学步园

1. +load的调用发生在程序的很早期:

1.1 C++ static initializers in your app (or framework or plugin) won't have run yet;

1.2
 frameworks you link to are guaranteed to be fully loaded by this point, so it's safe to use framework classes;

1.3 Your superclasses are also guaranteed to be fully loaded,
so they are safe to use as well;

1.4  there's
no autorelease pool present at loading time (usually) so you'll need to wrap your code in one if you're calling into Objective-C stuff;

1.5
如果在main class和该class的catergory中同时实现了+load，那么这两个+load方法都会被调用。

2.
+initialize的调用发生在第一个消息发送给它所在的class时：

2.1+initialize happens
once per class, and it happens the first time a message is sent to that class, its mechanism acts as the pseudocode below:

id objc_msgSend(id self, SEL _cmd, ...)
    {
        if(!self->class->initialized)
            [self->class initialize];
        ...send the message...
    }

2.2 (?)Because +initialize runs lazily, it's obviously not a good place
to put code to register a class that otherwise wouldn't get used.For example, NSValueTransformer or NSURLProtocol subclasses can't use +initialize to
register themselves with their superclasses, because you set up a chicken-and-egg situation.

2.3 There's
one more trick to +initialize. In my pseudocode above I wrote that the runtime does [self->class
initialize]. This implies that normal Objective-C dispatch semantics apply, and that if the class doesn't implement it, the superclass's +initialize will
run instead. That's exactly what happens. Because of this, you should always write your +initialize method to look like this:

   + (void)initialize
    {
        if(self == [WhateverClass class])
        {
            ...perform initialization...
        }
    }

Without that extra check, your initializations could run twice if you ever have a subclass that doesn't implement its own +initialize method.
This is not just a theoretical concern, even if you don't write any subclasses. Apple's Key-Value Observing creates dynamic
subclasses which don't override +initialize.

3.
以上参考自博客http://www.mikeash.com/pyblog/friday-qa-2009-05-22-objective-c-class-loading-and-initialization.html。实际测试结果：

3.1
+load的调用会引起+initialize的调用（当+initialize被实现了的时候）

3.2
某个实现了+initialize但没有实现+load的class，如果没有信息发送给它，则不会引起+initialize的调用；换句话说，如果该class实现了+load，则即使没有任何信息发送给它，+initialize也会在很早就被调用，紧接着会调用+load

3.3
subclass没有实现+initialize时，确实会引起superclass的+initialize两次调用。第一次调用是因为在调用子类的+initialize前，需要先调用父类的+initialize；第二次调用是发生在objc_msgSend函数打算调用子类的+initialize时，发现子类没有实现该方法，于是追溯到父类中，调用父类的+initialize