[swift-users] Defer with local variable's state

[Sebastian Hagedorn]

My bad. After double-checking my code, I realized that some branches were dispatching their work, which of course breaks that model. Sorry, and thanks for clarification that this is how it should works.


> On 11 Aug 2016, at 17:39, Joe Groff <jgroff at apple.com> wrote:
> 
>> 
>> On Aug 11, 2016, at 7:16 AM, Sebastian Hagedorn via swift-users <swift-users at swift.org> wrote:
>> 
>> We often write code that returns early, but has to make sure a bit of code (e.g., a completion handler) is always called whenever we return, which seems like a great use case for defer. I started to write this:
>> 
>> func execute(with completion: ((Bool) -> Void)?) {
>> 	var success = false
>> 	defer {
>> 		// should always execute with the current state of success at that time
>> 		completion?(success)
>> 	}
>> 
>> 	guard … else {
>> 		// completion is expected to be executed with false
>> 		return
>> 	}
>> 
>> 	success = true
>> 
>> 	// completion is expected to be executed with true
>> }
>> 
>> However, it seems that defer copies the state of success, which means any update to the variable is not respected, and the completion block is always called with false.
>> 
>> Is there a way to make this work? I could image to call a function within the defer block that evaluates the success (e.g., depending on the state of an instance variable), but using a local variable seems to encapsulate this a lot better.
> 
> This is a bug. Defer should track updates of the variable. Would you mind filing this at bugs.swift.org? Do you happen to know whether it reproduces only in debug or release builds, or both?
> 
> -Joe