iOS block原理研究

研究下block的具体实现，网上已经有过了，自己重新做下，也算是加深理解。

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

    @autoreleasepool {

        __block int age = 10;

        NSString *name = @"张三";

        void (^block)() = ^{

            int  a = 10;

            int b = 20;

            int c ;

            c =  a + b;

           NSLog(@"name %@",name);

            NSLog(@"age %d",age);

        };

        age  = 30;

        block();//10

}

终端中切换到项目的.m文件路径，执行  clang -rewrite-objc main.m ，会得到一个对应的main.cpp文件，文件蛮大，3M多，直接到文件的最后，会看到这样的代码

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

int a = 10;
int b = 20;

int c ;
c = a + b;

NSLog((NSString *)&__NSConstantStringImpl__var_folders_9p_zm9fm86j06j4ng_47_8jbbd40000gn_T_main_1ce142_mi_1,name);

NSLog((NSString *)&__NSConstantStringImpl__var_folders_9p_zm9fm86j06j4ng_47_8jbbd40000gn_T_main_1ce142_mi_2,age);
}
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->name, (void*)src->name, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->name, 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[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;

int age = 10;
NSString *name = (NSString *)&__NSConstantStringImpl__var_folders_9p_zm9fm86j06j4ng_47_8jbbd40000gn_T_main_1ce142_mi_0;
void (*block)() = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, name, age, 570425344));
age = 30;
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block); //block的执行

}
return 0;
}
static struct IMAGE_INFO { unsigned version; unsigned flag; } _OBJC_IMAGE_INFO = { 0, 2 };

其中 __block_impl 是一个block的内部结构，定义如下：

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

每个block会生成对应的结构体__main_block_impl_0 ，其中有个构造函数如下，

 __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, NSString *_name, int _age, int flags=0) : name(_name), age(_age）

 block内部访问的外部变量在这里都是值类型，看到这里应该很明白了吧！这也是为什么在block体里不能对外部变量进行修改。

如果要修改，需要加上__block，__block int age = 10，在看

struct __Block_byref_age_0 {
void *__isa;
__Block_byref_age_0 *__forwarding;
int __flags;
int __size;
int age;
};

struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
NSString *name;
__Block_byref_age_0 *age; // by ref
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, NSString *_name, __Block_byref_age_0 *_age, int flags=0) : name(_name), age(_age->__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_age_0 *age = __cself->age; // bound by ref
NSString *name = __cself->name; // bound by copy

int a = 10;
int b = 20;

int c ;
c = a + b;

NSLog((NSString *)&__NSConstantStringImpl__var_folders_9p_zm9fm86j06j4ng_47_8jbbd40000gn_T_main_f23869_mi_1,name);

NSLog((NSString *)&__NSConstantStringImpl__var_folders_9p_zm9fm86j06j4ng_47_8jbbd40000gn_T_main_f23869_mi_2,(age->__forwarding->age));
}
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->name, (void*)src->name, 3/*BLOCK_FIELD_IS_OBJECT*/);_Block_object_assign((void*)&dst->age, (void*)src->age, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->name, 3/*BLOCK_FIELD_IS_OBJECT*/);_Block_object_dispose((void*)src->age, 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[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;

__attribute__((__blocks__(byref))) __Block_byref_age_0 age = {(void*)0,(__Block_byref_age_0 *)&age, 0, sizeof(__Block_byref_age_0), 10};
NSString *name = (NSString *)&__NSConstantStringImpl__var_folders_9p_zm9fm86j06j4ng_47_8jbbd40000gn_T_main_f23869_mi_0;
void (*block)() = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, name, (__Block_byref_age_0 *)&age, 570425344));
(age.__forwarding->age) = 30;
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
}
return 0;
}
static struct IMAGE_INFO { unsigned version; unsigned flag; } _OBJC_IMAGE_INFO = { 0, 2 };

在这里可以看到，对加了block的外部变量会生成其对应的__Block_byref_age_0,传递的参数也变成了地址(__Block_byref_age_0 *)&age，变成了引用类型， 这样当外部变量被修改后，block中也被修改。