+(void) initialize in objective-c class static variables constructor
This +initialize method is described in The Objective-C Programming Language.
The runtime system sends an
initializemessage to every class object before the class receives any other messages and after its superclass has received theinitializemessage. This gives the class a chance to set up its runtime environment before it’s used. If no initialization is required, you don’t need to write aninitializemethod to respond to the message.
For example, when [DecimalPointButton alloc] is called, the runtime will check if [DecimalPointButton initialize] has been called. If not, it will +initialize the class. This ensure the backgroundImageDepressed image is ready before any instances of DecimalPointButton are constructed.