Objective-C中的對象,簡稱OC對象,主要可以分為3種

1. instance對象(實例對象)

• instance對象就是通過類alloc出來的對象,每次調(diào)用alloc都會產(chǎn)生新的instance對象
• 代碼表現(xiàn)

NSObject *object1 = [[NSObject alloc] init];
NSObject *object2 = [[NSObject alloc] init];

• object1, object2都是NSObject的instance對象(實例對象)
• 它們是不同的兩個對象,分別占據(jù)著兩塊不同的內(nèi)存
• instace對象在內(nèi)存中存儲的信息包括
  • isa指針
  • 其它成員變量

2. class對象(類對象)

• 代碼表現(xiàn)

NSObject *object1 = [[NSObject alloc] init];
NSObject *object2 = [[NSObject alloc] init];
Class objectClass1 = [object1 class];
Class objectClass2 = [object2 class];
Class objectClass3 = [NSObject class];
Class objectClass4 = object_getClass(object1); // Runtime
Class objectClass5 = object_getClass(object2); // Runtime

• objectClass1 ~ objectClass5都是NSObject的class對象(類對象),class方法返回的一直是class對象
• 它們都是同一個對象.每個類在內(nèi)存中有且只有一個class對象
• class對象在內(nèi)存中存儲的信息主要包括
  • isa指針
  • superclass指針
  • 類的屬性信息(@property),類的對象方法信息(instance method)
  • 類的協(xié)議信息(protocol),類的成員變量信息(ivar)

3. meta-class對象(元類對象)

• 代碼表現(xiàn)

Class objectMetaClass = object_getClass([NSObject class]); // Runtime

• objectMetaClass是NSObject的meta-class對象
• 每個類在內(nèi)存中有且只有一個meta-class對象
• meta-class對象和class對象的內(nèi)存結構是一樣的,但是用途不一樣,在內(nèi)存中存儲的信息主要包括
  • isa指針
  • superclass指針
  • 類的類方法信息(class method)
• 查看Class是否為meta-class

BOOL result = class_isMetaClass([NSObject class]); // Runtime

object_getClass的內(nèi)部實現(xiàn)

image.png

• 因為Objective-C和swift的混編問題,一些底層實現(xiàn)有所變化

Class objc_getClass(const char *aClassName)
{
    if (!aClassName) return Nil;

    // NO unconnected, YES class handler
    return look_up_class(aClassName, NO, YES);
}

Class look_up_class(const char *name, 
              bool includeUnconnected __attribute__((unused)), 
              bool includeClassHandler __attribute__((unused)))
{
    // 傳過來的類名為空的話,直接返回nil
    if (!name) return nil;

    // 定義Class結果result
    Class result;
    // 定義未實現(xiàn)過變量unrealized
    bool unrealized;
    {
        // 加鎖
        runtimeLock.lock();
        // 獲取非swift的Class
        result = getClassExceptSomeSwift(name);
        // 如果result有值,且result類的方法實現(xiàn)過了
        unrealized = result  &&  !result->isRealized();
        if (unrealized) {
            // 未實現(xiàn)的話,調(diào)用realizeClassMaybeSwiftAndUnlock實現(xiàn)一下,同時解鎖
            result = realizeClassMaybeSwiftAndUnlock(result, runtimeLock);
            // runtimeLock is now unlocked
        } else {
            // 解鎖
            runtimeLock.unlock();
        }
    }

    if (!result) {
        // Ask Swift about its un-instantiated classes.

