之前在看项目组件化方案的时候，看到蘑菇街的组件化方案中有在load方法中注册vc的方法。

所以想要完整了解一下load方法的具体细节，另外发现initialize方法基本上和load是成对出现的，所以顺便就一起看一下。

initialize

官方文档

Initializes the class before it receives its first message.
（在类收到第一条消息之前初始化它。）

官方文档中对initialize方法的描述就一种懒加载的形式，在类第一次收到消息的之前初始化。

源码

IMP _class_lookupMethodAndLoadCache3(id obj, SEL sel, Class cls)
{
    return lookUpImpOrForward(cls, sel, obj, 
                              YES/*initialize*/, NO/*cache*/, YES/*resolver*/);
}
IMP lookUpImpOrForward(Class cls, SEL sel, id inst, 
                       bool initialize, bool cache, bool resolver)
{
//如果initialize==Yes,说明需要初始化,并且该类没有进行过初始化,然后调用_class_initialize进行初始化
    if (initialize  &&  !cls->isInitialized()) {
        runtimeLock.unlockRead();
        _class_initialize (_class_getNonMetaClass(cls, inst));
        runtimeLock.read();
    }
}
void _class_initialize(Class cls)
{
    assert(!cls->isMetaClass());

    Class supercls;
    bool reallyInitialize = NO;

    // Make sure super is done initializing BEFORE beginning to initialize cls.
    // See note about deadlock above.
//在初始化之前先确保父类进行了初始化操作,如果没有就先初始化父类
    supercls = cls->superclass;
    if (supercls  &&  !supercls->isInitialized()) {
        _class_initialize(supercls);
    }
    
    // Try to atomically set CLS_INITIALIZING.
    {
        monitor_locker_t lock(classInitLock);
        if (!cls->isInitialized() && !cls->isInitializing()) {
            cls->setInitializing();
            reallyInitialize = YES;
        }
    }
    
    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);

        if (MultithreadedForkChild) {
            // LOL JK we don't really call +initialize methods after fork().
            performForkChildInitialize(cls, supercls);
            return;
        }

        if (PrintInitializing) {
            _objc_inform("INITIALIZE: thread %p: calling +[%s initialize]",
                         pthread_self(), cls->nameForLogging());
        }
//调用初始化发送,
        callInitialize(cls);

        if (PrintInitializing) {
            _objc_inform("INITIALIZE: thread %p: finished +[%s initialize]",
                             pthread_self(), cls->nameForLogging());
        }
        return;
    }
    
    else if (cls->isInitializing()) {
        if (_thisThreadIsInitializingClass(cls)) {
            return;
        } else if (!MultithreadedForkChild) {
            waitForInitializeToComplete(cls);
            return;
        } else {
            _setThisThreadIsInitializingClass(cls);
            performForkChildInitialize(cls, supercls);
        }
    }
    else if (cls->isInitialized()) {
        return;
    }
    
    else {
        // We shouldn't be here. 
        _objc_fatal("thread-safe class init in objc runtime is buggy!");
    }
}
void callInitialize(Class cls)
{
    ((void(*)(Class, SEL))objc_msgSend)(cls, SEL_initialize);
    asm("");
}

可以看到，调用时会先去查看父类是否存在，并且父类是否有初始化过，如果满足条件之后，就会递归去调用

    if (supercls  &&  !supercls->isInitialized()) {
        _class_initialize(supercls);
    }

最后可以看到调用了一个方法callInitialize(cls)
仔细看这个方法，其实内部就是一个objc_msgSend。

void callInitialize(Class cls)
{
    ((void(*)(Class, SEL))objc_msgSend)(cls, SEL_initialize);
    asm("");
}

方法实现

接下来我们通过一些代码来看一下initialize的执行顺序。

首先我们创建一个类ZZRInitizial，然后重写一下他的initialize方法。

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface ZZRInitizial : NSObject

@end

NS_ASSUME_NONNULL_END

--------------------------------------------------------------

#import "ZZRInitizial.h"

@implementation ZZRInitizial

+ (void)initialize
{
    NSLog(@"%s",__FUNCTION__);
}

@end

然后运行一下，发现，什么都没有打印，不过因为之前看过文档和源码，initialize是使用懒加载调用的，所以当我们没有用到它的时候没有打印也很正常。接下来我们试一下初始化一个实例。

    [ZZRInitizial new];

可以看到现在就打印出来了

2019-05-05 13:58:26.566212+0800 initializeDemo[86317:3926410] +[ZZRInitizial initialize]

接下来我们定义一个继承了ZZRInitizial的子类ZZRInitizialSubClass，然后再调用子类的初始化实例。

    [ZZRInitizialSubClass new];

