Using predefined function with parameters as completion block in swift

Question

EDIT: I thought the issue was Swift syntax, but the problem was that I did not know that the UIViewController.presentViewController specifies a signature for the completion handler that takes no input parameters.

I have to provide a completion handler to presentViewController. The completion handler I want to provide takes an input parameter. What is the syntax to handle this in swift?

If the handler did not take an input parameter it would look like this:

self.presentViewController(activityVC, animated: true, completion: myHandler)

But how would the above line look if myHandler was declared like this:

func myHandler(myParameter: AnyObject){
    ...
}

dispatch_block_t maybe? developer.apple.com/library/prerelease/ios/documentation/… — Kendel
– Kendel, Commented Jun 28, 2015 at 15:10
are you trying to pass in a function that expects one parameter as a completion block which does not provide a parameter? — luk2302
– luk2302, Commented Jun 28, 2015 at 15:13
That does not look like it relates to the question, I'll try to edit to make it more clear what I am asking about — Marmoy
– Marmoy, Commented Jun 28, 2015 at 15:14
Well the self.presentViewController uses (() -> Void) as its completion handler if that is what you want. If you want it to return a value you would use something like (() -> Int) — Kendel
– Kendel, Commented Jun 28, 2015 at 15:15
Ok, I'm not looking to return a value, but I guess the same answer goes for input values. The signature of the expected completion handler takes no inputs, is that correct? — Marmoy
– Marmoy, Commented Jun 28, 2015 at 15:19

Marmoy · Accepted Answer · 2015-06-28 15:26:08Z

1

I should have looked at the declaration of presentViewController before asking the question, but I am inexperienced with closures and thought any closure could be passed. It turns out that the issue is that presentViewController takes a very specific closure of the form

(() -> Void)?

The full signature is here:

func presentViewController(_ viewControllerToPresent: UIViewController,
              animated flag: Bool,
            completion completion: (() -> Void)?)

So the answer is that presentViewController cannot take a completion handler that itself takes another parameter.

answered Jun 28, 2015 at 15:26

Marmoy

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Comments

luk2302 · Accepted Answer · 2015-06-28 15:35:05Z

0

That does not work unfortunately. You cannot just pass in a function that has an invalid type for the completion block. It expects a closure without any parameters and with no return type / void. You are providing a function that expects a AnyObject and returns nothing / void. Therefore there is a mismatch between the expected (() -> Void) and the provided ((AnyObject) -> Void)

How would the program know what to pass in to myHandler as myParameter?

The only way I can think of to achieve this is to write a wrapper for the custom completion handler:

self.presentViewController(activityVC, animated: true, completion:{self.myHandler("")})

That way you provide a completion block that calls the myHandler but actually provides a value for myParameter.

edited Jun 28, 2015 at 15:35

answered Jun 28, 2015 at 15:21

luk2302

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2 Comments

Marmoy Over a year ago

You're right, taking a look at the declaration of UIViewController.presentViewController clearly shows why it can't be done. Another question is what the syntax would be if presentViewController expected a completion handler with an input parameter..

luk2302 Over a year ago

@Videre THEN it would be the same like if it would not expect one, then you would just write self.presentViewController(vc, animated: true, completion:myHandler) - iif myHandler expects a parameter of the same type and has a matching return type.

Kendel · Accepted Answer · 2015-06-28 15:19:00Z

0

I believe you are looking to do the following:

function myHandler(myParameter: (() -> Void)) {
//code here
}

Of course -> Void would change depending on if you want the function

answered Jun 28, 2015 at 15:19

Kendel

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