前文中所列舉的是最簡單的block铅忿，方便先理解block的結(jié)構(gòu)，下面講解一下在block中使用外部變量與block的作用域。
1.截獲自動(dòng)變量
OC源碼：
int i = 1;
void (^bBlock)() = ^{
NSLog(@"%d",i);
};
i=2;
bBlock();//輸出1,說明了在block聲明時(shí)已經(jīng)將自動(dòng)變量值復(fù)制到了block變量中,所以以后再修改被截獲的值時(shí)也無法影響到block的執(zhí)行部分了
轉(zhuǎn)換后的C源碼：
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截獲自動(dòng)變量
NSLog((NSString *)&__NSConstantStringImpl__var_folders_nf_mb711n6121z6123jpp8tbqrr0000gn_T_main_107bae_mi_0,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 argc, char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
int i = 1;
void (*bBlock)() = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, i));
i=2;
((void (*)(__block_impl *))((__block_impl *)bBlock)->FuncPtr)((__block_impl *)bBlock);//6.block調(diào)用
//(*bBlock->impl.FuncPtr)(bBlock)
}
return 0;
}
根據(jù)與前文中的代碼比較斑唬，可以發(fā)現(xiàn)，block的執(zhí)行函數(shù)中調(diào)用了外部變量i，轉(zhuǎn)換后的C源碼中恕刘，在構(gòu)造__main_block_impl_0結(jié)構(gòu)體時(shí)缤谎，就已經(jīng)將i的值傳遞進(jìn)去了，所以并不是在執(zhí)行block時(shí)才去獲取的i值褐着，而是聲明的時(shí)候就已經(jīng)將引用的外部變量值copy進(jìn)block了坷澡。所以也可以解釋為什么在block聲明后修改了i的值但是輸出的block結(jié)果還是之前獲取到的i值了。這就是截獲自動(dòng)變量的概念含蓉。
如果試圖在block中修改截取到的自動(dòng)變量的值频敛，如：
int i = 1;
void (^bBlock)() = ^{
i = i + 1；
NSLog(@"%d",i);
};
編譯器會(huì)給出
這個(gè)錯(cuò)誤馅扣，提示缺少了_ _block修飾符斟赚，有關(guān)__block修飾符的稍后在講，現(xiàn)在需要明確的就是block可以截獲自動(dòng)變量的值差油，且無法修改拗军。
但對(duì)于這三種變量是可以修改的，分別是全局變量厌殉，靜態(tài)全局變量和靜態(tài)變量
如：
int globalData = 1;
static int staticGlobalData = 1;
int main(int argc, char * argv[]) {
@autoreleasepool {
static int j = 1;
void (^bBlock)() = ^{
j = j+1;
globalData =globalData + 1;
staticGlobalData = staticGlobalData + 1;
NSLog(@"j=%d",j);
NSLog(@"globalData=%d",globalData);
NSLog(@"staticGlobalData=%d",staticGlobalData);
};
bBlock();
}
//輸出：
//j=2
//globalData=2
//staticGlobalData=2
對(duì)于全局變量與靜態(tài)全局變量而言由于作用域是全局食绿，所以可以在block中修改其值，但對(duì)于靜態(tài)變量而言公罕，轉(zhuǎn)換后的代碼如下：
int globalData = 1;
static int staticGlobalData = 1;
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
int *j;
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int *_j, int flags=0) : j(_j) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
int *j = __cself->j; // bound by copy注意器紧！
(*j) = (*j)+1;
globalData =globalData + 1;
staticGlobalData = staticGlobalData + 1;
NSLog((NSString *)&__NSConstantStringImpl__var_folders_nf_mb711n6121z6123jpp8tbqrr0000gn_T_main_0b992e_mi_0,(*j));
NSLog((NSString *)&__NSConstantStringImpl__var_folders_nf_mb711n6121z6123jpp8tbqrr0000gn_T_main_0b992e_mi_1,globalData);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_nf_mb711n6121z6123jpp8tbqrr0000gn_T_main_0b992e_mi_2,staticGlobalData);
