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iOS底层原理 runtime-object_class拾遗基础篇--(6)

runtime 基础知识

runtime是运行时,在运行的时候做一些事请,可以动态添加类和交换函数,那么有一个基础知识需要了解,arm64架构前,isa指针是普通指针,存储class和meta-class对象的内存地址,从arm64架构开始,对isa进行了优化,变成了一个union共用体,还是用位域来存储更多的信息,我们首先看一下isa指针的结构:

struct objc_object {
private:
    isa_t isa;
public:
    // ISA() assumes this is NOT a tagged pointer object
    Class ISA();
    // getIsa() allows this to be a tagged pointer object
    Class getIsa();
    //****
}


#include "isa.h"
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
};
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objc_object是结构体,包含了私有属性isa_t,isa_t isa是一个共用体,包含了ISA_BITFIELD是一个宏(结构体),bitsuintptr_t类型,uintptr_t其实是unsign long类型占用8字节,就是64位,我们进入到ISA_BITFIELD内部:

# 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)

# 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
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ISA_BITFIELDarm64x86是两种结构,存储了nonpointer,has_assoc,has_cxx_dtor,shiftcls,magic,weakly_referenced,deallocating,has_sidetable_rc,extra_rc这些信息,:1就占用了一位,:44就是占用了44位,:6就是占用了6位,:8就是占用了8位,那么共用体isa_t简化之后

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

    Class cls;
    uintptr_t bits;
    struct {
      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
    };
};
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isa_t是使用共用体结构,使用bits存储了结构体的数据,那么共用体是如何使用的?我们来探究一下

共用体基础知识

首先我们定义一个FYPerson,添加2个属性

@interface FYPerson : NSObject
@property (nonatomic,assign) BOOL rich;
@property (nonatomic,assign) BOOL tell;
@property (nonatomic,assign) BOOL handsome;
@end
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然后查看该类的实例占用空间大小

FYPerson *p=[[FYPerson alloc]init];
		p.handsome = YES;
		p.rich = NO;
		NSLog(@"大小:%zu",class_getInstanceSize(FYPerson.class));
		//16
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FYPerson定义了三个属性,占用空间是16字节,那么我们换一种方法实现这个三个属性的功能。 我们定义6个方法,3个set方法,3个get方法。

- (void)setTall:(BOOL)tall;
- (void)setRich:(BOOL)rich;
- (void)setHandsome:(BOOL)handsome;

- (BOOL)isTall;
- (BOOL)isRich;
- (BOOL)isHandsome;

//实现:
//使用0b00000000不是很易读,我们换成下边的写法1<<0
//#define FYHandsomeMask 0b00000001
//#define FYTallMask 0b00000010
//#define FYRichMask 0b00000001


#define FYHandsomeMask (1<<0)
#define FYTallMask (1<<1)
#define FYRichMask (1<<2)

@interface FYPerson()
{
	char _richTellHandsome;//0000 0000 rich tall handsome
}
@end


@implementation FYPerson

- (void)setRich:(BOOL)tall{
	if (tall) {
		_richTellHandsome = _richTellHandsome|FYRichMask;
	}else{
		_richTellHandsome = _richTellHandsome&~FYRichMask;
	}
	
}
- (void)setTall:(BOOL)tall{
	if (tall) {
		_richTellHandsome = _richTellHandsome|FYTallMask;
	}else{
		_richTellHandsome = _richTellHandsome&~FYTallMask;
	}
	
}
- (void)setHandsome:(BOOL)tall{
	if (tall) {
		_richTellHandsome = _richTellHandsome|FYHandsomeMask;
	}else{
		_richTellHandsome = _richTellHandsome&~FYHandsomeMask;

