- Stack (non-static) variables local to the enclosing lexical scope are captured as const variables.Their values are taken at the point of the block expression within the program. In nested blocks, the value is captured from the nearest enclosing scope.
void main1() { char *_para1="a"; printf("init _para1:%s,%p,%p\n",_para1,_para1,&_para1); void(^testBlock)(void)=^{ printf("exute _para1:%s,%p,%p\n",_para1,_para1,&_para1); }; _para1="b"; printf("before _para1:%s,%p,%p\n",_para1,_para1,&_para1); testBlock(); printf("after _para1:%s,%p,%p\n",_para1,_para1,&_para1); }
输出一下结果：
init _para1:a,0x97f70,0x27d70988 before _para1:b,0x97fa5,0x27d70988 exute _para1:a,0x97f70,0x165839e4 after _para1:b,0x97fa5,0x27d70988
struct __block_impl { void *isa; int Flags; int Reserved; void *FuncPtr; };
struct __main1_block_impl_0 { struct __block_impl impl; struct __main1_block_desc_0* Desc; char *_para1; __main1_block_impl_0(void *fp, struct __main1_block_desc_0 *desc, char *__para1, int flags=0) : _para1(__para1) { impl.isa = &_NSConcreteStackBlock; impl.Flags = flags; impl.FuncPtr = fp; Desc = desc; } };
//block实现的函数
static void __main1_block_func_0(struct __main1_block_impl_0 *__cself) { char *_para1 = __cself->_para1; // bound by copy printf("exute _para1:%s,%p,%p\n",_para1,_para1,&_para1); }
//block对象的描述信息（大小等等）
static struct __main1_block_desc_0 { size_t reserved; size_t Block_size; } __main1_block_desc_0_DATA = { 0, sizeof(struct __main1_block_impl_0)};
//这是objc测试函数test
void main1() { char *_para1="a"; printf("init _para1:%s,%p,%p\n",_para1,_para1,&_para1); void(*testBlock)(void)=(void (*)())&__main1_block_impl_0((void *)__main1_block_func_0, &__main1_block_desc_0_DATA, _para1); _para1="b"; printf("before _para1:%s,%p,%p\n",_para1,_para1,&_para1); ((void (*)(__block_impl *))((__block_impl *)testBlock)->FuncPtr)((__block_impl *)testBlock); printf("after _para1:%s,%p,%p\n",_para1,_para1,&_para1); }
简单分析block截获变量：
void(*testBlock)(void)=(void (*)())&__main1_block_impl_0((void *)__main1_block_func_0, &__main1_block_desc_0_DATA, _para1);
: _para1(__para1)
之后又对_para1做了修改，重新指向了 0x97fa5这块内存，但是不会影响block结构体成员_para1,因为成员_para1指向的是0x97f70。
((void (*)(__block_impl *))((__block_impl *)testBlock)->FuncPtr)((__block_impl *)testBlock);
其实还是调用了block对象里的函数对象（_block_imp1）的函数指针（FuncPtr） 所指向的函数__main1_block_func_0，并把block自己作为参数传递进去。
static void __main1_block_func_0(struct __main1_block_impl_0 *__cself) {
char *_para1 = __cself->_para1; // bound by copy
printf("exute _para1:%s,%p,%p\n",_para1,_para1,&_para1);
}
值得注意的的是，我们在执行之前对_para1修改，但是并没有对block内部产生影响（只是把局部变量_para1重新指向另一块内存（0x97fa5）），而在构造block的时候，block对象内部对截获的变量做了引用（引用的是0x97f70），放在（char *_para1）里，执行block的时候，直接调用函数__main1_block_func_0，函数内部，是引用了block结构体的成员_para1，所以会有这样的结果。
block 解析 - 局部变量,布布扣,bubuko.com
原文：http://www.cnblogs.com/NarutoYq/p/3873109.html