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A category might need to do a little more to install itself into the class system, we’ll show how in the next example. Here we will be extending NSDateFormatter.

A NSDateFormatter has several build-in behaviors that are specified by NSDateFormatterBehavior10_0 or NSDateFormatterBehavior10_4.

We’d like to extend this with a custom behavior MyDateFormatterBehavior and here is how to do it.

Create a subclass of NSDateFormatter

This will implement the desired behavior. How this is implemented is not shown. Lets just assume it’s a class called MyDateFormatter.

@interface MyDateFormatter : NSDateFormatter

Patch class into NSDateFormatter

NSDateFormatter has an extension mechanism in place called +mulleSetClass:forFormatterBehavior:, where we associate a custom class with the behavior.

We would like to do this ASAP, which is when our custom class gets loaded into the runtime. +load will have to be used, since no one will be calling our class directly.

The only thing we need to advertise to the user is our new constant value MyDateFormatterBehavior in a header somewhere for the new behavior:

Now comes the bad news, which is: it can be tricky. The following implementation is correct, but you have to know why.

@implementation MyDateFormatter

+ (void) load
   [NSDateFormatter mulleSetClass:self


As long as you are not using any other Objective-C class except your own class and its superclasses, your code is safe. But as soon you are using another class, even just @"some constant string" then you have to declare its usage beforehand. This can be tricky, when class-clusters come into play, where you may not even know the class (NSConstantString in this case for instance). Even worse, if you are messaging another class, this in turn may message other classes. And that may be hard to predict and maintain.

Fortunately you can declare a dependency on libraries, which in most cases should be Foundation or some equivalent.



before the +load method definition will be sufficient in most cases.

See bla bla for in depth


Now lets see how a category supports Plug And Play.