	}
}
- (BOOL)isRich{
	return !!(_richTellHandsome&FYRichMask);
}
- (BOOL)isTall{
	return !!(_richTellHandsome&FYTallMask);
}
- (BOOL)isHandsome{
	return !!(_richTellHandsome&FYHandsomeMask);
}
@end
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我们定义了一个char类型的变量_richTellHandsome,4字节,32位,可以存储32个bool类型的变量。赋值是使用_richTellHandsome = _richTellHandsome|FYRichMask,或_richTellHandsome = _richTellHandsome&~FYRichMask,取值是!!(_richTellHandsome&FYRichMask),前边加!!是转化成bool类型的,否则取值出来是1 or 2 or 4。我们再换一种思路将三个变量定义成一个结构体,取值和赋值都是可以直接操作的。

@interface FYPerson()
{
//	char _richTellHandsome;//0000 0000 rich tall handsome
	//位域
	struct{
		char tall : 1;//高度
		char rich : 1;//富有
		char handsome : 1; //帅
	} _richTellHandsome; // 0b0000 0000
	//使用2位 yes就是0b01 转化成1字节8位就是:0o0101 0101 结果是1
	//使用1位 yes就是0b1 转化成1字节8位就是:0o1111 1111 所以结果是-1
}
@end


@implementation FYPerson

- (void)setRich:(BOOL)tall{
	_richTellHandsome.rich = tall;
}
- (void)setTall:(BOOL)tall{
	_richTellHandsome.tall = tall;
}
- (void)setHandsome:(BOOL)tall{
	_richTellHandsome.handsome = tall;
}
- (BOOL)isRich{
	return !!_richTellHandsome.rich;
}
- (BOOL)isTall{
	return !!_richTellHandsome.tall;
}
- (BOOL)isHandsome{
	return !!_richTellHandsome.handsome;
}
@end
	
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结构体_richTellHandsome包含三个变量char tall : 1;,char rich : 1;,char handsome : 1。每一个变量占用空间为1位,3个变量占用3位。取值的时候使用!!(_richTellHandsome&FYHandsomeMask),赋值使用

if (tall) {
		_richTellHandsome = _richTellHandsome|FYHandsomeMask;
	}else{
		_richTellHandsome = _richTellHandsome&~FYHandsomeMask
	}
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我们采用位域来存储信息, 位域是指信息在存储时,并不需要占用一个完整的字节, 而只需占几个或一个二进制位。例如在存放一个开关量时,只有0和1 两种状态, 用一位二进位即可。为了节省存储空间,并使处理简便,C语言又提供了一种数据结构,称为“位域”或“位段”。所谓“位域”是把一个字节中的二进位划分为几 个不同的区域, 并说明每个区域的位数。每个域有一个域名,允许在程序中按域名进行操作。 这样就可以把几个不同的对象用一个字节的二进制位域来表示。

另外一个省空间的思路是使用联合, 使用union,可以更省空间,“联合”是一种特殊的类,也是一种构造类型的数据结构。在一个“联合”内可以定义多种不同的数据类型, 一个被说明为该“联合”类型的变量中,允许装入该“联合”所定义的任何一种数据,这些数据共享同一段内存,以达到节省空间的目的(还有一个节省空间的类型:位域)。 这是一个非常特殊的地方,也是联合的特征。另外,同struct一样,联合默认访问权限也是公有的,并且,也具有成员函数。

@interface FYPerson()
{
	union {
		char bits; //一个字节8位 ricH /tall/handsome都是占用的bits的内存空间
		struct{
			char tall : 1;//高度
			char rich : 1;//富有
			char handsome : 1; //帅
		}; // 0b0000 0000
	}_richTellHandsome;
}
@end


@implementation FYPerson

- (void)setRich:(BOOL)tall{
	if (tall) {
		_richTellHandsome.bits |= FYRichMask;
	}else{
		_richTellHandsome.bits &= ~FYRichMask;
	}
}
- (void)setTall:(BOOL)tall{
	if (tall) {
		_richTellHandsome.bits |= FYTallMask;
	}else{
		_richTellHandsome.bits &= ~FYTallMask;
	}
}
- (void)setHandsome:(BOOL)tall{
	if (tall) {
		_richTellHandsome.bits |= FYHandsomeMask;
	}else{
		_richTellHandsome.bits &= ~FYHandsomeMask;
	}
}
- (BOOL)isRich{
	return !!(_richTellHandsome.bits & FYRichMask);
}
- (BOOL)isTall{
	return !!(_richTellHandsome.bits & FYTallMask);
}
- (BOOL)isHandsome{
	return (_richTellHandsome.bits & FYHandsomeMask);
}
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使用联合共用体,达到省空间的目的,runtime源码中是用来很多union和位运算。 例如KVO 的NSKeyValueObservingOptions

