"Deferred Callback" in Objective C

Question

I believe what I'm trying to do is referred to as a "deferred callback" at least in some paradigms.

Basically I need a way to call a method after another method returns. Something like this:

- (void)someMethod:data
{
    [someObject doSomethingAfterCurrentMethodReturns]
}

I would like to be able to do this because a library for an external accessory that I have to use will crash the app if I do the thing I want to do within the method. This is noted in the documentation for the library. My current workaround involves an additional user interaction -- throw up UIAlertView and put the method behind a button click.

I've seen some libraries that seem to handle this. Is there a built in way to handle this in Objective C?

It's not, so "deferred callback" is probably not the perfect label. Hence the quotes. — anthropomo, Aug 16 '13 at 13:24

score 1 · Answer 1 · answered Dec 12 '18 at 17:03

I know this is old but wanted to provide an option that I like better than provided solution because I don't know if it is guaranteed to work and I didn't want to test it.

You are correct that you want defer equivalent. I too was looking for this. Other implementations suggest a custom class that leverage auto-release.

I liked this solution best as it is simple and inline:

- (void)foo {
// code to execute in foo
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
    // code to execute after foo() returns
});

score 1 · Answer 2 · answered Aug 17 '20 at 07:14

1

You can use

__attribute__((cleanup(...)))

answered Aug 17 '20 at 07:14

Chris Forever

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Please do not post only code as answer. Please explain your answer/implementation. – milanbalazs Aug 17 '20 at 10:43
1

There's more info about this here: https://pspdfkit.com/blog/2017/even-swiftier-objective-c/ – Koen. Nov 12 '20 at 12:28

score 0 · Accepted Answer · answered Aug 16 '13 at 13:21

0

Depending on the number of parameters the method has you could use performSelector:withObject:afterDelay:. Or you could start an NSTimer and call the method when it fires.

answered Aug 16 '13 at 13:21

Wain

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My fault: my method doesn't take an argument. And this doesn't work: `[self performSelector:@selector(myMethod) withObject:Nil afterDelay:50]` – anthropomo Aug 16 '13 at 13:44
And never mind, I was assuming NSTimeinterval would be in milliseconds! – anthropomo Aug 16 '13 at 13:50
Does it have to be specific delay time can it be with a condition to end of a loop or such – Ahmed El-Bermawy Sep 06 '20 at 20:34
If you have a specific condition why are you looking for a deferred call? – Wain Sep 08 '20 at 09:38

score 0 · Answer 4 · edited Mar 10 '16 at 21:54

0

I came up with a defer statement in ObjC similar to Go's defer. See my blog post and the GitHub repo. It would look like this:

- (void)someMethod:(id)data
{
  defer(^(){
    [someObject doSomethingAfterCurrentMethodReturns];
  });
  ///...
}

edited Mar 10 '16 at 21:54

ricardopereira

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answered Mar 06 '14 at 17:43

Alejandro

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"Deferred Callback" in Objective C

4 Answers4