源码

class_getInstanceMethod

Method class_getInstanceMethod(Class cls, SEL sel)
{
    if (!cls  ||  !sel) return nil;
    //查找方法的实现，或者转发
    lookUpImpOrForward(nil, sel, cls, LOOKUP_INITIALIZE | LOOKUP_RESOLVER);
    return _class_getMethod(cls, sel);
}

lookUpImpOrForward

IMP lookUpImpOrForward(id inst, SEL sel, Class cls, int behavior)
{
  //如果当前的behavior是 LOOKUP_INITIALIZE && cls未被initialized
    if ((behavior & LOOKUP_INITIALIZE)  &&  !cls->isInitialized()) {
        //调用 initializeNonMetaClass
        initializeNonMetaClass (_class_getNonMetaClass(cls, inst));
    }
}

initializeNonMetaClass

void initializeNonMetaClass(Class cls)
{
    //确保不是元类对象
    ASSERT(!cls->isMetaClass());   
    // 获取cls的父类
    supercls = cls->getSuperclass();  
    //递归调用，先调用父类的initialize
    if (supercls  &&  !supercls->isInitialized()) {
        initializeNonMetaClass(supercls);
    }
      //调用initialize方法
     callInitialize(cls);
}

callInitialize

void callInitialize(Class cls)
{
    //通过objc_msgSend方法调用，说明是正常的发送消息
    ((void(*)(Class, SEL))objc_msgSend)(cls, @selector(initialize));
    asm("");
}

总结

调用时刻

• 当类第一次接受到消息的时候调用

调用逻辑

• initialize可以通过class_getinstanceMethod获取调用轨迹
• class_getinstanceMethod中会判断cls是否需要initialize和是否已经被initialized
• initializeNonMetaClass中递归调用父类的initialize
• 最后通过callInitialize调用

调用顺序

• 如果分类实现了initialize，就会覆盖类本身的initialize
• 如果子类没有实现initialize，会调用父类的initialize，所以父类的initialize可能会被调用多次