objective-c：block

block是NSObject对象

​ 我们将object-c代码转换成c++，看看block的结构。执行 $ xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc -fobjc-arc main.m 将如下代码生成c++文件。

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        int a  = 10;
        void (^block)(void) = ^(void){
            NSLog(@"%d",a);
        };

        block();

        NSLog(@"%@", [block class]);

        NSLog(@"%@", [[block class] superclass]);

        NSLog(@"%@", [[[block class] superclass] superclass]);

        NSLog(@"%@", [[[[block class] superclass] superclass] superclass]);
    }
    return 0;
}

关键部分如下：

int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        int a = 10;
        void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, a));

        ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);

        NSLog((NSString *)&__NSConstantStringImpl__var_folders_6p_k8pdm4dd3831fkms4mrm7_7w0000gn_T_main_2e3b50_mi_1, ((Class (*)(id, SEL))(void *)objc_msgSend)((id)block, sel_registerName("class")));

        NSLog((NSString *)&__NSConstantStringImpl__var_folders_6p_k8pdm4dd3831fkms4mrm7_7w0000gn_T_main_2e3b50_mi_2, ((Class (*)(id, SEL))(void *)objc_msgSend)((id)((Class (*)(id, SEL))(void *)objc_msgSend)((id)block, sel_registerName("class")), sel_registerName("superclass")));

        NSLog((NSString *)&__NSConstantStringImpl__var_folders_6p_k8pdm4dd3831fkms4mrm7_7w0000gn_T_main_2e3b50_mi_3, ((Class (*)(id, SEL))(void *)objc_msgSend)((id)((Class (*)(id, SEL))(void *)objc_msgSend)((id)((Class (*)(id, SEL))(void *)objc_msgSend)((id)block, sel_registerName("class")), sel_registerName("superclass")), sel_registerName("superclass")));

        NSLog((NSString *)&__NSConstantStringImpl__var_folders_6p_k8pdm4dd3831fkms4mrm7_7w0000gn_T_main_2e3b50_mi_4, ((Class (*)(id, SEL))(void *)objc_msgSend)((id)((Class (*)(id, SEL))(void *)objc_msgSend)((id)((Class (*)(id, SEL))(void *)objc_msgSend)((id)((Class (*)(id, SEL))(void *)objc_msgSend)((id)block, sel_registerName("class")), sel_registerName("superclass")), sel_registerName("superclass")), sel_registerName("superclass")));
    }
    return 0;
}

先看block被调用的部分：((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);。它将block指针转成了__block_impl * 类型。

而在__main_block_impl_0的构造方法中，如下：

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int a;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _a, int flags=0) : a(_a) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

可以看到impl.isa = &_NSConcreteStackBlock这行代码中有isa指针，也就是说block指针指向的结构体有isa指针。我们再看看这部分代码：

  NSLog(@"%@", [block class]);

  NSLog(@"%@", [[block class] superclass]);

    NSLog(@"%@", [[[block class] superclass] superclass]);

    NSLog(@"%@", [[[[block class] superclass] superclass] superclass]);

它的打印结果如下：

2019-06-18 10:51:58.747875+0800 Block[71723:27889252] __NSMallocBlock__
2019-06-18 10:51:58.747891+0800 Block[71723:27889252] __NSMallocBlock
2019-06-18 10:51:58.747905+0800 Block[71723:27889252] NSBlock
2019-06-18 10:51:58.747917+0800 Block[71723:27889252] NSObject

可以看出，block是一个NSObject对象。它继承至NSBlock，而NSBlock继承至NSObject。

Block对变量的捕获

1、局部变量

​ 对于局部变量，block捕获的是值，也就是值传递：

int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        int a = 10;
        void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, a));
    }
    return 0;
}

2、静态变量

​ 对于static修饰的变量，block捕获的是变量的地址，也就是地址传递。普通指针也是值传递：

int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        static int a = 10;
        void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, &a));
    }
    return 0;
}

3、NSObject类型变量

​ 对于NSObject类型变量，block是地址捕获，默认是__strong类型：

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  NSObject *__strong obj;//强引用
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, NSObject *__strong _obj, int flags=0) : obj(_obj) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        NSObject *obj = ((NSObject *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSObject"), sel_registerName("new"));
        void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, obj, 570425344));
    }
    return 0;
}

所以，这种情况下，外部始终强引用block，会使block捕获的对象内存泄漏。

4、__block

​ __block int a = 10;转成c++，变成__attribute__((__blocks__(byref))) __Block_byref_a_0 a = {(void*)0,(__Block_byref_a_0 *)&a, 0, sizeof(__Block_byref_a_0), 10};。可以看出__block修饰的局部变量会封装成__Block_byref_a_0类型的结构体。而__Block_byref_a_0的结构如下：

struct __Block_byref_a_0 {
  void *__isa;
__Block_byref_a_0 *__forwarding;
 int __flags;
 int __size;
 int a;
};

block捕获的是参数a（__Block_byref_a_0结构体）中__forwarding参数的地址。

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_a_0 *a; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_a_0 *_a, int flags=0) : a(_a->__forwarding) {//_a是__Block_byref_a_0结构体的地址，这里捕获的是_a的__forwarding参数的地址
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

修改a的值的代码：a = 20转成c++，变成(a->__forwarding->a) = 20;。也就是说，修改 a的值变成修改__Block_byref_a_0结构体中a变量的值。

5、__weak

​ 用$ xcrun -sdk iphoneos clang -rewrite-objc -F UIKit -fobjc-arc -arch arm64 -stdlib=libc++ -mmacosx-version-min=10.7 -fobjc-runtime=macosx-10.7 -Wno-deprecated-declarations main.m命令将下面代码转成c++。

#import 

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSObject *obj = [NSObject new];
        __weak typeof(obj) weakObj = obj;
        void (^block)(void) = ^(void){
            __strong typeof(weakObj) strongObj = weakObj;
            NSLog(@"%@",strongObj);
        };
    }
    return 0;
}

关键代码如下：

static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  NSObject *__weak weakObj = __cself->weakObj; // bound by copy

            __attribute__((objc_ownership(strong))) typeof(weakObj) strongObj = weakObj;
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_6p_k8pdm4dd3831fkms4mrm7_7w0000gn_T_main_494bdc_mi_0,strongObj);
        }

int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        NSObject *obj = ((NSObject *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSObject"), sel_registerName("new"));
        __attribute__((objc_ownership(weak))) typeof(obj) weakObj = obj;
        void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, weakObj, 570425344));
    }
    return 0;
}

从void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, weakObj, 570425344));这一句可以看出block捕获的是weakObj。也就是一个弱引用。

而在__main_block_func_0(调用block执行的方法)中又将weakObj转换成了strongObj。

Block有三种类型

​ Block有三种类型，分别为__NSGlobalBlock__、__NSMallocBlock__和__NSStackBlock__。它们都继承至NSBlock。__NSGlobalBlock__存储在数据段， __NSMallocBlock__存储在堆中，__NSStackBlock__存储在栈中。