iOS class 和 instance 底層原理

1. 查看類的本質

// - 本質上OC對象就是結構體…
2. 實例對象, 類對象 元類對象的關係
// - 實例對象的 isa -> 類對象; 類對象的 isa -> 元類對象;
// - 子類(實例, 類, 元類)對象的 superClass -> 父類(實例, 類, 元類)對象; 父類(實例, 類, 元類)對象 的 superClass ->父類的父類(實例, 類, 元類)對象
// - 根類對象的 superClass -> nil; 根元類的 superClass -> 根類; 根元類的isa -> self;
// - 當實例對象調用對象方法時候, 會去實例對象的 isa->的類對象中去查找方法, 如果查找到了就調用, 否則去類對象->superClass 中去查找
// - 當類對象調用類方法時候, 會去類對象的 isa->的元類對象中去查找方法, 如果查找到了就調用, 否則去元類對象->superClass 中去查找

3. objc_class 結構體 超級好用;

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

#import <Foundation/Foundation.h>

#ifndef MJClassInfo_h
#define MJClassInfo_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對象佔用的內存空間
#ifdef __LP64__
    uint32_t reserved;
#endif
    const uint8_t * ivarLayout;
    const char * name;  // 類名
    method_list_t * baseMethodList;
    protocol_list_t * baseProtocols;
    const ivar_list_t * ivars;  // 成員變量列表
    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;    // 屬性列表
    const protocol_list_t * protocols;  // 協議列表
    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對象 */
struct mj_objc_object {
    void *isa;
};

/* 類對象 */
struct mj_objc_class : mj_objc_object {
    Class superclass;
    cache_t cache;
    class_data_bits_t bits;
public:
    class_rw_t* data() {
        return bits.data();
    }
    
    mj_objc_class* metaClass() {
        return (mj_objc_class *)((long long)isa & ISA_MASK);
    }
};

#endif /* MJClassInfo_h */