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?

I'm thinking of this pattern:

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

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?

ios automatic-ref-counting
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