how to pass a callback function in another function in swift?

Question

so i'm doing my first app, and i want to do a function that will be a uniform funciton to sevral places in the system, and not belong to only one specific class. Is there a way for me to pass a callback function as a parameter to another function ?

here is just a demonstration of what i mean.

ClassA {

   func classAcallbackFunction (displayString: String) {
     print (displayString)
   }


   ClassB().classBFunction(classAcallbackFunction())


}

ClassB {

   func classBfunction (myCallbackfunc: func) {
     mycallbackfunc("all is working !!!")
   }

}

LinusG. · Accepted Answer · 2018-08-23 07:08:10Z

4

The parameter you have declared is not correct. Replace it with something like this:

func classBFunction(_ completion: (String) -> Void) {
    completion("all working")
}

Like shared by regina_fallangi in the comments, callbacks are usually called completion handlers, which is why I replaced the names accordingly.

Extra Credit:

If you only want to optionally pass a function, you could do this:

func classBFunction(_ completion: ((String) -> Void)? = nil) {
    completion?("all working")
}

Now you could also just call classBFunction().

edited Aug 23, 2018 at 7:08

answered Aug 23, 2018 at 6:23

LinusG.

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4 Comments

regina_fallangi Over a year ago

A small nit: callback is the theoretical name and the one used predominantly in JavaScript. Swift uses completion, which gives it a different meaning. You should always call your completion, no matter what happens. If there is an error, the you should call completion(error), if there was success, completion(true), etc.

vadian Over a year ago

Another small nit: Underscore and parameter label are Swift 2 legacy, in Swift 3+ it's simply (String) -> Void as the parameter label is not used anyway.

Carson Holzheimer Over a year ago

If a completion should always be called, what should we call a high order function that isn't necessarily called, like a button click. A callback?

Carson Holzheimer Over a year ago

@vadian the parameter label may be important in situations where the type itself doesn't clarify what function should be passed.

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