Swift completion handler - escaping trailing closure - swift

Question

I've been reading quite a few articles now about swift functions with closures, trailing closures and escaping functions. They all seem to give examples which is sufficiently different that I'm not understanding what Im doing wrong with my own function.
My main problem is with how to execute a trailing closure function.
I've created this function to upload an image to firebase. It takes two inputs and are supposed to return a string (imageURL). I do belive this function is ok.
func uploadImageToFirebaseAndReturnImageURL(directory: String, image: UIImage!, handler: #escaping(_ imageURL: (ImageURL)) -> ()) {
let imageName = NSUUID().uuidString // create unique image name
if let uploadData = UIImagePNGRepresentation(image) {
DB_STORE.child(directory).putData(uploadData, metadata: nil, completion: { (metadata, error) in
if error != nil {
print(error)
return
}
if let profileImageUrl = metadata?.downloadURL()?.absoluteString {
let d = ImageURL(imageURL: profileImageUrl)
handler (d)
}
return
})
}
}
My issue is how to execute this function correctly when it comes to the handler.
I want to first execute the function then when complete I want to get the imageURL and use this variable into another nested function that upload this variable(String) into a firebase database.
uploadImageToFirebaseAndReturnImageURL(directory: "profileImage", image: selectedImageFromPicker!, handler: { imageURL in
guard let uid = Auth.auth().currentUser.uid else { print("User is not logged in"); return }
DataService.instance.updateUserWithProfileImageURL(uid: uid, imageURL: imageURL)
print("")
}
What am I doing wrong?

Answer 1

To pass a trailing closure you need to end/close your function call and omit the closure argument label. For instance:
func foo(first: Int, second: Int, handler: (Int) -> Void) {
...
}
call syntax:
foo(first: 10, second: 20) { result in
/* trailing closure body */
}
By the way, you should simply your handler argument declaration from:
handler: #escaping (_ imageURL: (ImageURL)) -> ()
to this:
handler: #escaping (ImageURL) -> Void
Using Void or () is matter of style since they are logically the same. I prefer the former ;)