然后可以看到打印出的方法

2019-05-05 14:02:42.331628+0800 initializeDemo[86412:3934965] +[ZZRInitizial initialize]
2019-05-05 14:02:42.331745+0800 initializeDemo[86412:3934965] +[ZZRInitizialSubClass initialize]

接下来我们再创建一个ZZRInitizial的Category，然后初始化一个实例

2019-05-05 14:22:32.726589+0800 initializeDemo[86608:3962561] +[ZZRInitizial(myCategory) initialize]

我们会发现调用的是Category中的方法，而没有调用原本类中的方法了。

总结

所以我们可以看到，initialize在类或者其子类的第一个方法被调用之前调用，并且使用懒加载的调用方式，即没有使用就不会调用。
并且因为是有系统调用，所以不需要再调用[super initialize]。
调用顺序上，会先调用父类的initialize，然后再调用原有类的initialize，如果有Category，并且Category中重写了initialize方法，则会调用Category中的initialize。

load

官方文档

Invoked whenever a class or category is added to the Objective-C runtime; implement this method to perform class-specific behavior upon loading.
（当类或者类别加入runtime时，实现该方法，可以在类加载的时候做一些类特有的操作）

以上是官方文档中对load方法的描述，也就是说当类被加入runtime，也就是被引用的时候，就会实现该方法。
另外，在每一个类、分类在程序运行的过程中除了手动调用之外，只会调用一次。

源码

load_images(const char *path __unused, const struct mach_header *mh)
{
    // Return without taking locks if there are no +load methods here.
    if (!hasLoadMethods((const headerType *)mh)) return;

    recursive_mutex_locker_t lock(loadMethodLock);

    // Discover load methods
    {
        rwlock_writer_t lock2(runtimeLock);
        prepare_load_methods((const headerType *)mh);
    }

    // Call +load methods (without runtimeLock - re-entrant)
    call_load_methods();
}

void prepare_load_methods(const headerType *mhdr)
{
    size_t count, i;

    runtimeLock.assertWriting();

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

    category_t **categorylist = _getObjc2NonlazyCategoryList(mhdr, &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);
    }
}
//递归查找父类,父类优先添加到集合中
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); 
}
//把类和类的load方法添加到loadable_classes数组中,数组中每一个元素都是一个结构体,结构体中包含类和load方法的IMP
static struct loadable_class *loadable_classes = nil;
struct loadable_class {
    Class cls;  // may be nil
    IMP method;
};
void add_class_to_loadable_list(Class cls)
{
    IMP method;

    loadMethodLock.assertLocked();

    method = cls->getLoadMethod();
    if (!method) return;  // Don't bother if cls has no +load method
    
    if (PrintLoading) {
        _objc_inform("LOAD: class '%s' scheduled for +load", 
                     cls->nameForLogging());
    }
    
    if (loadable_classes_used == loadable_classes_allocated) {
        loadable_classes_allocated = loadable_classes_allocated*2 + 16;
        loadable_classes = (struct loadable_class *)
            realloc(loadable_classes,
                              loadable_classes_allocated *
                              sizeof(struct loadable_class));
    }
    
    loadable_classes[loadable_classes_used].cls = cls;
    loadable_classes[loadable_classes_used].method = method;
    loadable_classes_used++;
}
static struct loadable_category *loadable_categories = nil;
struct loadable_category {
    Category cat;  // may be nil
    IMP method;
};
//处理分类
void add_category_to_loadable_list(Category cat)
{
    IMP method;

    loadMethodLock.assertLocked();

    method = _category_getLoadMethod(cat);

    // Don't bother if cat has no +load method
    if (!method) return;

    if (PrintLoading) {
        _objc_inform("LOAD: category '%s(%s)' scheduled for +load", 
                     _category_getClassName(cat), _category_getName(cat));
    }
    
    if (loadable_categories_used == loadable_categories_allocated) {
        loadable_categories_allocated = loadable_categories_allocated*2 + 16;
        loadable_categories = (struct loadable_category *)
            realloc(loadable_categories,
                              loadable_categories_allocated *
                              sizeof(struct loadable_category));
    }

    loadable_categories[loadable_categories_used].cat = cat;
    loadable_categories[loadable_categories_used].method = method;
    loadable_categories_used++;
}

从上边这段代码的函数名就可以看出来，主要是准备load方法。
这段代码处理了类的load方法，递归并获取父类的，把类和类的load方法的IMP存储到struct loadable_class结构体中，并把结构体添加到了loadable_classes数组中。
另外处理分类load方法，把分类和分类的load方法的IMP存储到loadable_category结构体中，并把结构体添加到了loadable_categories数组中。

然后接下来是重头戏call_load_methods()

