Objective-C的Block的实现原理

iOS在4.0之后就提供了对Block的支持.下图是苹果文档上的一张图,解释了Block的基本语法.

参考这篇文章http://blog.csdn.net/jasonblog/article/details/7756763,我依样画葫芦试试.

#include <stdio.h>int main(){return 0;}

使用clang处理之后的文件多了很多东西.从中我们能发现一个结构体的定义:

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

根据objc的命名规则,能看的出来这就是block的实现.FuncPtr应该是指向需要调用的函数.

接下来,写一个简单的Block:

#include <stdio.h>int main(){void(^blk)(void) = ^{ printf("Hello,block!\n"); };blk();return 0;}

使用clang处理之后,多了几行重要的代码:

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) { printf("Hello,block!\n"); }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)};

这里最开始定义了一个新的结构体__main_block_impl_0,他有2个成员变量以及一个构造函数.

其次定义了一个函数__main_block_func_0,再下来定义了新结构体__main_block_desc_0,并定义了该结构体的一个对象.

从这些新结构体以及函数的名字可以看出,是以出现函数+序列为命名依据.

接下来,我们看看main中对block的调用:

int main(){ void(*blk)(void) = (void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA); ((void (*)(__block_impl *))((__block_impl *)blk)->FuncPtr)((__block_impl *)blk); return 0;}

嘿嘿,是不是很头大,我也是研究了一会才有点头绪...总而言之,就是调用__main_block_impl_0的构造函数,生成一个block对象,再调用其FuncPtr指向的函数.

现在我们来试试访问局部变量:

#include <stdio.h>int main(){int i = 1024;void(^blk)(void) = ^{ printf("%d\n", i); };blk();return 0;}

处理之后:

struct __main_block_impl_0 {  struct __block_impl impl;  struct __main_block_desc_0* Desc;  int i;  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _i, int flags=0) : i(_i) {    impl.isa = &_NSConcreteStackBlock;    impl.Flags = flags;    impl.FuncPtr = fp;    Desc = desc;  }};static void __main_block_func_0(struct __main_block_impl_0 *__cself) {  int i = __cself->i; // bound by copy printf("%d\n", i); }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 i = 1024; void(*blk)(void) = (void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, i); ((void (*)(__block_impl *))((__block_impl *)blk)->FuncPtr)((__block_impl *)blk); return 0;}

可以看到__main_block_impl_0中多了一个i的成员变量.

接下来,我们试图在函数中更改i的值:

#include <stdio.h>int main(){int i = 1024;void(^blk)(void) = ^{ i = 0; printf("%d\n", i); };blk();return 0;}

clang时的错误提示:

可以看出clang提示需要添加__block标示符,从Block常参与的回调类工作可以解释这个问题:当使用Block作为回调处理时,假设定义该Block对象的FuncA中有一个叫v的局部变量,并且将v作为Block的参数传入.那么,如果在Block的执行在FuncA已经执行完成之后,那么FuncA的栈已经被回收,作为FuncA的局部变量的v也显然被释放了,所以在Block中直接修改auto类型的变量是不正确的.

如果我们将i设置为静态变量,就能正确通过处理,因为静态变量处于静态存储区,不存在栈释放后非法访问的问题.

#include <stdio.h>int main(){__block int i = 1024;void(^blk)(void) = ^{ i = 0; printf("%d\n", i); };blk();return 0;}

处理后:

struct __main_block_impl_0 {  struct __block_impl impl;  struct __main_block_desc_0* Desc;  int *i;  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int *_i, int flags=0) : i(_i) {    impl.isa = &_NSConcreteStackBlock;    impl.Flags = flags;    impl.FuncPtr = fp;    Desc = desc;  }};static void __main_block_func_0(struct __main_block_impl_0 *__cself) {  int *i = __cself->i; // bound by copy (*i) = 0; printf("%d\n", (*i)); }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(){ static int i = 1024; void(*blk)(void) = (void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, &i); ((void (*)(__block_impl *))((__block_impl *)blk)->FuncPtr)((__block_impl *)blk); return 0;}

现在,我们看看__block标示符处理的变量:

#include <stdio.h>int main(){__block int i = 1024;void(^blk)(void) = ^{ i = 0; printf("%d\n", i); };blk();return 0;}

处理后:

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) = 0; printf("%d\n", (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(){ __attribute__((__blocks__(byref))) __Block_byref_i_0 i = {(void*)0,(__Block_byref_i_0 *)&i, 0, sizeof(__Block_byref_i_0), 1024}; void(*blk)(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 *)blk)->FuncPtr)((__block_impl *)blk); return 0;}

可以很明显的看出,多了很多东西,第一个就是__block标示符修饰的变量对应的结构体

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

拥有一个__isa指针,以及一个指向自身类对象的指针__forwarding,而i则用保存数据.

在main函数中,我们能看到__Block_byref_i_0的初始化:

__attribute__((__blocks__(byref))) __Block_byref_i_0 i = {(void*)0,(__Block_byref_i_0 *)&i, 0, sizeof(__Block_byref_i_0), 1024};

可以发现__forwarding指针指向了自己.

同时多了两个新函数:

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*/);}

而__main_block_desc_0结构体多了copy和dispose两个成员变量分别指向以上两个函数.