block笔记(一):block的本质

简单概述

先说结论:

block是C语言的扩充功能,我们可以认为它是 带有自动变量的匿名函数

block是一个匿名的inline代码集合:

  • 参数列表,就像一个函数。
  • 是一个对象!
  • 有声明的返回类型
  • 可获得义词法范围的状态,。
  • 可选择性修改词法范围的状态。
  • 可以用相同的词法范围内定义的其它block共享进行修改的可能性
  • 在词法范围(堆栈框架)被破坏后,可以继续共享和修改词法范围(堆栈框架)中定义的状态

block的实现

在llvm的文件中,我找到了一份文档,Block_private.h,这里可以查看到block的实现情况

struct Block_layout {
    void *isa;
    int flags;
    int reserved;
    void (*invoke)(void *, ...);
    struct Block_descriptor *descriptor;
    /* Imported variables. */
};

struct Block_descriptor {
    unsigned long int reserved;
    unsigned long int size;
    void (*copy)(void *dst, void *src);
    void (*dispose)(void *);
};

里面的invoke就是指向具体实现的函数指针,当block被调用的时候,程序最终会跳转到这个函数指针指向的代码区。 而 Block_descriptor里面最重要的就是 copy函数和 dispose函数,从命名上可以推断出,copy函数是用来捕获变量并持有引用,而dispose函数是用来释放捕获的变量。函数捕获的变量会存储在结构体 Block_layout的后面,在invoke函数执行前全部读出。

按照惯例,使用 clang -rewrite-objc 将一个代码进行编译转换,将得到一份C++代码。刨除其他无用的代码:

struct __block_impl {
    void *isa;
    int Flags;
    int Reserved;
    void *FuncPtr;
};

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int flags=0) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
}

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};
int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
        (void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA);
    }
    return 0;
}

先看最直接的 __block_impl代码,

struct __block_impl {
    void *isa;
    int Flags;
    int Reserved;
    void *FuncPtr;
};

这里是一个结构体,里面的元素分别是

  • isa,指向所属类的指针,也就是block的类型
  • flags,标志变量,在实现block的内部操作时会用到
  • Reserved,保留变量
  • FuncPtr,block执行时调用的函数指针

接着, __main_block_impl_0因为包含了__block_impl,我们可以将它打开,直接看成

__main_block_impl_0{
    void *isa;
    int Flags;
    int Reserved;
    void *FuncPtr;
    struct __main_block_desc_0 *Desc;
}

这么一来,我们可以将block理解为,一个OC对象、一个函数。

崩溃地址

如果我们把block设置为nil,然后去调用,会发生什么?

void (^block)(void) = nil;
block();

当我们运行的时候,它会崩溃,报错信息为 Thread 1: EXC_BAD_ACCESS (code=1, address=0x10)。 我们这时候进行对比: 正常的block 置为nil的block 我们可以发现,当把block置为nil的时候,第四行的函数指针,被置为NULL,注意,这里是NULL而不是nil。 我们给一个对象发送nil消息是没有问题的,但是给如果是NULL就会发生崩溃。

nil:指向oc中对象的空指针 Nil:指向oc中类的空指针 NULL:指向其他类型的空指针,如一个c类型的内存指针 NSNull:在集合对象中,表示空值的对象 若obj为nil:[obj message]将返回NO,而不是NSException 若obj为NSNull:[obj message]将抛出异常NSException

它直接访问到了函数指针,因为前三位分别是void、int、int,大小分别是8、4、4,加一块就为16,所以在十六位中,就表示出0x10地址的崩溃。 如果是在32位的系统中,void的大小是4,崩溃的地址应该就是0x0c。

block的类型

我们可以知道,我们常见的block是有三种:

  • __NSGlobalBlock
  • __NSStackBlock
  • __NSMallocBlock
    void (^block)(void) = ^{
        NSLog(@"biboyang");
    };
    block();
    或
    static int age = 10;
        void(^block)(void) = ^{
            NSLog(@"Hello, World! %d",age);
        };
    block();

