__block修飾對象類型

int main(int argc, const char * argv[]) {
 __block NSString * any = [NSString stringWithFormat:@"1"];
 void (^test)() = ^ {
        NSLog(@"%@",any);
 };
 test();
 return 0;
}

編譯結(jié)果:

block(__block NSString)_code.png

結(jié)構(gòu)示意圖:

block(__block NSString)_map.png

__block修飾的對象數(shù)據(jù)類型的變量也能實現(xiàn)block內(nèi)外同呼吸共患難,iOS Block Part4已經(jīng)對同呼吸共患難的原理分析過了,這里就不做贅述了

int any;
NSString * any;
__block int any;
__block NSString * any;

前三種block捕獲的數(shù)據(jù)類型我們已經(jīng)講過,而本篇的__block NSString * any是這四種中最難.當(dāng)然它也是最后的boss,把它學(xué)會了blcok也就快通關(guān)了.

對比block(__block int與block(__block NSString * any) 編譯生成的struct __main_block_impl_0

block(__block int)

struct __main_block_impl_0 {
 struct __block_impl impl;
 struct __main_block_desc_0* Desc;
 __Block_byref_any_0 *any;
}

block(__block NSString * any)

struct __main_block_impl_0 {
 struct __block_impl impl;
 struct __main_block_desc_0* Desc;
 __Block_byref_any_0 *any;
}

看上去編譯生成的struct __main_block_impl_0結(jié)構(gòu)是一樣的.
其實不然,struct __main_block_impl_0的子結(jié)構(gòu)struct __Block_byref_any_0雙方有很大的不一樣.

block(__block int)

struct __Block_byref_any_0 {
    void *__isa;
    __Block_byref_any_0 *__forwarding;
 int __flags;
 int __size;
 int any;
};

block(__block NSString * any)

struct __Block_byref_any_0 {
    void *__isa;
    __Block_byref_any_0 *__forwarding;
 int __flags;
 int __size;
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 NSString *any;
};

block(__block NSString * any)編譯所得的struct __Block_byref_any_0多出兩個方法:

 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);

以上兩個方法對應(yīng)的是:

static void __Block_byref_id_object_copy_131(void *dst, void *src) {
 _Block_object_assign((char*)dst + 40, *(void * *) ((char*)src + 40), 131);
}

static void __Block_byref_id_object_dispose_131(void *src) {
 _Block_object_dispose(*(void * *) ((char*)src + 40), 131);
}

這兩個方法的作用是:block(__block NSString * any)編譯生成的struct __Block_byref_any_0->NSString *any拷貝與銷毀.關(guān)于拷貝的具體流程后面會說.現(xiàn)在做一個關(guān)于拷貝的梳理:

block(int any)
struct __main_block_impl_0需要拷貝

block(NSString * any)
struct __main_block_impl_0需要拷貝
NSString * any需要拷貝

block(__block int any)
struct __main_block_impl_0需要拷貝
struct __Block_byref_any_0需要拷貝

block(__block NSString * any)
struct __main_block_impl_0需要拷貝
struct __Block_byref_any_0需要拷貝
NSString * any需要拷貝

這是iOS Block Part2到iOS Block Part5所講編譯結(jié)果中所有提到的需要拷貝的地方.各種拷貝的具體實現(xiàn)請看下文.

參考文獻(xiàn):
Block技巧與底層解析 by tripleCC