前言

• 发布此文章主要是对自己所学知识的总结
• 通过文章的方式可以让自己对所学知识加深印象
• 方便日后需要的时候查看,如果有不对的地方欢迎指出
• 文笔不行,多多见谅

一直都是用印象笔记来记录自己的学习过程,但是笔记做完很久不看的话,就会忘记很多细节,所以才会选择再写一遍,加深一下印象,书读百遍其义自见.这段时间没有新需求,利用空闲时间巩固一下知识,免得空度时光

上一篇里针对Category的源码进行了解读,但是没有说到load和initialize这两个方法在分类中是怎么调用的,很早写的一篇iOS load和initialize认知里面提到了调用顺序,但是没有从底层原理进行分析,今天从源码中看看具体是怎么回事,理解了原理,对方法调用顺序也就理解起来更加容易.虽然网上已经有很多这样的文章,但人家的终归是人家的,只看不写印象还是不会深刻

一 : 源码解读load方法

load方法是在runtime加载类、分类时调用,每个类、分类在程序运行过程中只会调用一次,除非是手动调用,具体调用顺序通过源码一一解释

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();
}

prepare_load_methods((const headerType *)mh); : 准备load方法

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++;
}

上面代码主要做两件事:

• 处理类的load方法,递归获取父类的,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loadable_classes数组中
• 处理分类的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loadable_categories数组中

所有的类和分类都添加到了集合中,继续往下看,到了call_load_methods();
奉上代码:

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;
}

call_load_methods();主要做了两件事:

• 调用类的load方法,优先调用父类的load方法,因为父类是优先添加到集合中的
• 调用分类的load方法
  类的load方法通过(*load_method)(cls, SEL_load);直接是指针调用
  分类的load方法也是通过指针直接调用的,而不是通过objc_msgSend进行调用的
  但是有一点,如果是手动调用的话,和上面说的就不一样了,手动调用则是通过objc_msgSend进行调用的

Person *person = [[Person alloc]init];
[Person load];

编译成c++代码

        Person *person = ((Person *(*)(id, SEL))(void *)objc_msgSend)((id)((Person *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Person"), sel_registerName("alloc")), sel_registerName("init"));
        ((void (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Person"), sel_registerName("load"));

小节: 在调用load方法之前会把所有的类和分类添加到相应的数组中准备好(添加类的时候会通过递归的方法邮箱把父类添加到数组中),然后再通过遍历直接通过函数指针进行调用

二 : initialize

上面我们说了load方法是通过函数指针直接进行调用的,那initialize是怎么调用的呢?
initialize会在类第一次接收到消息的时候调用

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("");
}

一:首先会判断该类有没有进行过初始化,没有的话就进行初始化
二:在_class_initialize()函数里,判断父类有没有进行过初始化,如果没有就递归调用此方法,先初始化父类
三:调用callInitialize (),其实就是在发送消息调用objc_msgSend;

小结:从上面的代码可以看出,initialize其实就是消息发送,最终调用的是objc_msgSend,所以它就有以下的特点:

• 子类没有实现initialize方法会调用父类的initialize

• 分类的initialize会覆盖本类的方法

• 父类的initialize可能会调用多次哦

总结: load和initialize是有本质区别的,load是根据函数指针直接调用,而initialize走的是消息发送机制

本次分享到此为止,天天学习,好好向上!!