iOS底層探索003-isa分析

iOS底層探索-目錄

探索OC對象的本質(zhì)

我們知道平時編寫的Objective-C代碼,底層都是C/C++實現(xiàn)的,那么C/C++又是怎么實現(xiàn)的呢?

下面看一段我們非常熟悉的OC代碼:

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

@implementation Person
@end

通過Clang編譯成C/C++:

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

編譯成功后

extern "C" unsigned long OBJC_IVAR_$_Person$_name;
// Person 類對象
struct Person_IMPL {
    struct NSObject_IMPL NSObject_IVARS; //isa
    // 屬性
    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

這里可以看到: 類對象被轉(zhuǎn)化成了一個結(jié)構(gòu)體

在OC中進入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++代碼

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

小結(jié): 兩個方面都證實NSObject對象的本質(zhì)就是一個結(jié)構(gòu)體

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
}

點擊Class進入

typedef struct objc_class *Class;

Class是一個結(jié)構(gòu)體指針.所以isa也就是一個指針

通過這篇文章得知,對象在alloc過程中,開辟完空間需要關(guān)聯(lián)isa

isa的創(chuàng)建和關(guān)聯(lián)流程是怎樣的呢,下面我們來以前探索

對象創(chuàng)建過程中,關(guān)聯(lián)isa的方法

 obj->initInstanceIsa(cls, hasCxxDtor);

點擊進入initInstanceIsa
初始化一個isa

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

    initIsa(cls, true, hasCxxDtor);
}

進入initIsa
初始化isa函數(shù)

inline void 
objc_object::initIsa(Class cls, bool nonpointer, bool hasCxxDtor) 
{ 
    ASSERT(!isTaggedPointer()); 
    // 不存在,則創(chuàng)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笛匙、Meta-Class對象的內(nèi)存地址信息賦值
        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;
    }
}

進去isa_t
isa的結(jié)構(gòu)是一個union(聯(lián)合體)

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
};

進入ISA_BITFIELD(isa位域)

// 真機
# 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)
// 模擬器
# 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的大小為8/字節(jié)即為64位

isa初始化流程.png

isa結(jié)構(gòu)分析

通過上面分析得到,isa是一個union結(jié)構(gòu),用位域存儲信息

isa arm64 解釋

nonpointer:表示是否對isa指針開啟指針優(yōu)化 0:純isa指針，1:不止是類對象地址,isa中包含了類信息差导、對象的引用計數(shù)等
has_assoc：關(guān)聯(lián)對象標志位, 0沒有借跪，1存在
has_cxx_dtor：該對象是否有C++或者Objc的析構(gòu)器,如果有析構(gòu)函數(shù),則需要做析構(gòu)邏輯, 如果沒有,則可以更快的釋放對象
shiftcls：存儲類指針的值. 開啟指針優(yōu)化的情況下祭埂，在arm64架構(gòu)中有33位用來存儲類指針
magic：用于調(diào)試器判斷當前對象是真的對象還是沒有初始化的空間
weakly_referenced：對象是否被指向或者曾經(jīng)指向一個ARC的弱變量, 沒有弱引用的對象可以更快釋放
deallocating：標志對象是否正在釋放內(nèi)存
has_sidetable_rc:當對象引用技術(shù)大于10時, 則需要借用該變量存儲進位
extra_rc：當表示該對象的引用計數(shù)值, 實際上是引用計數(shù)值減1，例如，如果對象的引用計數(shù)為10卡儒，那么extra_rc為 9,如果引用計數(shù)大于10，則需要使用到下面的has_sidetable_rc

arm64&x86_64.png

通過上圖我們可以得知shiftcls存儲著類的地址信息

下面我們用x86_64驗證,Class信息存儲在3~46位

image.png
與ISA_MASK &運算

驗證4.png

查看源碼可知道

#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
    // 與ISA_MASK 與運算
    return (Class)(isa.bits & ISA_MASK);
#endif
}

對象關(guān)系分析

關(guān)系圖

總結(jié): 類和元類是一個閉環(huán), 實例指向類, 類指向元類, 元類指向跟元類, 跟元類指向自身, 根元類的父類是NSObject