iOS 捋一捋Category加载流程及+load

时间:2021-7-4 作者:qvyue

关于Category 的使用,大家肯定都手到擒来了。
今天就来盘它Category
面试的时候可能被问到

类的load方法中,能调用分类的方法吗?category如何被加载的?+load 方法调用顺序?

基于objc4-779.1源码,debug来看一看Category。
首先第一个问题类的load方法中,能调用分类的方法吗?,答案是肯定的,可以

  • 通过runtime的入口函数可以知道先加载完分类,后调用的load方法
我们通过捋清楚Category的加载来一步步解答。
void _objc_init(void)
{
    static bool initialized = false;
    if (initialized) return;
    initialized = true;
    
    // fixme defer initialization until an objc-using image is found?
    environ_init();
    tls_init();
    static_init();
    runtime_init();
    exception_init();
    cache_init();
    _imp_implementationWithBlock_init();

    _dyld_objc_notify_register(&map_images, load_images, unmap_image);
}

忽略掉一堆 init ,重点来看 _dyld_objc_notify_register(&map_images, load_images, unmap_image);
这个方法会注册3个事件并给出回调。
重点来看一下map_imagesload_images
从这俩个回调方法里看,你会发现Categorymap_images会加载完毕,而load_images会调用+load方法。这就解释了第一个问题:类的load方法中,能调用分类的方法

主要看点:map_images

map_images里来看看Category的加载。map_images最终会调用_read_images,这里就是核心部分了,从中间我们只看处理分类及类的部分

// Discover categories.
    for (EACH_HEADER) {
        bool hasClassProperties = hi->info()->hasCategoryClassProperties();

        auto processCatlist = [&](category_t * const *catlist) {
            for (i = 0; i name);
                Class cls = remapClass(cat->cls);
                locstamped_category_t lc{cat, hi};
                
                if (!cls) {
                    // Category's target class is missing (probably weak-linked).
                    // Ignore the category.
                    if (PrintConnecting) {
                        _objc_inform("CLASS: IGNORING category ???(%s) %p with "
                                     "missing weak-linked target class",
                                     cat->name, cat);
                    }
                    continue;
                }
                
                if (strcmp(cat->name, "HJJJJJ")==0) {
                    
                }
                // Process this category.
                if (cls->isStubClass()) {
                    // Stub classes are never realized. Stub classes
                    // don't know their metaclass until they're
                    // initialized, so we have to add categories with
                    // class methods or properties to the stub itself.
                    // methodizeClass() will find them and add them to
                    // the metaclass as appropriate.
                    if (cat->instanceMethods ||
                        cat->protocols ||
                        cat->instanceProperties ||
                        cat->classMethods ||
                        cat->protocols ||
                        (hasClassProperties && cat->_classProperties))
                    {
                        objc::unattachedCategories.addForClass(lc, cls);
                    }
                } else {
                    // First, register the category with its target class.
                    // Then, rebuild the class's method lists (etc) if
                    // the class is realized.
                    if (cat->instanceMethods ||  cat->protocols
                        ||  cat->instanceProperties)
                    {
                        if (cls->isRealized()) {
                            attachCategories(cls, &lc, 1, ATTACH_EXISTING);
                        } else {
                            objc::unattachedCategories.addForClass(lc, cls);
                        }
                    }
                    
                    if (cat->classMethods  ||  cat->protocols
                        ||  (hasClassProperties && cat->_classProperties))
                    {
                        if (cls->ISA()->isRealized()) {
                            attachCategories(cls->ISA(), &lc, 1, ATTACH_EXISTING | ATTACH_METACLASS);
                        } else {
                            objc::unattachedCategories.addForClass(lc, cls->ISA());
                        }
                    }
                }
            }
        };
        processCatlist(_getObjc2CategoryList(hi, &count));
        processCatlist(_getObjc2CategoryList2(hi, &count));
    }
for (EACH_HEADER) {
        classref_t const *classlist = 
            _getObjc2NonlazyClassList(hi, &count);
        for (i = 0; i isSwiftStable()) {
                if (cls->swiftMetadataInitializer()) {
                    _objc_fatal("Swift class %s with a metadata initializer "
                                "is not allowed to be non-lazy",
                                cls->nameForLogging());
                }
                // fixme also disallow relocatable classes
                // We can't disallow all Swift classes because of
                // classes like Swift.__EmptyArrayStorage
            }
            const char *tempChar;
            tempChar = object_getClassName(cls);
            if (strcmp(tempChar, "Person")==0) {
                