        // We use thread-local storage to prevent infinite recursion
        // if the hook function provokes another lookup of the same name
        // (for example, if the hook calls objc_allocateClassPair)

        auto *tls = _objc_fetch_pthread_data(true);

        // Stop if this thread is already looking up this name.
        for (unsigned i = 0; i < tls->classNameLookupsUsed; i++) {
            if (0 == strcmp(name, tls->classNameLookups[i])) {
                return nil;
            }
        }

        // Save this lookup in tls.
        if (tls->classNameLookupsUsed == tls->classNameLookupsAllocated) {
            tls->classNameLookupsAllocated =
                (tls->classNameLookupsAllocated * 2 ?: 1);
            size_t size = tls->classNameLookupsAllocated *
                sizeof(tls->classNameLookups[0]);
            tls->classNameLookups = (const char **)
                realloc(tls->classNameLookups, size);
        }
        tls->classNameLookups[tls->classNameLookupsUsed++] = name;

        // Call the hook.
        Class swiftcls = nil;
        if (GetClassHook.get()(name, &swiftcls)) {
            ASSERT(swiftcls->isRealized());
            result = swiftcls;
        }

        // Erase the name from tls.
        unsigned slot = --tls->classNameLookupsUsed;
        ASSERT(slot >= 0  &&  slot < tls->classNameLookupsAllocated);
        ASSERT(name == tls->classNameLookups[slot]);
        tls->classNameLookups[slot] = nil;
    }

    return result;
}

說說isa

• instance對象的isa指向class
  • 當調(diào)用對象方法時,通過instance的isa找到class,最后找到對象方法的實現(xiàn)進行調(diào)用
• class的isa指向meta-class
  • 當調(diào)用類方法時,通過class的isa找到meta-class,最后找到類方法的實現(xiàn)進行調(diào)用
• meta-class的isa指向基類的meta-class

說說superclass

• class的superclass指向meta-class

  • 如果沒有父類,superclass指針為nil

• meta-class的superclass指向基類的meta-class

  • 基類的meta-class的superclass指向基類的class

• 例子1(實例對象調(diào)用父類的對象方法),Student的instance實例調(diào)用Person的對象方法流程

  
  
  image.png

• 例子2(類對象調(diào)用父類的類方法),Student的class對象調(diào)用Person類的類方法流程

  
  
  image.png

經(jīng)典的isa和superclass圖解

image.png

• intance調(diào)用對象方法的軌跡

  • isa找到class,方法不存在,就通過superclass找父類,一直找,直到所有的父類找完都沒有這個對象方法的實現(xiàn)時,再經(jīng)過runtime的動態(tài)方法解析和消息轉(zhuǎn)發(fā),如果都沒有,就會報unrecognized selector sent to instance 0xxxxxxxxx

• class調(diào)用類方法的軌跡

  • isa找meta-class,方法不存在,就通過superclass找父類
  • 這種有一種特殊情況??,找到meta-class的都沒有找到時,因為meta-class的superclass指向基類的meta-class的,所以會調(diào)用基類的meta-class的類方法.這里不用奇怪明明調(diào)用的是類方法,最后卻調(diào)用了基類的對象方法.
    • 示例代碼如下

    #import <Foundation/Foundation.h>
    #import <objc/objc.h>
    
    @interface XYPerson : NSObject
    
    + (void)test;
    
    @end
    
    @implementation XYPerson
    
    @end
    
    @interface NSObject (Test)
    
    + (void)test;
    
    @end
    
    @implementation NSObject (Test)
    
    - (void)test
    {
        NSLog(@"-[NSObject test] - %p", self);
    }
    
    @end
    
    int main(int argc, const char * argv[]) {
        @autoreleasepool {
            // [XYPerson class] - 0x1000041e0
            NSLog(@"[XYPerson class] - %p", [XYPerson class]);
            // [NSObject class] - 0x7fff90dd4118
            NSLog(@"[NSObject class] - %p", [NSObject class]);
    
            // -[NSObject test] - 0x1000041e0
            [XYPerson test];
    //        objc_msgSend([XYPerson class], @selector(test));
    
            // -[NSObject test] - 0x7fff90dd4118
            [NSObject test];
    //        objc_msgSend([NSObject class], @selector(test));
        }
        return 0;
    }
    

說說isa和superclass細節(jié)

