Runtime底层Class是个什么?

Class

• 在苹果开源的Runtime源码中可以找到这些数据结构

//
//  ysClassInfo.h
//  TestClass
//
//  Created by ys Lee on 2018/3/8.
//  Copyright © 2018年 ys Lee. All rights reserved.
//

#import <Foundation/Foundation.h>

#ifndef ysClassInfo_h
#define ysClassInfo_h

# if __arm64__
#   define ISA_MASK        0x0000000ffffffff8ULL
# elif __x86_64__
#   define ISA_MASK        0x00007ffffffffff8ULL
# endif

#if __LP64__
typedef uint32_t mask_t;
#else
typedef uint16_t mask_t;
#endif
typedef uintptr_t cache_key_t;

struct bucket_t {
    cache_key_t _key;
    IMP _imp;
};

struct cache_t {
    bucket_t *_buckets;
    mask_t _mask;
    mask_t _occupied;
};

struct entsize_list_tt {
    uint32_t entsizeAndFlags;
    uint32_t count;
};

struct method_t {
    SEL name;
    const char *types;
    IMP imp;
};

struct method_list_t : entsize_list_tt {
    method_t first;
};

struct ivar_t {
    int32_t *offset;
    const char *name;
    const char *type;
    uint32_t alignment_raw;
    uint32_t size;
};

struct ivar_list_t : entsize_list_tt {
    ivar_t first;
};

struct property_t {
    const char *name;
    const char *attributes;
};

struct property_list_t : entsize_list_tt {
    property_t first;
};

struct chained_property_list {
    chained_property_list *next;
    uint32_t count;
    property_t list[0];
};

typedef uintptr_t protocol_ref_t;
struct protocol_list_t {
    uintptr_t count;
    protocol_ref_t list[0];
};

struct class_ro_t {
    uint32_t flags;
    uint32_t instanceStart;
    uint32_t instanceSize;  // instance对象占用的内存空间
#ifdef __LP64__
    uint32_t reserved;
#endif
    const uint8_t * ivarLayout;
    const char * name;  // 类名
    method_list_t * baseMethodList;
    protocol_list_t * baseProtocols;
    const ivar_list_t * ivars;  // 成员变量列表
    const uint8_t * weakIvarLayout;
    property_list_t *baseProperties;
};

struct class_rw_t {
    uint32_t flags;
    uint32_t version;
    const class_ro_t *ro;
    method_list_t * methods;    // 方法列表
    property_list_t *properties;    // 属性列表
    const protocol_list_t * protocols;  // 协议列表
    Class firstSubclass;
    Class nextSiblingClass;
    char *demangledName;
};

#define FAST_DATA_MASK          0x00007ffffffffff8UL
struct class_data_bits_t {
    uintptr_t bits;
public:
    class_rw_t* data() {
        return (class_rw_t *)(bits & FAST_DATA_MASK);
    }
};

/* OC对象 */
struct objc_object {
    void *isa;
};

/* 类对象 */
struct objc_class : objc_object {
    Class superclass;
    cache_t cache;
    class_data_bits_t bits;
public:
    class_rw_t* data() {
        return bits.data();
    }
    
    objc_class* metaClass() {
        return (objc_class *)((long long)isa & ISA_MASK);
    }
};

#endif /* ysClassInfo_h */

isa 指针具体指向哪里呢?

• 下图总结的非常到位

isa-superclass.png

• 再来一张图解释清楚

20190409224709.jpg

方法调用轨迹

instance的isa指向class

class的isa指向meta-class

meta-class的isa指向基类的meta-class

class的superclass指向父类的class
如果没有父类，superclass指针为nil

meta-class的superclass指向父类的meta-class
基类的meta-class的superclass指向基类的class

instance调用对象方法的轨迹
isa找到class，方法不存在，就通过superclass找父类

class调用类方法的轨迹
isa找meta-class，方法不存在，就通过superclass找父类