答:
查看官方文档,描述如下:(额,翻译的不是很好,将就下吧)
Invoked whenever a class or category is added to the Objective-C runtime :implement this method to perform class-specific behavior upon loading.
The load message is sent to classes or catetories that are both dynamically loaded and statically linked ,but only if the newly loaded class or category implements a methor that can respond.