像是这种,没有对外捕获变量的,就是GlobaBlock。

    int b = 10;
    void(^block2)(void) = ^{
        NSLog(@"Hello, World! %d",b);
    };
    block2();

这种block,在MRC中,即是StackBlock。在ARC中,因为编译器做了优化,自动进行了copy,这种就是MallocBlock了。 之所以做这种优化的原因很好理解: 如果StackBlock访问了一个auto变量,因为自己是存在Stack上的,所以变量也就会被保存在栈上。但是因为栈上的数据是由系统自动进行管理的,随时都有可能被回收。非常容易造成野指针的问题。 怎么解决呢?复制到堆上就好了! ARC也是如此做的。它会自动将栈上的block复制到堆上,所以,ARC下的block的属性关键词其实使用strong和copy都不会有问题,不过为了习惯,还是使用copy为好。

Blcok的类 副本源的配置存储域 复制效果
__NSStackBlock
__NSGlobalBlock 程序的数据区域 无用
__NSMallocBlock 引用计数增加

系统默认调用copy方法把Block赋复制的四种情况 1.手动调用copy 2.Block是函数的返回值 3.Block被强引用,Block被赋值给__strong或者id类型 4.调用系统API入参中含有usingBlcok的Cocoa方法或者GCD的相关API

ARC环境下,一旦Block赋值就会触发copy,__block就会copy到堆上,Block也是__NSMallocBlock。ARC环境下也是存在__NSStackBlock的时候,这种情况下,__block就在栈上。

如何截获变量

这里直接拿冰霜的文章来用

#import <Foundation/Foundation.h>

int global_i = 1;

static int static_global_j = 2;

int main(int argc, const char * argv[]) {
   
    static int static_k = 3;
    int val = 4;
    
    void (^myBlock)(void) = ^{
        global_i ++;
        static_global_j ++;
        static_k ++;
        NSLog(@"Block中 global_i = %d,static_global_j = %d,static_k = %d,val = %d",global_i,static_global_j,static_k,val);
    };
    
    global_i ++;
    static_global_j ++;
    static_k ++;
    val ++;
    NSLog(@"Block外 global_i = %d,static_global_j = %d,static_k = %d,val = %d",global_i,static_global_j,static_k,val);
    
    myBlock();
    
    return 0;
}

运行结果

Block 外  global_i = 2,static_global_j = 3,static_k = 4,val = 5
Block 中  global_i = 3,static_global_j = 4,static_k = 5,val = 4

转换的结果为

int global_i = 1;

static int static_global_j = 2;

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int *static_k;
  int val;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int *_static_k, int _val, int flags=0) : static_k(_static_k), val(_val) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  int *static_k = __cself->static_k; // bound by copy
  int val = __cself->val; // bound by copy

        global_i ++;
        static_global_j ++;
        (*static_k) ++;
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_45_k1d9q7c52vz50wz1683_hk9r0000gn_T_main_6fe658_mi_0,global_i,static_global_j,(*static_k),val);
    }

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};


int main(int argc, const char * argv[]) {

    static int static_k = 3;
    int val = 4;

    void (*myBlock)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, &static_k, val));

    global_i ++;
    static_global_j ++;
    static_k ++;
    val ++;
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_45_k1d9q7c52vz50wz1683_hk9r0000gn_T_main_6fe658_mi_1,global_i,static_global_j,static_k,val);

