在iOS中，所有的类都继承自NSObject，在这个类中，有两个类方法，分别是 +load 与 +initialize，他们承担着不同的任务，今天详细的解读一下它们各自的意义以及调用原理。

+load

load方法会先调用所有父类+load，然后调用子类+load，最后调用分类 +load，分类的调用顺序由 Compile Source决定。

从 runtime的源码来解开+load方法调用的原理。

首先**void prepare_load_methods(header_info *hi) **函数：

void prepare_load_methods(header_info *hi)
{
    size_t count, i;

    rwlock_assert_writing(&runtimeLock);

    classref_t *classlist =
        _getObjc2NonlazyClassList(hi, &count);
    for (i = 0; i < count; i++) {
        schedule_class_load(remapClass(classlist[i]));
    }

    category_t **categorylist = _getObjc2NonlazyCategoryList(hi, &count);
    for (i = 0; i < count; i++) {
        category_t *cat = categorylist[i];
        Class cls = remapClass(cat->cls);
        if (!cls) continue;  // category for ignored weak-linked class
        realizeClass(cls);
        assert(cls->ISA()->isRealized());
        add_category_to_loadable_list(cat);
    }
}

函数主要作用是准备好被调用的类和分类，具体实现方法在 schedule_class_load(remapClass(classlist[i]));函数中。

static void schedule_class_load(Class cls)
{
    if (!cls) return;
    assert(cls->isRealized());  // _read_images should realize

    if (cls->data()->flags & RW_LOADED) return;

    // Ensure superclass-first ordering
    schedule_class_load(cls->superclass);

    add_class_to_loadable_list(cls);
    cls->setInfo(RW_LOADED);
}

在 schedule_class_load(cls->superclass);中可以看出，每次都传入父类作为参数递归的调用，决定了方法的调用顺序，父类有限，待方法调用完成，会在全局变量 loadable_classes 和 loadable_categories 中。

当类准备好，接下来调用+load方法，void call_load_methods(void) 函数

void call_load_methods(void)
{
    static BOOL loading = NO;
    BOOL more_categories;

    recursive_mutex_assert_locked(&loadMethodLock);

    // Re-entrant calls do nothing; the outermost call will finish the job.
    if (loading) return;
    loading = YES;

    void *pool = objc_autoreleasePoolPush();

    do {
        // 1. Repeatedly call class +loads until there aren't any more
        while (loadable_classes_used > 0) {
            call_class_loads();
        }

        // 2. Call category +loads ONCE
        more_categories = call_category_loads();

        // 3. Run more +loads if there are classes OR more untried categories
    } while (loadable_classes_used > 0  ||  more_categories);

    objc_autoreleasePoolPop(pool);

    loading = NO;
}

其中，主类的调用顺序优于分类的调用，具体调用方法在 call_class_loads(); 和 call_category_loads();中。

static void call_class_loads(void)
{
    int i;

    // Detach current loadable list.
    struct loadable_class *classes = loadable_classes;
    int used = loadable_classes_used;
    loadable_classes = nil;
    loadable_classes_allocated = 0;
    loadable_classes_used = 0;

    // Call all +loads for the detached list.
    for (i = 0; i < used; i++) {
        Class cls = classes[i].cls;
        load_method_t load_method = (load_method_t)classes[i].method;
        if (!cls) continue;

        if (PrintLoading) {
            _objc_inform("LOAD: +[%s load]\n", cls->nameForLogging());
        }
        (*load_method)(cls, SEL_load);
    }

    // Destroy the detached list.
    if (classes) _free_internal(classes);
}

在这个方法中可以看到，使用方法首地址进行直接调用，并非使用objc_msgSend，通过这个方式，我们可以在分类中做一些事情，例如 Method Swizzling

+initialize

+initialize 方法会在类收到第一个消息时调用，是一个懒加载的方式，如果一直没收到消息，则永不调用。这种设计节省了资源。

在 objc-runtime-new.mm中我们可以看到。

IMP lookUpImpOrForward(Class cls, SEL sel, id inst,
                       bool initialize, bool cache, bool resolver)
{
    ...
        rwlock_unlock_write(&runtimeLock);
    }

    if (initialize  &&  !cls->isInitialized()) {
        _class_initialize (_class_getNonMetaClass(cls, inst));
        // If sel == initialize, _class_initialize will send +initialize and 
        // then the messenger will send +initialize again after this 
        // procedure finishes. Of course, if this is not being called 
        // from the messenger then it won't happen. 2778172
    }

    // The lock is held to make method-lookup + cache-fill atomic 
    // with respect to method addition. Otherwise, a category could 
    ...
}

当向类发送消息时，如果类没有初始化，则会调用初始化方法 void _class_initialize(Class cls)

void _class_initialize(Class cls)
{
    ...
    Class supercls;
    BOOL reallyInitialize = NO;

    // Make sure super is done initializing BEFORE beginning to initialize cls.
    // See note about deadlock above.
    supercls = cls->superclass;
    if (supercls  &&  !supercls->isInitialized()) {
        _class_initialize(supercls);
    }

    // Try to atomically set CLS_INITIALIZING.
    monitor_enter(&classInitLock);
    if (!cls->isInitialized() && !cls->isInitializing()) {
        cls->setInitializing();
        reallyInitialize = YES;
    }
    monitor_exit(&classInitLock);

    if (reallyInitialize) {
        // We successfully set the CLS_INITIALIZING bit. Initialize the class.

        // Record that we're initializing this class so we can message it.
        _setThisThreadIsInitializingClass(cls);

        // Send the +initialize message.
        // Note that +initialize is sent to the superclass (again) if 
        // this class doesn't implement +initialize. 2157218
        if (PrintInitializing) {
            _objc_inform("INITIALIZE: calling +[%s initialize]",
                         cls->nameForLogging());
        }

        ((void(*)(Class, SEL))objc_msgSend)(cls, SEL_initialize);

        if (PrintInitializing) {
            _objc_inform("INITIALIZE: finished +[%s initialize]",
    ...
}

这里我们可以看出，其实+initialize本质为objc_msgSend，走的是方法调用的流程，如子类没有实现+initialize，则会调用父类+initialize方法，如分类实现了+initialize方法，则会覆盖主类方法（这里的覆盖不是真正的覆盖，主类的方法还存在方法列表中，只是runtime命中对应方法后不会继续向下搜索，则直接调用该方法）。

我们可以避免父类+initialize方法多次执行

+ (void)initialize {
  if (self == [ClassName self]) {
    // ... do the initialization ...
  }
}

总结:

参考链接：
http://blog.leichunfeng.com/blog/2015/05/02/objective-c-plus-load-vs-plus-initialize/
https://www.mikeash.com/pyblog/friday-qa-2009-05-22-objective-c-class-loading-and-initialization.html
http://blog.iderzheng.com/objective-c-load-vs-initialize/
http://nshipster.com/method-swizzling/