1. __block修飾基礎類型
int main(int argc, const char * argv[]) {
__block int any = 1;
void (^test)() = ^ {
NSLog(@"%d",any);
};
test();
return 0;
}
}
老規(guī)矩顯示編譯結果,后是示意圖:
編譯結果:
結構示意圖:
block(int)
struct __main_block_impl_0 {
struct __block_impl impl;//block方法實現
struct __main_block_desc_0* Desc;//block的描述
int any;//block捕獲了外部變量
};
block(__block int)
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
__Block_byref_any_0 *any;
}
對比捕獲int與捕獲__block int的block編譯生成的struct __main_block_impl_0.
int any變成了__Block_byref_any_0 *any.
結構如下:
struct __Block_byref_any_0 {
void *__isa;
__Block_byref_any_0 *__forwarding;
int __flags;
int __size;
int any;
};
struct __Block_byref_any_0也就是實現__block修飾變量可以在block內外同呼吸,共患難的關鍵.
當然還可以注意到:
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*);
}
struct __main_block_desc_0也有拷貝和銷毀的方法.
這個拷貝的方法是供堆上struct __Block_byref_any_0拷貝用.
當然這個銷毀方法也是供堆上struct __Block_byref_any_0銷毀用的.
2. __block如何實現同呼吸,共患難
2.1 首先我們看看不帶__block,外圍變量與內部的是什么關系吧.
回看int any:
int any = 1;
void (*test)() = (
(void (*)())&__main_block_impl_0(
(void *)__main_block_func_0,
&__main_block_desc_0_DATA,
any
)
);
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _any, int flags=0) : any(_any) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
1.生成 棧上的struct __main_block_impl_0
在調用__main_block_impl_0函數時,傳入any,注意__main_block_impl_0函數末尾的any(_any),完成了_any=any,一個最簡單的C語言函數調用就完成了,生成一個struct __main_block_impl_0.
2.拷貝 棧上的struct __main_block_impl_0到堆上
前面已經說過棧上的struct __main_block_impl_0會被完整的拷貝到堆上.
block外圍的int any <值等于> 棧上struct __main_block_impl_0->int any
棧上struct __main_block_impl_0->int any <值等于> 堆上struct __main_block_impl_0->int any
所以不用__block修飾基本數據類型的變量,block內外用的變量的關系就是C語言函數調用時實參與形參的關系,其中一方值的改變對于對方沒有任何影響.
不用
__block修飾的對象數據類型的變量也是同理,就不做贅述了
2.2 帶__block,外圍變量與內部變量的是什么關系呢
回看__block int:
__attribute__((__blocks__(byref))) __Block_byref_any_0 any = {
(void*)0,
(__Block_byref_any_0 *)&any,
0,
sizeof(__Block_byref_any_0),
1
};
void (*test)() = (
(void (*)())&__main_block_impl_0(
(void *)__main_block_func_0,
&__main_block_desc_0_DATA,
(__Block_byref_any_0 *)&any,
570425344
)
);
struct __Block_byref_any_0 {
void *__isa;
__Block_byref_any_0 *__forwarding;
int __flags;
int __size;
int any;
};
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
__Block_byref_any_0 *any; // by ref
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_any_0 *_any, int flags=0) : any(_any->__forwarding) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
可以看到,因為__block的作用,int any被轉化為__Block_byref_any_0 any
而且在__main_block_impl_0調用時傳入的是__Block_byref_any_0 any結構體的地址
__Block_byref_any_0 * _any是形參,
any(_any->__forwarding)完成了_any->__forwarding指向了實參__Block_byref_any_0 * any
在看block內部使用到變量,
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
__Block_byref_any_0 *any = __cself->any; // bound by ref
NSLog((NSString *)&__NSConstantStringImpl__var_folders_qp_p2pj3jmj65n39jgl4wx9_l9w0000gn_T_main_ad101a_mi_0,(any->__forwarding->any));
}
在看block外使用到變量,
NSLog((NSString *)&__NSConstantStringImpl__var_folders_qp_p2pj3jmj65n39jgl4wx9_l9w0000gn_T_main_d1ce15_mi_1,(any.__forwarding->any));
ps:關于block外使用到變量,圖片block(__block int)_code所展示的編譯結果是沒有block外使用到變量.
block外使用到變量代碼如下,筆者就不再編譯貼出結果了,如果對block外使用到變量有異議可以用如下代碼編譯看看.
int main(int argc, const char * argv[]) {
__block int any = 1;
void (^test)() = ^ {
NSLog(@"%d",any);//block內使用到變量
};
test();
NSLog(@"%d",any);//block外使用到變量
return 0;
}
}
可以看到,自__block int any之后所有在block外用到any的地方都被換成了(any->__forwarding->any)
(any->__forwarding->any)要繞明白,前一個any是一個struct __Block_byref_any_0,后一個any是struct __Block_byref_any_0內的int any
1.生成 棧上的struct __main_block_impl_0
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
__Block_byref_any_0 *any;
}
struct __Block_byref_any_0 {
void *__isa;
__Block_byref_any_0 *__forwarding;
int __flags;
int __size;
int any;
};
注意此時struct __Block_byref_any_0內__forwarding的指向 看圖:
此時struct __Block_byref_any_0的__forwarding指向自己本身.
2.拷貝 棧上的struct __main_block_impl_0拷貝到堆上
一個struct __main_block_impl_0在棧上,
一個struct __main_block_impl_0在堆上.
看圖:
回到這個式子
(any->__forwarding->any),前一個any一定是"棧上的__Block_byref_any_0結構體any
經過拷貝生成堆block的同時,
棧上的struct __main_block_impl_0 any->__forwarding指向了堆上的struct __main_block_impl_0 any,
堆上的struct __main_block_impl_0 any->__forwarding指向自己.
block內:堆上的struct __main_block_impl_0 any->__forwarding->int any ==> 堆上struct __main_block_impl_0 any->int any;
block外:棧上的struct __main_block_impl_0 any->__forwarding->int any ==> 堆上struct __main_block_impl_0 any->int any;
一模一樣,又是同一個地址,所以就可以同呼吸共患難了!
參考文獻:
Block技巧與底層解析 by tripleCC
