NSObject +load and +initialize - What do they do?
I'm interested in understanding the circumstances leading a developer to override +initialize or +load. Documentation makes it clear these methods are called for you by the Objective-C runtime, but that's really all that is clear from the documentation of those methods. :-)
My curiosity comes from looking at Apple's example code - MVCNetworking. Their model class has a +(void) applicationStartup method. It does some housekeeping on the filesystem, reads NSDefaults, etc etc... and, after trying to grok NSObject's class methods, it seems like this janitorial work might be okay to put into +load.
I did modify the MVCNetworking project, removing the call in App Delegate to +applicationStartup, and putting the housekeeping bits into +load... my computer didn't catch fire, but that doesn't mean it's correct! I'm hoping to gain an understanding of any subtleties, gotchas, and whatnots around a custom setup method you have to call versus +load or +initialize.
For +load documentation says:
The load message is sent to classes and categories that are both dynamically loaded and statically linked, but only if the newly loaded class or category implements a method that can respond.
This sentence is kludgey and difficult to parse if you don't know the precise meaning of all the words. Help!
What is meant by "both dynamically loaded and statically linked?" Can something be dynamically loaded AND statically linked, or are they mutually exclusive?
"...the newly loaded class or category implements a method that can respond" What method? Respond how?
As for +initialize, documentation says:
initialize it is invoked only once per class. If you want to perform independent initialization for the class and for categories of the class, you should implement load methods.
I take this to mean, "if your trying to setup the class... don't use initialize." Okay, fine. When or why would I override initialize then?
Answer
The load message
The runtime sends the load message to each class object, very soon after the class object is loaded in the process's address space. For classes that are part of the program's executable file, the runtime sends the load message very early in the process's lifetime. For classes that are in a shared (dynamically-loaded) library, the runtime sends the load message just after the shared library is loaded into the process's address space.
Furthermore, the runtime only sends load to a class object if that class object itself implements the load method. Example:
@interface Superclass : NSObject
@end
@interface Subclass : Superclass
@end
@implementation Superclass
+ (void)load {
NSLog(@"in Superclass load");
}
@end
@implementation Subclass
// ... load not implemented in this class
@end
The runtime sends the load message to the Superclass class object. It does not send the load message to the Subclass class object, even though Subclass inherits the method from Superclass.
The runtime sends the load message to a class object after it has sent the load message to all of the class's superclass objects (if those superclass objects implement load) and all of the class objects in shared libraries you link to. But you don't know which other classes in your own executable have received load yet.
Every class that your process loads into its address space will receive a load message, if it implements the load method, regardless of whether your process makes any other use of the class.
You can see how the runtime looks up the load method as a special case in the _class_getLoadMethod of objc-runtime-new.mm, and calls it directly from call_class_loads in objc-loadmethod.mm.
The runtime also runs the load method of every category it loads, even if several categories on the same class implement load. This is unusual. Normally, if two categories define the same method on the same class, one of the methods will “win” and be used, and the other method will never be called.
The initialize Method
The runtime calls the initialize method on a class object just before sending the first message (other than load or initialize) to the class object or any instances of the class. This message is sent using the normal mechanism, so if your class doesn't implement initialize, but inherits from a class that does, then your class will use its superclass's initialize. The runtime will send the initialize to all of a class's superclasses first (if the superclasses haven't already been sent initialize).
Example:
@interface Superclass : NSObject
@end
@interface Subclass : Superclass
@end
@implementation Superclass
+ (void)initialize {
NSLog(@"in Superclass initialize; self = %@", self);
}
@end
@implementation Subclass
// ... initialize not implemented in this class
@end
int main(int argc, char *argv[]) {
@autoreleasepool {
Subclass *object = [[Subclass alloc] init];
}
return 0;
}
This program prints two lines of output:
2012-11-10 16:18:38.984 testApp[7498:c07] in Superclass initialize; self = Superclass
2012-11-10 16:18:38.987 testApp[7498:c07] in Superclass initialize; self = Subclass
Since the system sends the initialize method lazily, a class won't receive the message unless your program actually sends messages to the class (or a subclass, or instances of the class or subclasses). And by the time you receive initialize, every class in your process should have already received load (if appropriate).
The canonical way to implement initialize is this:
@implementation Someclass
+ (void)initialize {
if (self == [Someclass class]) {
// do whatever
}
}
The point of this pattern is to avoid Someclass re-initializing itself when it has a subclass that doesn't implement initialize.
The runtime sends the initialize message in the _class_initialize function in objc-initialize.mm. You can see that it uses objc_msgSend to send it, which is the normal message-sending function.
Further reading
Check out Mike Ash's Friday Q&A on this topic.