iOS Class 源码的基础研究（一）

一直对 ios 类 OC/Swift 的类本质理解总感觉是模糊的，好像知道但又不是特别清楚，周末花一些时间整理备忘。

一. 错误版本： runtime.h中 objc_class 的定义（Objc2 已经弃用， 此版本也是网上分析源码用的最多的一种，需警惕！）

#if !OBJC_TYPES_DEFINED

/// An opaque type that represents a method in a class definition.
typedef struct objc_method *Method;

/// An opaque type that represents an instance variable.
typedef struct objc_ivar *Ivar;

/// An opaque type that represents a category.
typedef struct objc_category *Category;

/// An opaque type that represents an Objective-C declared property.
typedef struct objc_property *objc_property_t;

struct objc_class {
    Class isa  OBJC_ISA_AVAILABILITY;

#if !__OBJC2__
    Class super_class                                        OBJC2_UNAVAILABLE;
    const char *name                                         OBJC2_UNAVAILABLE;
    long version                                             OBJC2_UNAVAILABLE;
    long info                                                OBJC2_UNAVAILABLE;
    long instance_size                                       OBJC2_UNAVAILABLE;
    struct objc_ivar_list *ivars                             OBJC2_UNAVAILABLE;
    struct objc_method_list **methodLists                    OBJC2_UNAVAILABLE;
    struct objc_cache *cache                                 OBJC2_UNAVAILABLE;
    struct objc_protocol_list *protocols                     OBJC2_UNAVAILABLE;
#endif

} OBJC2_UNAVAILABLE;
/* Use `Class` instead of `struct objc_class *` */

#endif

（看起来确实简单明了，然并卵）
------------------------------------黄金分割线------------------------------------

二. 正确版本
源码 objc-private.h 文件中定义如下：

typedef struct objc_class *Class;
typedef struct objc_object *id;

源码 objc-runtime-new.h 中关于 objc_class 中的定义代码如下：

struct objc_class : objc_object {
    // Class ISA;
    Class superclass;
    cache_t cache;             // formerly cache pointer and vtable
    class_data_bits_t bits;    // class_rw_t * plus custom rr/alloc flags

    class_rw_t *data() { 
        return bits.data();
    }
    ...
};

//swift 类
struct swift_class_t : objc_class {
    uint32_t flags;
    uint32_t instanceAddressOffset;
    uint32_t instanceSize;
    uint16_t instanceAlignMask;
    uint16_t reserved;

    uint32_t classSize;
    uint32_t classAddressOffset;
    void *description;
    // ...

    void *baseAddress() {
        return (void *)((uint8_t *)this - classAddressOffset);
    }
};

类肯定有对应的成员变量列表，属性列表，协议列表，它们在哪里？
在 Objc2.0 中，存储位置在 class_rw_t 中，其中的 class_ro_t 理解是类常规定义的部分，class_rw_t 包含常规定义的部分和动态方法加载的部分;

struct class_rw_t {
    uint32_t flags;
    uint32_t version;

    const class_ro_t *ro;

    method_array_t methods;
    property_array_t properties;
    protocol_array_t protocols;

    Class firstSubclass;
    Class nextSiblingClass;

    char *demangledName;
    ...
};

struct class_ro_t {
    uint32_t flags;
    uint32_t instanceStart;
    uint32_t instanceSize;
#ifdef __LP64__
    uint32_t reserved;
#endif

    const uint8_t * ivarLayout;
    
    const char * name;
    method_list_t * baseMethodList;
    protocol_list_t * baseProtocols;
    const ivar_list_t * ivars;

    const uint8_t * weakIvarLayout;
    property_list_t *baseProperties;

    method_list_t *baseMethods() const {
        return baseMethodList;
    }
};

分类在哪里？我还要靠它提高工作效率呢？
继续阅读源码 objc-runtime-new.h 文件发现方法：

static void addUnattachedCategoryForClass(category_t *cat, Class cls, 
                                          header_info *catHeader)
{
    runtimeLock.assertWriting();

    // DO NOT use cat->cls! cls may be cat->cls->isa instead
    NXMapTable *cats = unattachedCategories();
    category_list *list;

    list = (category_list *)NXMapGet(cats, cls);
    if (!list) {
        list = (category_list *)
            calloc(sizeof(*list) + sizeof(list->list[0]), 1);
    } else {
        list = (category_list *)
            realloc(list, sizeof(*list) + sizeof(list->list[0]) * (list->count + 1));
    }
    list->list[list->count++] = (locstamped_category_t){cat, catHeader};
    NXMapInsert(cats, cls, list);
}

struct locstamped_category_t {
    category_t *cat;
    struct header_info *hi;
};

struct locstamped_category_list_t {
    uint32_t count;
#if __LP64__
    uint32_t reserved;
#endif
    locstamped_category_t list[0];
};

struct category_t {
    const char *name;
    classref_t cls;
    struct method_list_t *instanceMethods;
    struct method_list_t *classMethods;
    struct protocol_list_t *protocols;
    struct property_list_t *instanceProperties;

    method_list_t *methodsForMeta(bool isMeta) {
        if (isMeta) return classMethods;
        else return instanceMethods;
    }

    property_list_t *propertiesForMeta(bool isMeta) {
        if (isMeta) return nil; // classProperties;
        else return instanceProperties;
    }
};

又看到了熟悉的老朋友：

    struct method_list_t *instanceMethods;
    struct method_list_t *classMethods;
    struct protocol_list_t *protocols;
    struct property_list_t *instanceProperties;

至此，我们基本清楚了 objc_class 的结构，对应的成员变量，属性，方法，协议列表的存储变量；以及category 的存储；