typedef NS_OPTIONS(NSUInteger, NSKeyValueObservingOptions){
        NSKeyValueObservingOptionNew = 0x01,
    NSKeyValueObservingOptionOld = 0x02,
    NSKeyValueObservingOptionInitial = 0x04,
    NSKeyValueObservingOptionPrior = 0x08
}
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这个NSKeyValueObservingOptions使用位域,当传进去的时候NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld,则传进去的值为0x3,转化成二进制就是0b11,则两位都是1可以包含2个值。 那么我们来设计一个简单的可以使用或来传值的枚举

typedef enum {
	FYOne = 1,//  0b 0001
	FYTwo = 2,//  0b 0010
	FYTHree = 4,//0b 0100
	FYFour = 8,// 0b 1000
}FYOptions;

- (void)setOptions:(FYOptions )ops{
	if (ops &FYOne) {
		NSLog(@"FYOne is show");
	}
	if (ops &FYTwo) {
		NSLog(@"FYTwo is show");
	}
	if (ops &FYTHree) {
		NSLog(@"FYTHree is show");
	}
	if (ops &FYFour) {
		NSLog(@"FYFour is show");
	}
}

[self setOptions:FYOne|FYTwo|FYTHree];

//输出是:
FYOne is show
FYTwo is show
FYTHree is show

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这是一个名字为FYOptions的枚举,第一个是十进制是1,二进制是0b 0001,第二个十进制是2,二进制是0b 0010,第三个十进制是4,二进制是0b 0100,第四个十进制是8,二进制是0b 1000。 那么我们使用的时候可以FYOne|FYTwo|FYTHree,打包成一个值,相当于1|2|4 = 7,二进制表示是0b0111,后三位都是1,可以通过&mask取出对应的每一位的数值。

Class的结构

isa详解 – 位域存储的数据及其含义

参数 含义
nonpointer 0->代表普通的指针,存储着Class、Meta-Class对象的内存地址。1->代表优化过,使用位域存储更多的信息
has_assoc 是否有设置过关联对象,如果没有,释放时会更快
has_cxx_dtor 是否有C++的析构函数(.cxx_destruct),如果没有,释放时会更快
shiftcls 存储着Class、Meta-Class对象的内存地址信息
magic 用于在调试时分辨对象是否未完成初始化
weakly_referenced 是否有被弱引用指向过,如果没有,释放时会更快
deallocating 对象是否正在释放
extra_rc 里面存储的值是引用计数器减1
has_sidetable_rc 引用计数器是否过大无法存储在isa中
如果为1,那么引用计数会存储在一个叫SideTable的类的属性中

class结构

struct fy_objc_class : xx_objc_object {
	Class superclass;
	cache_t cache;
	class_data_bits_t bits;
public:
	class_rw_t* data() {
		return bits.data();
	}
	
	fy_objc_class* metaClass() { // 提供metaClass函数,获取元类对象
		// 上一篇我们讲解过,isa指针需要经过一次 & ISA_MASK操作之后才得到真正的地址
		return (fy_objc_class *)((long long)isa & ISA_MASK);
	}
};
struct class_rw_t {
	uint32_t flags;
	uint32_t version;
	const class_ro_t *ro;//只读 数据
	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;
};


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;
};
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class_ro_t是只读的,class_rw_t是读写的,在源码中runtime->Source->objc-runtime-new.mm->static Class realizeClass(Class cls) 1869行


    const class_ro_t *ro;
    class_rw_t *rw;
    Class supercls;
    Class metacls;
    bool isMeta;

    if (!cls) return nil;
    //如果已注册 就返回
    if (cls->isRealized()) return cls;
    assert(cls == remapClass(cls));

