转载

Weak-Strong-Dance真的安全吗？

绝大多数iOS开发者用过 block ，并且知道用 __weak 的方式去解决循环引用的问题。而进阶一些的开发者则了解 Weak-Strong-Dance ，那么什么是 Weak-Strong-Dance ？它能保证block执行是的 “安全” 吗？

Weak-Strong-Dance

看看下面两段代码的区别，你就明白什么是 Weak-Strong-Dance 了。

- (void)test {
    __weak typeof(self) weakSelf = self;
    self.block = ^{
        [weakSelf copy];
    };
}

- (void)test {
    __weak typeof(self) weakSelf = self;
    self.block = ^{
        __strong typeof(self) strongSelf = weakSelf;
        [strongSelf copy];
    };
}

也就是在用 __weak 解决循环引用的前提下，在 block 内部用 __strong 持有对象，试图解决 “在多线程下，可能weakSelf指向的对象会在 Block 执行前被废弃，导致各种各样的问题，比如说KVO，传入nil可是会crash呢” ，如下代码

__weak typeof(self) weakSelf = self;
self.handler = ^{
    typeof(weakSelf) strongSelf = weakSelf;
    [strongSelf.obserable removeObserver:strongSelf
                              forKeyPath:kObservableProperty];
};

此时，你可能会这样认为， self 所指向对象的引用计数变成 2，即使主线程中的 self 因为超出作用于而释放，对象的引用计数依然为 1，避免了对象的销毁。

思维纠正

它真的能解决在多线程下，可能 weakSelf 指向的对象会在 Block 执行前被废弃而导致的问题吗？

答案当然是否定的，让我们来看看demo：

不用 Weak-Strong-Dance ：

#import "TestBlock.h"

@interface TestBlock ()

@property (nonatomic, strong) dispatch_block_t block;

@end

@implementation TestBlock

- (void)test {
    __weak typeof(self) weakSelf = self;
    self.block = ^{
        [weakSelf copy];
    };
}

@end

看看用clang改写后的代码，这里就只贴关键代码了：

// @interface TestBlock ()

// @property (nonatomic, strong) dispatch_block_t block;

/* @end */


// @implementation TestBlock


  struct __TestBlock__test_block_impl_0 {
  struct __block_impl impl;
  struct __TestBlock__test_block_desc_0* Desc;
  TestBlock *const __weak weakSelf;
  __TestBlock__test_block_impl_0(void *fp, struct __TestBlock__test_block_desc_0 *desc, TestBlock *const __weak _weakSelf, int flags=0) : weakSelf(_weakSelf) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __TestBlock__test_block_func_0(struct __TestBlock__test_block_impl_0 *__cself) {
  TestBlock *const __weak weakSelf = __cself->weakSelf; // bound by copy

        ((id (*)(id, SEL))(void *)objc_msgSend)((id)weakSelf, sel_registerName("copy"));
    }
static void __TestBlock__test_block_copy_0(struct __TestBlock__test_block_impl_0*dst, struct __TestBlock__test_block_impl_0*src) {_Block_object_assign((void*)&dst->weakSelf, (void*)src->weakSelf, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static void __TestBlock__test_block_dispose_0(struct __TestBlock__test_block_impl_0*src) {_Block_object_dispose((void*)src->weakSelf, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static struct __TestBlock__test_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __TestBlock__test_block_impl_0*, struct __TestBlock__test_block_impl_0*);
  void (*dispose)(struct __TestBlock__test_block_impl_0*);
} __TestBlock__test_block_desc_0_DATA = { 0, sizeof(struct __TestBlock__test_block_impl_0), __TestBlock__test_block_copy_0, __TestBlock__test_block_dispose_0};

static void _I_TestBlock_test(TestBlock * self, SEL _cmd) {
    __attribute__((objc_ownership(weak))) typeof(self) weakSelf = self;
    ((void (*)(id, SEL, dispatch_block_t))(void *)objc_msgSend)((id)self, sel_registerName("setBlock:"), ((void (*)())&__TestBlock__test_block_impl_0((void *)__TestBlock__test_block_func_0, &__TestBlock__test_block_desc_0_DATA, weakSelf, 570425344)));
}


static void(* _I_TestBlock_block(TestBlock * self, SEL _cmd) )(){ return (*(__strong dispatch_block_t *)((char *)self + OBJC_IVAR_$_TestBlock$_block)); }
static void _I_TestBlock_setBlock_(TestBlock * self, SEL _cmd, dispatch_block_t block) { (*(__strong dispatch_block_t *)((char *)self + OBJC_IVAR_$_TestBlock$_block)) = block; }
// @end

代码很长，解释下：

在 struct __TestBlock__test_block_impl_0 里头，我们能看到 TestBlock *const __weak weakSelf; 这代表在 block 内部是以 弱引用 的方式捕获 self 的，这没毛病。重点来了，看这一段代表 block 具体实现的代码块

static void __TestBlock__test_block_func_0(struct __TestBlock__test_block_impl_0 *__cself) {
  TestBlock *const __weak weakSelf = __cself->weakSelf; // bound by copy

        ((id (*)(id, SEL))(void *)objc_msgSend)((id)weakSelf, sel_registerName("copy"));
    }

这里可以看到如果此时外部废弃了 self ，的确会导致 block 内部访问成nil的情况。

那么如果用了 Weak-Strong-Dance 呢？

__weak typeof(self) weakSelf = self;
self.block = ^{
    __strong typeof(self) strongSelf = weakSelf;
    [strongSelf copy];
};

看看clang改写后会有什么区别：

  struct __TestBlock__test_block_impl_0 {
  struct __block_impl impl;
  struct __TestBlock__test_block_desc_0* Desc;
  TestBlock *const __weak weakSelf;
  __TestBlock__test_block_impl_0(void *fp, struct __TestBlock__test_block_desc_0 *desc, TestBlock *const __weak _weakSelf, int flags=0) : weakSelf(_weakSelf) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __TestBlock__test_block_func_0(struct __TestBlock__test_block_impl_0 *__cself) {
  TestBlock *const __weak weakSelf = __cself->weakSelf; // bound by copy

        __attribute__((objc_ownership(strong))) typeof(self) strongSelf = weakSelf;
        ((id (*)(id, SEL))(void *)objc_msgSend)((id)strongSelf, sel_registerName("copy"));
    }