传递一个__weak对象？ [英] Messaging a __weak object?

问题描述

如果我向弱对象发送消息会怎样？发送消息是否拥有该对象并将其保留在内存中直到返回？

What happens if I send a message to a weak object? Does sending the message possess the object and hold it in memory until return?

我正在考虑这种模式：

__weak MyObject *weakSelf = self;
dispatch_async(dispatch_get_main_queue(), ^{
    [weakSelf doSomeAction];
});

假设 weakSelf 是非零的消息被发送，可能在 doSomeAction 工作时被解除分配，或者保证在 doSomeAction 返回之前保持有效？

Assuming weakSelf is non-nil when the message is sent, might it be deallocated while doSomeAction is working or is it guaranteed to remain valid until doSomeAction returns?

推荐答案

来自 Clang ARC文档：

阅读 在对象左值上执行左值到右值转换时发生。

Reading occurs when performing a lvalue-to-rvalue conversion on an object lvalue.

• 对于 __弱对象，保留当前的指针，然后在当前的完整表达式结束时释放。这必须相对于赋值和指针的最终版本以原子方式执行。

• For __weak objects, the current pointee is retained and then released at the end of the current full-expression. This must execute atomically with respect to assignments and to the final release of the pointee.

消息传递a弱引用对变量执行左值到右值的转换，这意味着弱引用的值将被保留，然后在当前的完整表达式（基本上是语句）结束时释放。它基本上等同于分配一个强变量，其范围仅适用于当前语句，然后传递该强变量。

Messaging a weak reference performs an lvalue-to-rvalue conversion on the variable, which means the value of the weak reference will be retained and then released at the end of the current full-expression (basically, the statement). It's basically equivalent to assigning to a strong variable whose scope only lasts for the current statement, and then messaging that strong variable.

这里的内容是如果您想要发送消息a弱变量一次，永远不要再触摸它，你不关心在弱引用结束的情况下评估方法参数的副作用 nil ，然后继续直接向弱引用发送消息。但是如果你需要两次引用弱引用（在单独的语句中），或者评估参数的副作用很重要，那么你应该分配一个强变量并测试非 - nil 继续之前。

The takeaway here is if you want to message a weak variable once, and never touch it again, and you don't care about the side-effects of evaluating the arguments to the method in the case where the weak reference ends up nil, then go ahead and message the weak reference directly. But if you need to refer to the weak reference twice (in separate statements), or the side-effects of evaluating the arguments do matter, then you should assign to a strong variable and test for non-nil before proceeding.

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