Overriding methods using categories in Objective-C

Question

Can I use a class category to override a method that is already implemented using a category? Like this:

1) Original method

-(BOOL) method {
  return true;
}

2) Overrided method

-(BOOL) method {
  NSLog(@"error?"); 
  return true; 
}

Will this work, or is this illegal?

Answer 1

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.