    ((void (*)(__block_impl *))((__block_impl *)myBlock)->FuncPtr)((__block_impl *)myBlock);

    return 0;
}

首先全局变量global_i和静态全局变量static_global_j的值增加,以及它们被Block捕获进去,这一点很好理解,因为是全局的,作用域很广,所以Block捕获了它们进去之后,在Block里面进行++操作,Block结束之后,它们的值依旧可以得以保存下来。 在__main_block_impl_0中,可以看到静态变量static_k和自动变量val,被Block从外面捕获进来,成为__main_block_impl_0这个结构体的成员变量了。 在执行Block语法的时候,Block语法表达式所使用的自动变量的值是被保存进了Block的结构体实例中,也就是Block自身中。 这么来就清晰了很多,自动变量是以值传递方式传递到Block的构造函数里面去的。Block只捕获Block中会用到的变量。由于只捕获了自动变量的值,并非内存地址,所以Block内部不能改变自动变量的值。

修改自动变量

截获变量有两种方法__block和指针法(不过__block法归根结底,其实也是操作指针)。 这里描述一下指针法:

    NSMutableString * str = [[NSMutableString alloc]initWithString:@"Hello,"];
    
    void (^myBlock)(void) = ^{
        [str appendString:@"World!"];
        NSLog(@"Block中 str = %@",str);
    };
    NSLog(@"Block外 str = %@",str);
    myBlock();
    
    const char *text = "hello";
    void(^block)(void) = ^{
        printf("%caaaaaaaaaaa\n",text[2]);
    };
    block();

直接操作指针去进行截获,不过一般来讲,这种方法多用于C语言数组的时候。使用OC的时候多数是使用__block。

这里写一个__block的捕获代码,使用刚才的方法再来一次:

1.普通非对象的变量

struct __Block_byref_i_0 {
  void *__isa;
__Block_byref_i_0 *__forwarding;
 int __flags;
 int __size;
 int i;
};

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_i_0 *i; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_i_0 *_i, int flags=0) : i(_i->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_i_0 *i = __cself->i; // bound by ref

        (i->__forwarding->i) ++;
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_45_k1d9q7c52vz50wz1683_hk9r0000gn_T_main_3b0837_mi_0,(i->__forwarding->i));
    }
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->i, (void*)src->i, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->i, 8/*BLOCK_FIELD_IS_BYREF*/);}

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __main_block_impl_0*, struct __main_block_impl_0*);
  void (*dispose)(struct __main_block_impl_0*);
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};
int main(int argc, const char * argv[]) {
    __attribute__((__blocks__(byref))) __Block_byref_i_0 i = {(void*)0,(__Block_byref_i_0 *)&i, 0, sizeof(__Block_byref_i_0), 0};

    void (*myBlock)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_i_0 *)&i, 570425344));

    ((void (*)(__block_impl *))((__block_impl *)myBlock)->FuncPtr)((__block_impl *)myBlock);

    return 0;
}

我们可以发现这里多了两个结构体

struct __Block_byref_i_0 {
  void *__isa;
__Block_byref_i_0 *__forwarding;
 int __flags;
 int __size;
 int i;
};

这个实例内,包含了 __isa 指针、一个标志位 __flags 、一个记录大小的 __size 。最最重要的,多了一个 __forwarding 指针和 val 变量. 这里长话短说,出来了一个新的 __forwarding指针,指向了结构体实例本身在内存的地址。

block通过指针的持续传递,将使用的自动变量值保存到了block的结构体实例中。在block体内修改 __block0变量,通过一系列指针指向关系,最终指向了__Block_byref_age_0结构体内与局部变量同名同类型的那个成员,并成功修改变量值。

在栈中, __forwarding指向了自己本身,但是如果复制到了堆上,__forwarding就指向复制到堆上的block,而堆上的block中的 __forwarding这时候指向了自己。

2.对象的变量

//以下代码是在ARC下执行的
#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
     
    __block id block_obj = [[NSObject alloc]init];
    id obj = [[NSObject alloc]init];

    NSLog(@"block_obj = [%@ , %p] , obj = [%@ , %p]",block_obj , &block_obj , obj , &obj);
    
    void (^myBlock)(void) = ^{
        NSLog(@"***Block中****block_obj = [%@ , %p] , obj = [%@ , %p]",block_obj , &block_obj , obj , &obj);
    };
    
    myBlock();
   
    return 0;
}

转换之后

struct __Block_byref_block_obj_0 {
  void *__isa;
__Block_byref_block_obj_0 *__forwarding;
 int __flags;
 int __size;
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 id block_obj;
};

