iOS Block Part5:block捕获__block对象

__block修饰对象类型

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

编译结果:

block(__block NSString)_code.png

结构示意图:

block(__block NSString)_map.png

__block修饰的对象数据类型的变量也能实现block内外同呼吸共患难,iOS Block Part4已经对同呼吸共患难的原理分析过了,这里就不做赘述了

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

前三种block捕获的数据类型我们已经讲过,而本篇的__block NSString * any是这四种中最难.当然它也是最后的boss,把它学会了blcok也就快通关了.

对比block(__block int与block(__block NSString * any) 编译生成的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;
}

看上去编译生成的struct __main_block_impl_0结构是一样的.
其实不然,struct __main_block_impl_0的子结构struct __Block_byref_any_0双方有很大的不一样.

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)编译所得的struct __Block_byref_any_0多出两个方法:

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

以上两个方法对应的是:

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);
}

这两个方法的作用是:block(__block NSString * any)编译生成的struct __Block_byref_any_0->NSString *any拷贝与销毁.关于拷贝的具体流程后面会说.现在做一个关于拷贝的梳理:

block(int any)
struct __main_block_impl_0需要拷贝

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

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

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

这是iOS Block Part2到iOS Block Part5所讲编译结果中所有提到的需要拷贝的地方.各种拷贝的具体实现请看下文.

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参考文献:
Block技巧与底层解析 by tripleCC