            }
            realizeClassWithoutSwift(cls, nil);
        }
    }

通过断点会发现 分类部分的代码 objc::unattachedCategories.addForClass(lc, cls);,只是将class和其对应的category做了一个映射,插入到map中。真正要看的还是realizeClassWithoutSwift 这个方法。

这个地方又会牵扯到几种情况:分类的懒加载和非懒加载,类的懒加载和非懒加载。

区分懒加载和非懒加载,按苹果的意思就是实现了+load方法的类/分类是非懒加载,否则是懒加载
这就会产生4种情况,我们一种一种的来看。
老规矩,先创建个Person类,再创建个Person+HJJJJJ分类
分类添加2方法

-(void)sayYes;
-(void)sayNo;

1. 非懒加载类,非懒加载分类。

通过realizeClassWithoutSwift一直往后走 ,最终走到attachCategories,附加分类的东西。
在方法内部增加 断点,通过分类名字来断。

    const char *tempChar;
    tempChar = cls->nameForLogging();
    if (strcmp(tempChar, "Person")==0) {
        
    }
    /// 依次读取每一个category,将其methods,property,protocol添加到mlists,proplist,protolist中存储
    for (uint32_t i = 0; i methodsForMeta(isMeta);
        //printf("22-%sn",entry.cat->name);
        if (mlist) {
            
            if (strcmp(entry.cat->name, "HJJJJJ")==0) {
                
            }
            if (mcount == ATTACH_BUFSIZ) {
                prepareMethodLists(cls, mlists, mcount, NO, fromBundle);
                rw->methods.attachLists(mlists, mcount);
                mcount = 0;
            }
            mlists[ATTACH_BUFSIZ - ++mcount] = mlist;
            fromBundle |= entry.hi->isBundle();
        }

打印一下从分类里取出来的方法method_list_t *mlist = entry.cat->methodsForMeta(isMeta),元类取的是类方法,类取的是实例方法。

iOS 捋一捋Category加载流程及+load
isMeta=false

我添加了2个实例方法,那么来看看isMeta为false,会发现从分类里取出的sayYes和sayNo方法, 而后会通过attachLists添加到rw的methods里。在之后打印rw的methods 看看

iOS 捋一捋Category加载流程及+load
rw

所以流程是map_images->_read_images->realizeClassWithoutSwift->methodizeClass->objc::unattachedCategories.attachToClass->attachCategories->rw->methods.attachLists

2. 非懒加载类,懒加载分类。

发现并没有走attachCategories,往前打断点,会发现methodizeClass方法里直接attachLists 到rw里了

  // Install methods and properties that the class implements itself.
    method_list_t *list = ro->baseMethods();
    if (list) {
        const char *tempChar;
        tempChar = cls->nameForLogging();
        if (strcmp(tempChar, "Person")==0) {
            
        }
        prepareMethodLists(cls, &list, 1, YES, isBundleClass(cls));
        rw->methods.attachLists(&list, 1);
    }

打印结果:

iOS 捋一捋Category加载流程及+load
rw1

流程是map_images->_read_images->realizeClassWithoutSwift->methodizeClass->rw->methods.attachLists

3. 懒加载类,非懒加载分类。

既然是懒加载的类,那么只有在用到的时候才会加载了,map_images则不会断住了,通过attachCategories里的断点,看看堆栈信息。

iOS 捋一捋Category加载流程及+load
image.png

可以看到是在load_images里加载的,在调用load方法之前会做一些处理,prepare_load_methods里会对非懒加载类和非懒加载分类进行处理。
流程是load_images->prepare_load_methods->realizeClassWithoutSwift->methodizeClass->objc::unattachedCategories.attachToClass->attachCategories->rw->methods.attachLists