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  id obj;
  __Block_byref_block_obj_0 *block_obj; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, id _obj, __Block_byref_block_obj_0 *_block_obj, int flags=0) : obj(_obj), block_obj(_block_obj->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_block_obj_0 *block_obj = __cself->block_obj; // bound by ref
  id obj = __cself->obj; // bound by copy

        NSLog((NSString *)&__NSConstantStringImpl__var_folders_45_k1d9q7c52vz50wz1683_hk9r0000gn_T_main_e64910_mi_1,(block_obj->__forwarding->block_obj) , &(block_obj->__forwarding->block_obj) , obj , &obj);
    }
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->block_obj, (void*)src->block_obj, 8/*BLOCK_FIELD_IS_BYREF*/);_Block_object_assign((void*)&dst->obj, (void*)src->obj, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->block_obj, 8/*BLOCK_FIELD_IS_BYREF*/);_Block_object_dispose((void*)src->obj, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __main_block_impl_0*, struct __main_block_impl_0*);
  void (*dispose)(struct __main_block_impl_0*);
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};


int main(int argc, const char * argv[]) {

    __attribute__((__blocks__(byref))) __Block_byref_block_obj_0 block_obj = {(void*)0,(__Block_byref_block_obj_0 *)&block_obj, 33554432, sizeof(__Block_byref_block_obj_0), __Block_byref_id_object_copy_131, __Block_byref_id_object_dispose_131, ((NSObject *(*)(id, SEL))(void *)objc_msgSend)((id)((NSObject *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSObject"), sel_registerName("alloc")), sel_registerName("init"))};

    id obj = ((NSObject *(*)(id, SEL))(void *)objc_msgSend)((id)((NSObject *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSObject"), sel_registerName("alloc")), sel_registerName("init"));
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_45_k1d9q7c52vz50wz1683_hk9r0000gn_T_main_e64910_mi_0,(block_obj.__forwarding->block_obj) , &(block_obj.__forwarding->block_obj) , obj , &obj);

    void (*myBlock)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, obj, (__Block_byref_block_obj_0 *)&block_obj, 570425344));

    ((void (*)(__block_impl *))((__block_impl *)myBlock)->FuncPtr)((__block_impl *)myBlock);

    return 0;
}

在转换出来的源码中,我们也可以看到,Block捕获了__block,并且强引用了,因为在__Block_byref_block_obj_0结构体中,有一个变量是id block_obj,这个默认也是带__strong所有权修饰符的。

根据打印出来的结果来看,ARC环境下,Block捕获外部对象变量,是都会copy一份的,地址都不同。只不过带有__block修饰符的变量会被捕获到Block内部持有。

在ARC中,对于声明为__block的外部对象,在block内部会进行retain,以至于在block环境内能安全的引用外部对象。

实例变量的问题

之前一直没有想到过一个问题: 我们知道不应该在block中使用实例变量,是因为会发生循环引用;那为什么会发生循环引用呢? 受谈谈ivar的直接访问的启发,我也开始探索一下这里的原因。 写如下的代码:


#import <Foundation/Foundation.h>
#import "objc/runtime.h"

typedef void(^MyBlock)(void);

@interface MyObject : NSObject
@property (nonatomic) NSUInteger BRInteger;
@property (nonatomic, copy) NSString *BRString;
@property (nonatomic, copy) MyBlock BRBlock;

- (void)inits;

@end

@implementation MyObject
- (void)inits
{
    self.BRBlock = ^{
        _BRInteger = 5;
        _BRString = @"Balaeniceps_rex";
    };
}
@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        MyObject *object = [MyObject new];
        [object inits];
    }
    return 0;
}