• 代碼示意

// MJPerson類對象的地址：0x00000001000014c8
// MJPerson實例對象的isa：0x001d8001000014c9(為什么地址有差別,需要要與上一個 ISA_MASK 才是真實的值)
// isa & ISA_MASK：0x00000001000014c8

MJPerson *person = [[MJPerson alloc] init];

Class personClass = [MJPerson class];

Class personMetaClass = object_getClass(personClass);

• 從64bit開始,isa需要進行一次位運算,才能計算出真實的isa地址值

  • isa & ISA_MASK

  • ISA_MASK的值

    
    
    image.png

• superclass不需要做此操作,直接就是對應的地址值

isa和superclass-class和meta-class的結構

• 代碼結構

/// Represents an instance of a class.
struct objc_object {
    Class _Nonnull isa  OBJC_ISA_AVAILABILITY;
};

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() const {
        return bits.data();
    }
    ...
}

struct class_rw_t {
    // Be warned that Symbolication knows the layout of this structure.
    uint32_t flags;
    uint16_t witness;
#if SUPPORT_INDEXED_ISA
    uint16_t index;
#endif

    explicit_atomic<uintptr_t> ro_or_rw_ext;

    Class firstSubclass;
    Class nextSiblingClass;
};

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

    union {
        const uint8_t * ivarLayout;
        Class nonMetaclass;
    };

    explicit_atomic<const char *> name;
    // With ptrauth, this is signed if it points to a small list, but
    // may be unsigned if it points to a big list.
    void *baseMethodList;
    protocol_list_t * baseProtocols;
    const ivar_list_t * ivars;

    const uint8_t * weakIvarLayout;
    property_list_t *baseProperties;
};

• 圖片結構

  
  
  image.png

模擬class內(nèi)部的結構

#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對象占用的內(nèi)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(xié)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 */

// objective-c++
#import <Foundation/Foundation.h>
#import <objc/runtime.h>
#import "MJClassInfo.h"

// MJPerson
@interface MJPerson : NSObject <NSCopying>
{
@public
    int _age;
}
@property (nonatomic, assign) int no;
- (void)personInstanceMethod;
+ (void)personClassMethod;
@end

@implementation MJPerson

- (void)test
{
    
}

- (void)personInstanceMethod
{
    
}
+ (void)personClassMethod
{
    
}
- (id)copyWithZone:(NSZone *)zone
{
    return nil;
}
@end

// MJStudent
@interface MJStudent : MJPerson <NSCoding>
{
@public
    int _weight;
}
@property (nonatomic, assign) int height;
- (void)studentInstanceMethod;
+ (void)studentClassMethod;
@end

@implementation MJStudent
- (void)test
{
    
}
- (void)studentInstanceMethod
{
    
}
+ (void)studentClassMethod
{
    
}
- (id)initWithCoder:(NSCoder *)aDecoder
{
    return nil;
}

- (void)encodeWithCoder:(NSCoder *)aCoder
{
    
}
@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        MJStudent *stu = [[MJStudent alloc] init];
        stu->_weight = 10;
        
        mj_objc_class *studentClass = (__bridge mj_objc_class *)([MJStudent class]);
        mj_objc_class *personClass = (__bridge mj_objc_class *)([MJPerson class]);
        
        class_rw_t *studentClassData = studentClass->data();
        class_rw_t *personClassData = personClass->data();
        
        class_rw_t *studentMetaClassData = studentClass->metaClass()->data();
        class_rw_t *personMetaClassData = personClass->metaClass()->data();

        NSLog(@"1111");
    }
    return 0;
}

面試題

1. 對象的isa指針指向哪里?
   • instance對象的isa指向class對象
   • class對象的isa指向meta-class對象
   • meta-class對象的isa指向基類的meta-class對象
2. OC的類信息存放在哪里?
   • 對象方法,屬性,成員變量,協(xié)議信息,存放在class對象中
   • 類方法,存放在meta-class對象中
   • 成員變量的具體值,存放在instance對象