    // fixme verify class is not in an un-dlopened part of the shared cache?
//只读ro
    ro = (const class_ro_t *)cls->data();
    if (ro->flags & RO_FUTURE) {
        // This was a future class. rw data is already allocated.
        rw = cls->data();//初始化ro
        ro = cls->data()->ro;
        cls->changeInfo(RW_REALIZED|RW_REALIZING, RW_FUTURE);
    } else {
        // Normal class. Allocate writeable class data.
        //初始化 rw 
        rw = (class_rw_t *)calloc(sizeof(class_rw_t), 1);
        rw->ro = ro;
        rw->flags = RW_REALIZED|RW_REALIZING;
        //指针指向rw 一开始是指向ro的
        cls->setData(rw);
    }

    isMeta = ro->flags & RO_META;

    rw->version = isMeta ? 7 : 0;  // old runtime went up to 6
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开始cls->data指向的是ro,初始化之后,指向的rw,rw->ro指向的是原来的roclass_rw_t中的method_array_t是存储的方法列表,我们进入到method_array_t看下它的数据结构:

class method_array_t : 
    public list_array_tt<method_t, method_list_t> 
{
    typedef list_array_tt<method_t, method_list_t> Super;

 public:
    method_list_t **beginCategoryMethodLists() {
        return beginLists();
    }
    
    method_list_t **endCategoryMethodLists(Class cls);

    method_array_t duplicate() {
        return Super::duplicate<method_array_t>();
    }
};
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method_array_t是一个类,存储了method_t二维数组,那么我们看下method_t的结构

struct method_t {
    SEL name;
    const char *types;
    MethodListIMP imp;

    struct SortBySELAddress :
        public std::binary_function<const method_t&,const method_t&, bool>
    {
        bool operator() (const method_t& lhs,
                         const method_t& rhs)
        { return lhs.name < rhs.name; }
    };
};
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method_t是存储了3个变量的结构体,SEL是方法名,types是编码(方法返回类型,参数类型), imp函数指针(函数地址)。

SEL
  • SEL代表方法\函数名,一般叫做选择器,底层结构跟char *类似
  • 可以通过@selector()和sel_registerName()获得
  • 可以通过sel_getName()和NSStringFromSelector()转成字符串
  • 不同类中相同名字的方法,所对应的方法选择器是相同的
Type Encoding

iOS中提供了一个叫做@encode的指令,可以将具体的类型转成字符编码,官方网站插件encodeing

code Meaning
c A char
i An int
s A short
l A long
l is treated as a 32-bit quantity on 64-bit programs.
q A long long
C An unsigned char
I An unsigned int
S An unsigned short
L An unsigned long
Q An unsigned long long
f A float
d A double
B A C++ bool or a C99 _Bool
v A void
* A character string (char *)
@ An object (whether statically typed or typed id)
# A class object (Class)
: A method selector (SEL)
[array type] An array
{name=type...} A structure
(name=type...) A union
bnum A bit field of num bits
^type A pointer to type
? An unknown type (among other things, this code is used for function pointers)

我们通过一个例子来了解encode

-(void)test:(int)age heiht:(float)height{
}


FYPerson *p=[[FYPerson alloc]init];
	SEL sel = @selector(test:heiht:);
	Method m1= class_getInstanceMethod(p.class, sel);
	const char *type = method_getTypeEncoding(m1);
	NSLog(@"%s",type);
	
	//输出
	v24@0:8i16f20
	//0id 8 SEL 16 int 20 float = 24
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v24@0:8i16f20是encoding的值,我们来分解一下,前边是v24是函数返回值是void,所有参数占用了24字节,@0:8是从第0开始,长度是8字节的位置,i16是从16字节开始的int类型,f20是从20字节开始,类型是float