使用 clang -rewrite-objc -fobjc-arc -stdlib=libc++ -mmacosx-version-min=10.7 -fobjc-runtime=macosx-10.7 -Wno-deprecated-declarations main.m命令进行转换。得到以下的代码(为了简便,将代码做了省略):

typedef void(*MyBlock)(void);


#ifndef _REWRITER_typedef_MyObject
#define _REWRITER_typedef_MyObject
typedef struct objc_object MyObject;
typedef struct {} _objc_exc_MyObject;
#endif

//对于每个ivar,都有对应的全局变量
extern "C" unsigned long OBJC_IVAR_$_MyObject$_BRInteger;
extern "C" unsigned long OBJC_IVAR_$_MyObject$_BRString;
extern "C" unsigned long OBJC_IVAR_$_MyObject$_BRBlock;
//内部的结构
struct MyObject_IMPL {
    struct NSObject_IMPL NSObject_IVARS;
    NSUInteger _BRInteger;
    NSString *__strong _BRString;
    __strong MyBlock _BRBlock;
};

// @property (nonatomic) NSUInteger BRInteger;
// @property (nonatomic, copy) NSString *BRString;
// @property (nonatomic, copy) MyBlock BRBlock;

// - (void)inits;

/* @end */


// @implementation MyObject

struct __MyObject__inits_block_impl_0 {
    struct __block_impl impl;
    struct __MyObject__inits_block_desc_0* Desc;
    MyObject *const __strong self;
    
    //注意这里捕捉了self
    __MyObject__inits_block_impl_0(void *fp, struct __MyObject__inits_block_desc_0 *desc, MyObject *const __strong _self, int flags=0) : self(_self) {
        impl.isa = &_NSConcreteStackBlock;
        impl.Flags = flags;
        impl.FuncPtr = fp;
        Desc = desc;
    }
};

//block的函数方法(也就是方法layout中第四行的那个)
static void __MyObject__inits_block_func_0(struct __MyObject__inits_block_impl_0 *__cself) {
    MyObject *const __strong self = __cself->self; // bound by copy
    //这里是通过self的地址,那倒全局变量的偏移去获取实例变量的地址
    (*(NSUInteger *)((char *)self + OBJC_IVAR_$_MyObject$_BRInteger)) = 5;
    (*(NSString *__strong *)((char *)self + OBJC_IVAR_$_MyObject$_BRString)) = (NSString *)&__NSConstantStringImpl__var_folders_m1_05zb_zbd1g1f8k27nc6yn_th0000gn_T_main_e9db32_mi_0;
}
static void __MyObject__inits_block_copy_0(struct __MyObject__inits_block_impl_0*dst, struct __MyObject__inits_block_impl_0*src) {_Block_object_assign((void*)&dst->self, (void*)src->self, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static void __MyObject__inits_block_dispose_0(struct __MyObject__inits_block_impl_0*src) {_Block_object_dispose((void*)src->self, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static struct __MyObject__inits_block_desc_0 {
    size_t reserved;
    size_t Block_size;
    void (*copy)(struct __MyObject__inits_block_impl_0*, struct __MyObject__inits_block_impl_0*);
    void (*dispose)(struct __MyObject__inits_block_impl_0*);
} __MyObject__inits_block_desc_0_DATA = { 0, sizeof(struct __MyObject__inits_block_impl_0), __MyObject__inits_block_copy_0, __MyObject__inits_block_dispose_0};

static void _I_MyObject_inits(MyObject * self, SEL _cmd) {
    ((void (*)(id, SEL, MyBlock))(void *)objc_msgSend)((id)self, sel_registerName("setBRBlock:"), ((void (*)())&__MyObject__inits_block_impl_0((void *)__MyObject__inits_block_func_0, &__MyObject__inits_block_desc_0_DATA, self, 570425344)));
}