}
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, char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
static int j = 1;
void (*bBlock)() = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, &j));
((void (*)(__block_impl *))((__block_impl *)bBlock)->FuncPtr)((__block_impl *)bBlock);
}
return 0;
}
在代碼中標(biāo)注“注意”的地方即可看到區(qū)別，當(dāng)block中訪問的自動(dòng)變量為靜態(tài)變量時(shí)楼眷，加入到__main_block_func_0結(jié)構(gòu)體中的為其指針铲汪，所以可以通過(*j) = (*j)+1;的方式來修改靜態(tài)變量的值，這就是超出變量作用域時(shí)使用變量的最簡單的方法罐柳。
2.__block說明符
對(duì)于__block說明符掌腰，我們都知道加上了它就可以在block中修改引用的外部變量的值，但是具體是怎么實(shí)現(xiàn)的呢张吉？下面附上源碼說明
OC源碼：
__block int i = 1;
void (^bBlock)() = ^{
NSLog(@"%d",i);
i=3;
NSLog(@"%d",i);
};
i=2;
bBlock();//輸出2齿梁，3,__block關(guān)鍵字復(fù)制的是自動(dòng)變量的指針
C源碼：
struct __Block_byref_i_0 {//標(biāo)注3
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; //標(biāo)注1
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_i_0 *_i, int flags=0) : i(_i->__forwarding) {//標(biāo)注2
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
NSLog((NSString *)&__NSConstantStringImpl__var_folders_nf_mb711n6121z6123jpp8tbqrr0000gn_T_main_37e40a_mi_0,(i->__forwarding->i));
(i->__forwarding->i)=3;//標(biāo)注4
NSLog((NSString *)&__NSConstantStringImpl__var_folders_nf_mb711n6121z6123jpp8tbqrr0000gn_T_main_37e40a_mi_1,(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};//標(biāo)注5
int main(int argc, char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
__attribute__((__blocks__(byref))) __Block_byref_i_0 i = {(void*)0,(__Block_byref_i_0 *)&i, 0, sizeof(__Block_byref_i_0), 1};
void (*bBlock)() = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_i_0 *)&i, 570425344));
(i.__forwarding->i)=2;
((void (*)(__block_impl *))((__block_impl *)bBlock)->FuncPtr)((__block_impl *)bBlock);
}
return 0;
}
從輸出結(jié)果來看，加上了_ _block說明符后肮蛹，就好像變量的作用域包含了block內(nèi)勺择，再來看轉(zhuǎn)換出來的C源碼：
標(biāo)注1處可以看到，對(duì)比未加 __block說明符伦忠，在__main_block_impl_0結(jié)構(gòu)體重多出了__Block_byref_i_0類型的i變量省核。
標(biāo)注2處可以看到在構(gòu)造函數(shù)中，將i的__forwarding指針傳遞進(jìn)了__main_block_impl_0結(jié)構(gòu)體昆码，而不是像之前未加__block時(shí)傳遞的僅為簡單的值气忠。
標(biāo)注3處來詳細(xì)的看__Block_byref_i_0結(jié)構(gòu)體邻储，這就是__block變量變?yōu)榈慕Y(jié)構(gòu)體實(shí)例。其中int i相當(dāng)于變量i的值也就是被使用的值旧噪，__forwarding指針持有指向該實(shí)例自身的指針吨娜，通過成員變量__forwarding來訪問成員變量。所以在之后的main函數(shù)中舌菜，對(duì)i值進(jìn)行修改時(shí)(i.__forwarding->i)=2;萌壳，也是通過__forwarding指針來實(shí)現(xiàn)的，也就是說用__block說明符聲明變量后日月，這個(gè)變量已經(jīng)變成了一個(gè)block變量袱瓮，無論是否在block內(nèi)調(diào)用，對(duì)于這個(gè)變量的操作都得通過它自己約定的__forwarding來訪問爱咬，也就可實(shí)現(xiàn)無論是否在block的函數(shù)中尺借，都可正常修改i值。
