How could I create a function with a completion handler in Swift? - function

Question

I was just curious as to how I would approach this. If I had a function, and I wanted something to happen when it was fully executed, how would I add this into the function? Thanks

Answer 1

Say you have a download function to download a file from network, and want to be notified when download task has finished.
typealias CompletionHandler = (success:Bool) -> Void
func downloadFileFromURL(url: NSURL,completionHandler: CompletionHandler) {
// download code.
let flag = true // true if download succeed,false otherwise
completionHandler(success: flag)
}
// How to use it.
downloadFileFromURL(NSURL(string: "url_str")!, { (success) -> Void in
// When download completes,control flow goes here.
if success {
// download success
} else {
// download fail
}
})

Answer 2

I had trouble understanding the answers so I'm assuming any other beginner like myself might have the same problem as me.
My solution does the same as the top answer but hopefully a little more clear and easy to understand for beginners or people just having trouble understanding in general.
To create a function with a completion handler
func yourFunctionName(finished: () -> Void) {
print("Doing something!")
finished()
}
to use the function
override func viewDidLoad() {
yourFunctionName {
//do something here after running your function
print("Tada!!!!")
}
}
Your output will be
Doing something
Tada!!!

Answer 3

Simple Example:
func method(arg: Bool, completion: (Bool) -> ()) {
print("First line of code executed")
// do stuff here to determine what you want to "send back".
// we are just sending the Boolean value that was sent in "back"
completion(arg)
}
How to use it:
method(arg: true, completion: { (success) -> Void in
print("Second line of code executed")
if success { // this will be equal to whatever value is set in this method call
print("true")
} else {
print("false")
}
})

Answer 4

Swift 5.0 + , Simple and Short
example:
Style 1
func methodName(completionBlock: () -> Void) {
print("block_Completion")
completionBlock()
}
Style 2
func methodName(completionBlock: () -> ()) {
print("block_Completion")
completionBlock()
}
Use:
override func viewDidLoad() {
super.viewDidLoad()
methodName {
print("Doing something after Block_Completion!!")
}
}
Output
block_Completion
Doing something after Block_Completion!!

Answer 5

We can use Closures for this purpose. Try the following
func loadHealthCareList(completionClosure: (indexes: NSMutableArray)-> ()) {
//some code here
completionClosure(indexes: list)
}
At some point we can call this function as given below.
healthIndexManager.loadHealthCareList { (indexes) -> () in
print(indexes)
}
Please refer the following link for more information regarding Closures.
https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html

Answer 6

In addition to above : Trailing closure can be used .
downloadFileFromURL(NSURL(string: "url_str")!) { (success) -> Void in
// When download completes,control flow goes here.
if success {
// download success
} else {
// download fail
}
}

Answer 7

In case you need result values in your completion handler, it's a good idea to include labels proceeded with underscores, like this...
func getAccountID(account: String, completionHandler: (_ id: String?, _ error: Error?) -> ()) {
// Do something and return values in the completion handler
completionHandler("123", nil)
}
...because when you type this function, Xcode will automatically fill in the result value labels, like this:
getAccountID(account: inputField.stringValue) { id, error in
}

Answer 8

I'm a little confused about custom made completion handlers. In your example:
Say you have a download function to download a file from network,and want to be notified when download task has finished.
typealias CompletionHandler = (success:Bool) -> Void
func downloadFileFromURL(url: NSURL,completionHandler: CompletionHandler) {
// download code.
let flag = true // true if download succeed,false otherwise
completionHandler(success: flag)
}
Your // download code will still be ran asynchronously. Why wouldn't the code go straight to your let flag = true and completion Handler(success: flag) without waiting for your download code to be finished?

Answer 9

//MARK: - Define
typealias Completion = (_ success:Bool) -> Void
//MARK: - Create
func Call(url: NSURL, Completion: Completion) {
Completion(true)
}
//MARK: - Use
Call(url: NSURL(string: "http://")!, Completion: { (success) -> Void in
if success {
//TRUE
} else {
//FALSE
}
})