Super simple trailing closure syntax in Swift

Question

I am trying to follow the example in the Swift docs for a trailing closure.

This is the function:

func someFunctionThatTakesAClosure(closure: () -> Void) {
    // function body goes here
    print("we do something here and then go back")//does not print
}

And I call it here.

        print("about to call function")//prints ok
        someFunctionThatTakesAClosure(closure: {
            print("we did what was in the function and can now do something else")//does not print
        })
        print("after calling function")//prints ok

The function, however, is not getting called. What is wrong with the above?

Here's the Apple example:

func someFunctionThatTakesAClosure(closure: () -> Void) { // function body goes here }

// Here's how you call this function without using a trailing closure:

someFunctionThatTakesAClosure(closure: { // closure's body goes here })

A closure is just a value. Just like any other value, you can "ignore" its existence and nothing would happen. Making anything happen would require you to actually use the value. — Alexander
– Alexander, Commented Mar 23, 2019 at 0:27

vacawama · Accepted Answer · 2019-03-23 00:19:31Z

1

Here is your example fixed:

func someFunctionThatTakesAClosure(closure: () -> Void) {
    // function body goes here
    print("we do something here and then go back")

    // don't forget to call the closure
    closure()
}


print("about to call function")

// call the function using trailing closure syntax
someFunctionThatTakesAClosure() {
    print("we did what was in the function and can now do something else")
}

print("after calling function")

Output:

about to call function
we do something here and then go back
we did what was in the function and can now do something else
after calling function

answered Mar 23, 2019 at 0:19

vacawama

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Comments

Shehata Gamal · Accepted Answer · 2019-03-23 00:46:50Z

1

Docs isn't very clear in explanation you need

print("1")
someFunctionThatTakesAClosure() {  // can be also  someFunctionThatTakesAClosure { without ()
    print("3") 

}

func someFunctionThatTakesAClosure(closure: () -> Void) { 
   print("2") 

   /// do you job here and line blow will get you back
    closure()
}

the trailing closure is meant for completions like when you do a network request and finally return the response like this

func someFunctionThatTakesAClosure(completion:  @escaping ([String]) -> Void) { 
   print("inside the function body") 
   Api.getData { 
      completion(arr)
   }
}

And to call

print("Before calling the function")
someFunctionThatTakesAClosure { (arr) in
  print("Inside the function callback  / trailing closure " , arr)
}
print("After calling the function")

what you missed to read

edited Mar 23, 2019 at 0:46

answered Mar 23, 2019 at 0:09

Shehata Gamal

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1 Comment

user6631314 Over a year ago

Thanks, I saw that but read it as though you could use either one. Thx for clarifying! I upvoted answer.

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