iOS底层探索003-isa分析

iOS底层探索-目录

探索OC对象的本质

我们知道平时编写的Objective-C代码,底层都是C/C++实现的,那么C/C++又是怎么实现的呢?

下面看一段我们非常熟悉的OC代码:

@interface Person : NSObject
@property (nonatomic, copy) NSString *name;
@end

@implementation Person
@end

通过Clang编译成C/C++:

clang -rewrite-objc main.m -o main.cpp

编译成功后

extern "C" unsigned long OBJC_IVAR_$_Person$_name;
// Person 类对象
struct Person_IMPL {
    struct NSObject_IMPL NSObject_IVARS; //isa
    // 属性
    NSString *_name;
};

// @property (nonatomic, copy) NSString *name;
/* @end */


// @implementation Person

// get 方法
static NSString * _I_Person_name(Person * self, SEL _cmd) { return (*(NSString **)((char *)self + OBJC_IVAR_$_Person$_name)); }

extern "C" __declspec(dllimport) void objc_setProperty (id, SEL, long, id, bool, bool);
// set 方法
static void _I_Person_setName_(Person * self, SEL _cmd, NSString *name) { objc_setProperty (self, _cmd, __OFFSETOFIVAR__(struct Person, _name), (id)name, 0, 1); }
// @end

这里可以看到: 类对象被转化成了一个结构体

在OC中进入NSObject


@interface NSObject <NSObject> {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-interface-ivars"
    Class isa  OBJC_ISA_AVAILABILITY;
#pragma clang diagnostic pop
}

C++代码

struct Person_IMPL {
    struct NSObject_IMPL NSObject_IVARS;
    NSString *_name;
};

小结: 两个方面都证实NSObject对象的本质就是一个结构体

isa流程分析

@interface NSObject <NSObject> {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-interface-ivars"
    Class isa  OBJC_ISA_AVAILABILITY;
#pragma clang diagnostic pop
}

点击Class进入

typedef struct objc_class *Class;

Class是一个结构体指针.所以isa也就是一个指针

通过这篇文章得知,对象在alloc过程中,开辟完空间需要关联isa

isa的创建和关联流程是怎样的呢,下面我们来以前探索

对象创建过程中,关联isa的方法

 obj->initInstanceIsa(cls, hasCxxDtor);

点击进入initInstanceIsa
初始化一个isa

inline void 
objc_object::initInstanceIsa(Class cls, bool hasCxxDtor)
{
    ASSERT(!cls->instancesRequireRawIsa());
    ASSERT(hasCxxDtor == cls->hasCxxDtor());

    initIsa(cls, true, hasCxxDtor);
}

进入initIsa
初始化isa函数

inline void 
objc_object::initIsa(Class cls, bool nonpointer, bool hasCxxDtor) 
{ 
    ASSERT(!isTaggedPointer()); 
    // 不存在,则创建isa
    if (!nonpointer) {
        isa = isa_t((uintptr_t)cls);
    } else {
        ASSERT(!DisableNonpointerIsa);
        ASSERT(!cls->instancesRequireRawIsa());

        isa_t newisa(0);
#if SUPPORT_INDEXED_ISA
        ASSERT(cls->classArrayIndex() > 0);
        newisa.bits = ISA_INDEX_MAGIC_VALUE;
        // isa.magic is part of ISA_MAGIC_VALUE
        // isa.nonpointer is part of ISA_MAGIC_VALUE
        newisa.has_cxx_dtor = hasCxxDtor;
        newisa.indexcls = (uintptr_t)cls->classArrayIndex();
#else
        newisa.bits = ISA_MAGIC_VALUE;
        // isa.magic is part of ISA_MAGIC_VALUE
        // isa.nonpointer is part of ISA_MAGIC_VALUE
        newisa.has_cxx_dtor = hasCxxDtor;
        // Class、Meta-Class对象的内存地址信息赋值
        newisa.shiftcls = (uintptr_t)cls >> 3;
#endif
        // This write must be performed in a single store in some cases
        // (for example when realizing a class because other threads
        // may simultaneously try to use the class).
        // fixme use atomics here to guarantee single-store and to
        // guarantee memory order w.r.t. the class index table
        // ...but not too atomic because we don't want to hurt instantiation
        isa = newisa;
    }
}