方法缓存

Class内部结构中有个方法缓存(cache_t),用散列表(哈希表)来缓存曾经调用过的方法,可以提高方法的查找速度。 我们来到cache_t内部

struct cache_t {
    struct bucket_t *_buckets;//散列表
    mask_t _mask;//散列表长度-1
    mask_t _occupied;//已经存储的方法数量
}

struct bucket_t {
#if __arm64__
    MethodCacheIMP _imp;
    cache_key_t _key;
#else
    cache_key_t _key;//SEL作为key 
    MethodCacheIMP _imp; //函数地址
#endif
}
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散列表的数据结构表格所示

索引 bucket_t
0 bucket_t(_key,_imp)
1 bucket_t(_key,_imp)
2 bucket_t(_key,_imp)
3 bucket_t(_key,_imp)
4 bucket_t(_key,_imp)
... ...

通过cache_getImp(cls, sel)获取IMP。具体在cache_t::find函数中

bucket_t * cache_t::find(cache_key_t k, id receiver)
{
    assert(k != 0);

    bucket_t *b = buckets();
    mask_t m = mask();
	//key&mask 得到索引
    mask_t begin = cache_hash(k, m);
    mask_t i = begin;
    do {
        if (b[i].key() == 0  ||  b[i].key() == k) {
            return &b[i];
        }
    } while ((i = cache_next(i, m)) != begin);

    // hack
    Class cls = (Class)((uintptr_t)this - offsetof(objc_class, cache));
    cache_t::bad_cache(receiver, (SEL)k, cls);
}

// Class points to cache. SEL is key. Cache buckets store SEL+IMP.
// Caches are never built in the dyld shared cache.

static inline mask_t cache_hash(cache_key_t key, mask_t mask) 
{
    return (mask_t)(key & mask);
}
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首先获取buckets()获取butket_t,然后获取_mask,通过 cache_hash(k, m)获取第一次访问的索引icache_hash通过(mask_t)(key & mask)得出具体的索引,当第一次成功获取到butket_t则直接返回,否则执行cache_next(i, m)获取下一个索引,直到获取到或者循环一遍结束。 那么我们来验证一下已经执行的函数的确是存在cache中的,我们自定义了class_rw_t

#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;

#if __arm__  ||  __x86_64__  ||  __i386__
// objc_msgSend has few registers available.
// Cache scan increments and wraps at special end-marking bucket.
#define CACHE_END_MARKER 1
static inline mask_t cache_next(mask_t i, mask_t mask) {
    return (i+1) & mask;
}

#elif __arm64__
// objc_msgSend has lots of registers available.
// Cache scan decrements. No end marker needed.
#define CACHE_END_MARKER 0
static inline mask_t cache_next(mask_t i, mask_t mask) {
    return i ? i-1 : mask;
}

#else
#error unknown architecture
#endif

struct bucket_t {
    cache_key_t _key;
    IMP _imp;
};

struct cache_t {
    bucket_t *_buckets;
    mask_t _mask;
    mask_t _occupied;
    
    IMP imp(SEL selector)
    {
        mask_t begin = _mask & (long long)selector;
        mask_t i = begin;
        do {
            if (_buckets[i]._key == 0  ||  _buckets[i]._key == (long long)selector) {
                return _buckets[i]._imp;
            }
        } while ((i = cache_next(i, _mask)) != begin);
        return NULL;
    }
};

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 mj_objc_object {
    void *isa;
};

/* 类对象 */
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
复制代码

测试代码是

FYPerson *p = [[FYPerson alloc]init];
		Method test1Method = class_getInstanceMethod(p.class, @selector(test));
		Method test2Method = class_getInstanceMethod(p.class, @selector(test2));
		IMP imp1= method_getImplementation(test1Method);
		IMP imp2= method_getImplementation(test2Method);

		mj_objc_class *cls = (__bridge mj_objc_class *)p.class;
		NSLog(@"-----");
		[p test];
		[p test2];
		cache_t cache = cls->cache;
		bucket_t *buck = cache._buckets;
		
		
		for (int i = 0; i <= cache._mask; i ++) {
			bucket_t item = buck[i];
			if (item._key != 0) {
				NSLog(@"key:%lu imp:%p",item._key,item._imp);
			}
		}
		
