitute for a subclass. There are several significant shortcomings to using a category to override methods:
When a category overrides an inherited
method, the method in the category
can, as usual, invoke the inherited
implementation via a message to
However, if a category overrides a
method that exists in the category’s
class, there is no way to invoke the
A category cannot reliably override methods declared in another category of the same class.
This issue is of particular significance because many of the Cocoa classes are implemented using categories. A framework-defined method you try to override may itself have been implemented in a category, and so which implementation takes precedence is not defined.
The very presence of some category methods may cause behavior changes across all frameworks. For example, if you override the
windowWillClose:delegate method in a category on NSObject, all window delegates in your program then respond using the category method; the behavior of all your instances of NSWindow may change. Categories you add on a framework class may cause mysterious changes in behavior and lead to crashes.