3. 懒加载类,懒加载分类

这种情况需要第一个发送消息时才会加载,在main方法[Person new]一下,看看断点的堆栈信息。

iOS 捋一捋Category加载流程及+load
image1.png

流程是第一次发送消息,调用objc_msgSend->lookUpImpOrForward->realizeClassWithoutSwift->methodizeClass->rw->methods.attachLists

再来看看attachLists这个方法是如何将list合二为一的。

void attachLists(List* const * addedLists, uint32_t addedCount) {
        if (addedCount == 0) return;

        if (hasArray()) {
            // many lists -> many lists
            uint32_t oldCount = array()->count;
            uint32_t newCount = oldCount + addedCount;
            setArray((array_t *)realloc(array(), array_t::byteSize(newCount)));
            array()->count = newCount;
            memmove(array()->lists + addedCount, array()->lists, 
                    oldCount * sizeof(array()->lists[0]));
            memcpy(array()->lists, addedLists, 
                   addedCount * sizeof(array()->lists[0]));
        }
        else if (!list  &&  addedCount == 1) {
            // 0 lists -> 1 list
            list = addedLists[0];
        } 
        else {
            // 1 list -> many lists
            List* oldList = list;
            uint32_t oldCount = oldList ? 1 : 0;
            uint32_t newCount = oldCount + addedCount;
            setArray((array_t *)malloc(array_t::byteSize(newCount)));
            array()->count = newCount;
            if (oldList) array()->lists[addedCount] = oldList;
            memcpy(array()->lists, addedLists, 
                   addedCount * sizeof(array()->lists[0]));
        }
    }

这里主要看memmove 和 memcpy这俩函数,memmove将原来的list往后移了addedCount,而memcpy则是将新的addedLists复制到了原来list的前面。这就说明了分类覆盖主类的同名方法并不是真正的覆盖,而是分类的方法被排到了前面,当方法查找时,则返回了分类的方法实现

调用被分类插队的同名方法

在分类和主类都添加sayYes方法,再在分类sayNo方法里输入以下代码

unsigned int methodCount = 0;
    Method *methodList = class_copyMethodList([self class], &methodCount);
    
    IMP lastIMP = nil;
    for (NSInteger i = 0; i 

会发现主类和分类的sayYes都走了

iOS 捋一捋Category加载流程及+load
sayYes
顺便捋一捋+load方法的顺序

call_load_methods 之前,会调用prepare_load_methods去发现+load方法,而这里面会对非懒加载类和非懒加载分类做处理。

classref_t const *classlist = 
        _getObjc2NonlazyClassList(mhdr, &count);
    for (i = 0; i cls);
        //printf("%s--%sn",cls->nameForLogging(),cat->name);
        if (!cls) continue;  // category for ignored weak-linked class
        if (cls->isSwiftStable()) {
            _objc_fatal("Swift class extensions and categories on Swift "
                        "classes are not allowed to have +load methods");
        }
        realizeClassWithoutSwift(cls, nil);
        ASSERT(cls->ISA()->isRealized());
        add_category_to_loadable_list(cat);
    }

进去schedule_class_load方法,里面有一段代码

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

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

在将类的+load方法添加进类的加载列表的时候,会schedule_class_load先将父类的+load加进去,这样保证父类在子类之前调用+load,然后添加分类的+load方法到分类的加载列表,而分类的+load加载顺序则看文件编译的顺序了。

真正调用+load方法的是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;
}

从代码里可以看出来,首先循环调用类的load方法,再调用分类的load方法。

验证一下

iOS 捋一捋Category加载流程及+load
1

iOS 捋一捋Category加载流程及+load
1.1

iOS 捋一捋Category加载流程及+load
2

iOS 捋一捋Category加载流程及+load
2.2

这里可能想到 分类里的+load为啥没有’覆盖’ 类的+load方法打印呢,这里并没有做什么特别的处理,分类的+load确实’覆盖’ 了类的+load方法前面了。
但是call_load_methods的时候,并没有走消息转发的机制(objc_msgSend),而是直接通过函数指针直接调用的((*load_method)(cls, @selector(load));),所以只要实现了就会调用。

总结以下load的调用顺序:父类先于子类调用,主类先于分类调用,分类则看文件编译顺序

分析就到这里,有误请大佬指正。

end

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