Overriding methods using categories in Objective-C

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 super.
    However, if a category overrides a
    method that exists in the category’s
    class, there is no way to invoke the
    original implementation

  • 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.

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