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

    loadMethodLock.assertLocked();

    // 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;
}
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(classes);
}
static bool call_category_loads(void)
{
    int i, shift;
    bool new_categories_added = NO;
    
    // Detach current loadable list.
    struct loadable_category *cats = loadable_categories;
    int used = loadable_categories_used;
    int allocated = loadable_categories_allocated;
    loadable_categories = nil;
    loadable_categories_allocated = 0;
    loadable_categories_used = 0;

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

        cls = _category_getClass(cat);
        if (cls  &&  cls->isLoadable()) {
            if (PrintLoading) {
                _objc_inform("LOAD: +[%s(%s) load]\n", 
                             cls->nameForLogging(), 
                             _category_getName(cat));
            }
            (*load_method)(cls, SEL_load);
            cats[i].cat = nil;
        }
    }

    // Compact detached list (order-preserving)
    shift = 0;
    for (i = 0; i < used; i++) {
        if (cats[i].cat) {
            cats[i-shift] = cats[i];
        } else {
            shift++;
        }
    }
    used -= shift;

    // Copy any new +load candidates from the new list to the detached list.
    new_categories_added = (loadable_categories_used > 0);
    for (i = 0; i < loadable_categories_used; i++) {
        if (used == allocated) {
            allocated = allocated*2 + 16;
            cats = (struct loadable_category *)
                realloc(cats, allocated *
                                  sizeof(struct loadable_category));
        }
        cats[used++] = loadable_categories[i];
    }

    // Destroy the new list.
    if (loadable_categories) free(loadable_categories);

    // Reattach the (now augmented) detached list. 
    // But if there's nothing left to load, destroy the list.
    if (used) {
        loadable_categories = cats;
        loadable_categories_used = used;
        loadable_categories_allocated = allocated;
    } else {
        if (cats) free(cats);
        loadable_categories = nil;
        loadable_categories_used = 0;
        loadable_categories_allocated = 0;
    }

    if (PrintLoading) {
        if (loadable_categories_used != 0) {
            _objc_inform("LOAD: %d categories still waiting for +load\n",
                         loadable_categories_used);
        }
    }

    return new_categories_added;
}

这段代码主要调用了类的load方法，优先调用父类的load方法，然后调用分类的load方法。
调用父类的load方法主要是通过(*load_method)(cls, SEL_load);方法直接调用指针。
但是手动调用和自动调用的区别就是手动调用的时候是通过objc_msgSend而不是指针。

方法实现

同样的，我们新建一个类ZZRLoad

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface ZZRLoad : NSObject

@end

NS_ASSUME_NONNULL_END

----------------------------------------

#import "ZZRLoad.h"

@implementation ZZRLoad

+ (void)load
{
    NSLog(@"%s",__FUNCTION__);
}

@end

然后运行代码，我们就会发现，ZZRLoad的load方法就已经执行了，而且是在最前边执行的。

2019-05-05 15:34:56.613609+0800 initializeDemo[87221:4058674] +[ZZRLoad load]

接下来基于ZZRLoad创建一个子类ZZRLoadSubClass，然后运行会发现：

2019-05-05 15:38:25.778688+0800 initializeDemo[87271:4065322] +[ZZRLoad load]
2019-05-05 15:38:25.779203+0800 initializeDemo[87271:4065322] +[ZZRLoadSubClass load]

两个方法都执行了load。

然后基于ZZRLoad创建一个分类，运行后会发现：

2019-05-05 15:39:26.135793+0800 initializeDemo[87297:4067390] +[ZZRLoad load]
2019-05-05 15:39:26.136449+0800 initializeDemo[87297:4067390] +[ZZRLoadSubClass load]
2019-05-05 15:39:26.136542+0800 initializeDemo[87297:4067390] +[ZZRLoad(myCategory) load]

三个也都执行了。

总结

Load方法会在runtime加载类、分类的时候调用，每个类、分类的load方法在程序运行的过程中只会调用一次（你手动调用的不算数）。由于load函数是系统自动加载的，因此不需要调用父类的load函数，否则父类的load函数会多次执行。

区别

系统中为了保障线程安全，在load方法内部使用了锁，所以我们在使用的时候尽量需要在load方法中添加太多逻辑，防止线程阻塞。
对弈initialize方法中主要用来对一些不方便在编译期初始化的对象进行赋值。

参考资料

load - NSObject | Apple Developer Documentation
initialize - NSObject | Apple Developer Documentation
iOS类方法load和initialize详解 - 掘金
【OC底层】Category、+load方法、+initialize方法原理 - 这酸爽！ - 博客园
iOS - + initialize 与 +load - 简书
通过源码查看load和initialize - 简书