static NSUInteger _I_MyObject_BRInteger(MyObject * self, SEL _cmd) { return (*(NSUInteger *)((char *)self + OBJC_IVAR_$_MyObject$_BRInteger)); }
static void _I_MyObject_setBRInteger_(MyObject * self, SEL _cmd, NSUInteger BRInteger) { (*(NSUInteger *)((char *)self + OBJC_IVAR_$_MyObject$_BRInteger)) = BRInteger; }

static NSString * _I_MyObject_BRString(MyObject * self, SEL _cmd) { return (*(NSString *__strong *)((char *)self + OBJC_IVAR_$_MyObject$_BRString)); }
extern "C" __declspec(dllimport) void objc_setProperty (id, SEL, long, id, bool, bool);

static void _I_MyObject_setBRString_(MyObject * self, SEL _cmd, NSString *BRString) { objc_setProperty (self, _cmd, __OFFSETOFIVAR__(struct MyObject, _BRString), (id)BRString, 0, 1); }

static void(* _I_MyObject_BRBlock(MyObject * self, SEL _cmd) )(){ return (*(__strong MyBlock *)((char *)self + OBJC_IVAR_$_MyObject$_BRBlock)); }
static void _I_MyObject_setBRBlock_(MyObject * self, SEL _cmd, MyBlock BRBlock) { objc_setProperty (self, _cmd, __OFFSETOFIVAR__(struct MyObject, _BRBlock), (id)BRBlock, 0, 1); }
// @end

int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
        
        MyObject *object = ((MyObject *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("MyObject"), sel_registerName("new"));
        ((void (*)(id, SEL))(void *)objc_msgSend)((id)object, sel_registerName("inits"));
    }
    return 0;
}

我们可以发现,每个实例变量都是被创建了对应的全局变量:

extern "C" unsigned long OBJC_IVAR_$_MyObject$_BRInteger;
extern "C" unsigned long OBJC_IVAR_$_MyObject$_BRString;
extern "C" unsigned long OBJC_IVAR_$_MyObject$_BRBlock;

下面是block的layout中的第四排的函数调用方法。

//block的函数方法(也就是方法layout中第四行的那个)
static void __MyObject__inits_block_func_0(struct __MyObject__inits_block_impl_0 *__cself) {
    MyObject *const __strong self = __cself->self; // bound by copy
    //这里是通过self的地址,那倒全局变量的偏移去获取实例变量的地址
    (*(NSUInteger *)((char *)self + OBJC_IVAR_$_MyObject$_BRInteger)) = 5;
    (*(NSString *__strong *)((char *)self + OBJC_IVAR_$_MyObject$_BRString)) = (NSString *)&__NSConstantStringImpl__var_folders_m1_05zb_zbd1g1f8k27nc6yn_th0000gn_T_main_e9db32_mi_0;
}

通过这里,我们其实也能发现,这里是通过self的偏移去获取实例变量的地址,也是和self息息相关的。 如果这个还不会证明实例变量中的self的作用的话,我们接着往下看;

struct __MyObject__inits_block_impl_0 {
    struct __block_impl impl;
    struct __MyObject__inits_block_desc_0* Desc;
    MyObject *const __strong self;
    
    //注意这里捕捉了self
    __MyObject__inits_block_impl_0(void *fp, struct __MyObject__inits_block_desc_0 *desc, MyObject *const __strong _self, int flags=0) : self(_self) {
        impl.isa = &_NSConcreteStackBlock;
        impl.Flags = flags;
        impl.FuncPtr = fp;
        Desc = desc;
    }
};

在这个方法里,我们可以发现,在block当中,其实也引用到MyObject,是一个强引用的self!而block的构造函数中也多次引用了self。 我们如果了解过property的话,也会知道实例变量是在编译期就确定地址了。内部实现的全局变量就代表了地址的offset。

参考

Blocks Programming Topics

Working with Blocks

fuckingblocksyntax.com