关于 block 持有 self 的探究

前言

我们知道 block 中使用 self.xxx 的时候，block 会持有 self。

那么假如我们通过 _xxx 直接引用属性变量是否可以避免循环引用呢？

编写测试代码

(完整代码在最后)

- (void)startDemo
{
    // 通过 self.xxx 方式访问
    void (^block0)() = ^{
        NSLog(@"zhangqingyu %@", self.value1);
    };
    block0();

    // 通过 _xxx 方式访问
    void (^block1)() = ^{
        NSLog(@"zhangqingyu %@", _value2);
    };
    block1();

    // 临时变量
    void (^block2)() = ^{
        NSString *value3 = @"Fucking block!";
        NSLog(@"zhangqingyu %@", value3);
    };
    block2();
}

了解 Clang 命令

Clang 是一个C、C++、Objective-C和Objective-C++编程语言的编译器前端。

它采用了底层虚拟机（LLVM）作为其后端。

它的目标是提供一个GNU编译器套装（GCC）的替代品。

使用 Clang 编译上面的 Demo

clang -fobjc-arc -framework Foundation main.m -o test

执行结果

执行结果

clang -fobjc-arc -framework Foundation -rewrite-objc main.m

执行结束后，可以看到多出一个 cpp 文件

cpp文件阅读

通过 self.xxx 方式访问

通过源码，我们可以找到 我们在 OC 中的 Block 被编译成以下的结构体。

结构体中可以看到一条属性QYBlockDemo *const __strong self;

这也解释了为什么当 self 持有 block ，block 引用 self 会引起循环引用。

struct __QYBlockDemo__startDemo_block_impl_0 {
  struct __block_impl impl;
  struct __QYBlockDemo__startDemo_block_desc_0* Desc;
  QYBlockDemo *const __strong self;
  __QYBlockDemo__startDemo_block_impl_0(void *fp, struct __QYBlockDemo__startDemo_block_desc_0 *desc, QYBlockDemo *const __strong _self, int flags=0) : self(_self) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

static void __QYBlockDemo__startDemo_block_func_0(struct __QYBlockDemo__startDemo_block_impl_0 *__cself) {
  QYBlockDemo *const __strong self = __cself->self; // bound by copy

        NSLog((NSString *)&__NSConstantStringImpl__var_folders_9w_6_s3z1fs4mg055_c5w4008pw0000gn_T_main_3c98bd_mi_2, ((NSString *(*)(id, SEL))(void *)objc_msgSend)((id)self, sel_registerName("value1")));
    }

通过 _xxx 方式访问

可以看到 block 仍然持有了 self

通过这种方式并不能避免循环引用

struct __QYBlockDemo__startDemo_block_impl_1 {
  struct __block_impl impl;
  struct __QYBlockDemo__startDemo_block_desc_1* Desc;
  QYBlockDemo *const __strong self;
  __QYBlockDemo__startDemo_block_impl_1(void *fp, struct __QYBlockDemo__startDemo_block_desc_1 *desc, QYBlockDemo *const __strong _self, int flags=0) : self(_self) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

...

static void __QYBlockDemo__startDemo_block_func_1(struct __QYBlockDemo__startDemo_block_impl_1 *__cself) {
  QYBlockDemo *const __strong self = __cself->self; // bound by copy

        NSLog((NSString *)&__NSConstantStringImpl__var_folders_9w_6_s3z1fs4mg055_c5w4008pw0000gn_T_main_3c98bd_mi_3, (*(NSString *__strong *)((char *)self + OBJC_IVAR_$_QYBlockDemo$_value2)));
    }

临时变量

正常的临时变量时 不会持有 self

struct __QYBlockDemo__startDemo_block_impl_2 {
  struct __block_impl impl;
  struct __QYBlockDemo__startDemo_block_desc_2* Desc;
  __QYBlockDemo__startDemo_block_impl_2(void *fp, struct __QYBlockDemo__startDemo_block_desc_2 *desc, int flags=0) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

...

static void __QYBlockDemo__startDemo_block_func_2(struct __QYBlockDemo__startDemo_block_impl_2 *__cself) {

        NSString *value3 = (NSString *)&__NSConstantStringImpl__var_folders_9w_6_s3z1fs4mg055_c5w4008pw0000gn_T_main_3c98bd_mi_4;
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_9w_6_s3z1fs4mg055_c5w4008pw0000gn_T_main_3c98bd_mi_5, value3);
    }

结论

• 在 block 中使用 self, 会使 block 持有 self
• 通过下划线_xxx的方式直接访问属性变量，仍然会使 block 持有 self。

完整代码

#import <Foundation/Foundation.h>

@interface QYBlockDemo : NSObject

@property (nonatomic, strong) NSString *value1;

@property (nonatomic, strong) NSString *value2;

- (void)startDemo;

@end

@implementation QYBlockDemo

- (id)init
{
    self = [super self];
    if (self)
    {
        _value1 = @"Hello, World!";
        _value2 = @"Hello, Objective-C!";
    }
    return self;
}

- (void)startDemo
{
    // 通过 self.xxx 方式访问
    void (^block0)() = ^{
        NSLog(@"zhangqingyu %@", self.value1);
    };
    block0();

    // 通过 _xxx 方式访问
    void (^block1)() = ^{
        NSLog(@"zhangqingyu %@", _value2);
    };
    block1();

    // 临时变量
    void (^block2)() = ^{
        NSString *value3 = @"Fucking block!";
        NSLog(@"zhangqingyu %@", value3);
    };
    block2();
}

@end

int main (int argc, const char * argv[])
{
    @autoreleasepool
    {
        QYBlockDemo *demo = [[QYBlockDemo alloc] init];
        [demo startDemo];
    }
    return 0;
}