		
		//输出
p imp1
(IMP) $0 = 0x0000000100000df0 (day11-runtime1`-[FYPerson test] at FYPerson.m:12)
(lldb) p imp2
(IMP) $1 = 0x0000000100000e20 (day11-runtime1`-[FYPerson test2] at FYPerson.m:15)
p/d @selector(test)             //输出 test方法的sel地址
(SEL) $6 = 140734025103231 "test"
(lldb) p/d @selector(test2)     //输出 test2方法的sel地址
(SEL) $7 = 4294971267 "test2"

key1:140733954181041 imp1:0x7fff59fc4cd1
key2:4294971267 imp2:0x100000e20         //对应test2
key3:140734025103231 imp3:0x100000df0    //对应test1
复制代码

可以看出来IMP1IMP2key1key2分别对应了bucket_t中的key2,key3imp2imp3

static void cache_fill_nolock(Class cls, SEL sel, IMP imp, id receiver)
{
    cacheUpdateLock.assertLocked();

    //当initialized 没有执行完毕的时候不缓存
    if (!cls->isInitialized()) return;

    // Make sure the entry wasn't added to the cache by some other thread 
    // before we grabbed the cacheUpdateLock.
    if (cache_getImp(cls, sel)) return;

    cache_t *cache = getCache(cls);
    cache_key_t key = getKey(sel);

    // Use the cache as-is if it is less than 3/4 full
    mask_t newOccupied = cache->occupied() + 1;
    mask_t capacity = cache->capacity();
    if (cache->isConstantEmptyCache()) {
        // Cache is read-only. Replace it.
        cache->reallocate(capacity, capacity ?: INIT_CACHE_SIZE);
    }
    else if (newOccupied <= capacity / 4 * 3) {
        // Cache <= 3/4 
    }
    else {
        扩容 之后,缓存清空
        cache->expand();
    }
//bucket_t 最小是4,当>3/4时候,扩容,空间扩容之后是之前的2️倍。
    bucket_t *bucket = cache->find(key, receiver);
    if (bucket->key() == 0) cache->incrementOccupied();
    bucket->set(key, imp);
}
复制代码

cache_t初始化是大小是4,当大于3/4时,进行扩容,扩容之后是之前的2倍,数据被清空,cacha->_occupied恢复为0。 验证代码如下:

FYPerson *p = [[FYPerson alloc]init];
mj_objc_class *cls = (__bridge mj_objc_class *)p.class;
NSLog(@"-----");
[p test];
/*
 key:init imp:0x7fff58807c2d
 key:class imp:0x7fff588084b7
 key:(null) imp:0x0
 key:test imp:0x100000bf0
 Program ended with exit code: 0
 */
[p test2]; //当执行该函数的时候
/*
 key:(null) imp:0x0
 key:(null) imp:0x0
 key:(null) imp:0x0
 key:(null) imp:0x0
 key:(null) imp:0x0
 key:(null) imp:0x0
 key:test2 imp:0x100000c20
 key:(null) imp:0x0
 */

cache_t cache = cls->cache;
bucket_t *buck = cache._buckets;


for (int i = 0; i <= cache._mask; i ++) {
	bucket_t item = buck[i];
//            if (item._key != 0) {
////                printf("key:%s imp:%p \n",(const char *)item._key,item._imp);
//            }
    printf("key:%s imp:%p \n",(const char *)item._key,item._imp);

}
复制代码

总结

  • arm64之后isa使用联合体用更少的空间存储更多的数据,arm64之前存储class和meta-class指针。
  • 函数执行会先从cache中查找,没有的话,当再次找到该函数会添加到cache中
  • class->cache查找bucket_t的key需要先&_mask之后再判断是否有该key
  • cache扩容在大于3/4进行2倍扩容,扩容之后,旧数据删除,imp个数清空
  • class->rw在初始化中讲class_ro_t值赋值给rw,然后rw->ro指向之前的ro

资料下载


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