The +initialize
method is called automatically the first time a class is used, before any class methods are used or instances are created. You should never call +initialize
yourself.
I also wanted to pass along a tidbit I learned that can bite you down the road: +initialize
is inherited by subclasses, and is also called for each subclasses that doesn't implement an +initialize
of their own. This can be especially problematic if you naively implement singleton initialization in +initialize
. The solution is to check the type of the class variable like so:
+ (void) initialize {
if ([self class] == [MyParentClass class]) {
// Once-only initializion
}
// Initialization for this class and any subclasses
}
All classes that descend from NSObject have both +class
and -class
methods that return the Class
object. Since there is only one Class object for each class, we do want to test equality with the ==
operator. You can use this to filter what should happen only once ever, versus once for each distinct class in a hierarchy (which may not yet exist) below a given class.
On a tangential topic, it's worth learning about the following related methods, if you haven't already:
Edit: Check out this post by @bbum that explains more about +initialize
: http://www.friday.com/bbum/2009/09/06/iniailize-can-be-executed-multiple-times-load-not-so-much/