进去isa_t
isa的结构是一个union(联合体)

union isa_t {
    isa_t() { }
    isa_t(uintptr_t value) : bits(value) { }

    Class cls;
    uintptr_t bits;
#if defined(ISA_BITFIELD)
    struct {
        ISA_BITFIELD;  // defined in isa.h
    };
#endif
};

进入ISA_BITFIELD(isa位域)

// 真机
# if __arm64__ 
#   define ISA_MASK        0x0000000ffffffff8ULL
#   define ISA_MAGIC_MASK  0x000003f000000001ULL
#   define ISA_MAGIC_VALUE 0x000001a000000001ULL
#   define ISA_BITFIELD                                                      \
      uintptr_t nonpointer        : 1;                                       \
      uintptr_t has_assoc         : 1;                                       \
      uintptr_t has_cxx_dtor      : 1;                                       \
      uintptr_t shiftcls          : 33; /*MACH_VM_MAX_ADDRESS 0x1000000000*/ \
      uintptr_t magic             : 6;                                       \
      uintptr_t weakly_referenced : 1;                                       \
      uintptr_t deallocating      : 1;                                       \
      uintptr_t has_sidetable_rc  : 1;                                       \
      uintptr_t extra_rc          : 19
#   define RC_ONE   (1ULL<<45)
#   define RC_HALF  (1ULL<<18)
// 模拟器
# elif __x86_64__
#   define ISA_MASK        0x00007ffffffffff8ULL
#   define ISA_MAGIC_MASK  0x001f800000000001ULL
#   define ISA_MAGIC_VALUE 0x001d800000000001ULL
#   define ISA_BITFIELD                                                        \
      uintptr_t nonpointer        : 1;                                         \
      uintptr_t has_assoc         : 1;                                         \
      uintptr_t has_cxx_dtor      : 1;                                         \
      uintptr_t shiftcls          : 44; /*MACH_VM_MAX_ADDRESS 0x7fffffe00000*/ \
      uintptr_t magic             : 6;                                         \
      uintptr_t weakly_referenced : 1;                                         \
      uintptr_t deallocating      : 1;                                         \
      uintptr_t has_sidetable_rc  : 1;                                         \
      uintptr_t extra_rc          : 8
#   define RC_ONE   (1ULL<<56)
#   define RC_HALF  (1ULL<<7)

# else
#   error unknown architecture for packed isa
# endif

isa的大小为8/字节即为64位

isa初始化流程.png

isa结构分析

通过上面分析得到,isa是一个union结构,用位域存储信息

isa arm64 解释

nonpointer:表示是否对isa指针开启指针优化 0:纯isa指针，1:不止是类对象地址,isa中包含了类信息、对象的引用计数等
has_assoc：关联对象标志位, 0没有，1存在
has_cxx_dtor：该对象是否有C++或者Objc的析构器,如果有析构函数,则需要做析构逻辑, 如果没有,则可以更快的释放对象
shiftcls：存储类指针的值. 开启指针优化的情况下，在arm64架构中有33位用来存储类指针
magic：用于调试器判断当前对象是真的对象还是没有初始化的空间
weakly_referenced：对象是否被指向或者曾经指向一个ARC的弱变量, 没有弱引用的对象可以更快释放
deallocating：标志对象是否正在释放内存
has_sidetable_rc:当对象引用技术大于10时, 则需要借用该变量存储进位
extra_rc：当表示该对象的引用计数值, 实际上是引用计数值减1，例如，如果对象的引用计数为10，那么extra_rc为 9,如果引用计数大于10，则需要使用到下面的has_sidetable_rc

arm64&x86_64.png

通过上图我们可以得知shiftcls存储着类的地址信息

下面我们用x86_64验证,Class信息存储在3~46位

image.png
与ISA_MASK &运算

验证4.png

查看源码可知道

#if SUPPORT_NONPOINTER_ISA

inline Class 
objc_object::ISA() 
{
    ASSERT(!isTaggedPointer()); 
#if SUPPORT_INDEXED_ISA
    if (isa.nonpointer) {
        uintptr_t slot = isa.indexcls;
        return classForIndex((unsigned)slot);
    }
    return (Class)isa.bits;
#else
    // 与ISA_MASK 与运算
    return (Class)(isa.bits & ISA_MASK);
#endif
}

对象关系分析

关系图

总结: 类和元类是一个闭环, 实例指向类, 类指向元类, 元类指向跟元类, 跟元类指向自身, 根元类的父类是NSObject