swift : Closure declaration as like block declaration

Question

We can declare block as below in Objective-C.

typedef void (^CompletionBlock) (NSString* completionReason);

I'm trying to do this in swift, it give error.

func completionFunction(NSString* completionReason){ }
typealias CompletionBlock = completionFunction

Error : Use of undeclared 'completionFunction'

Definition :

var completion: CompletionBlock = { }

How to do this?

Update:

According to @jtbandes's answer, I can create closure with multiple arguments as like

typealias CompletionBlock = ( completionName : NSString, flag : Int) -> ()

@Downvotters: Please explain what is wrong with this question? — Mani, Jun 10 '14 at 08:29
there are a number of people feels we should learn swift language first and then write. Foolish feelings... — Janak Nirmal, Jun 11 '14 at 10:10
@Mani Hi, would you mind also showing how you would assign that type alias to a `var` (i.e., `var completion: CompletionBlock = {....}` using multiple arguments? Thanks. — Unheilig, Apr 17 '15 at 08:14

jtbandes · Accepted Answer · 2023-08-16T15:39:31.570

144

typealias CompletionBlock = (NSString?) -> Void
// or
typealias CompletionBlock = (result: NSData?, error: NSError?) -> Void

var completion: CompletionBlock = { reason in print(reason) }
var completion: CompletionBlock = { result, error in print(error) }

Note that the parentheses around the input type are only required as of Swift 3+.

edited Aug 16 '23 at 15:39

answered Jun 10 '14 at 05:56

jtbandes

If it take more than two arguments, how can we do that? Would you please direct me to reference in that book or anyother? – Mani Jun 10 '14 at 06:00
Best book for Swift: Apple Inc. “The Swift Programming Language.” iBooks. https://itun.es/ca/jEUH0.l – David Skrundz Jun 10 '14 at 06:02
@NSArray I know only one book is available for swift. But I'm asking about that corresponding chapter with topic... :) – Mani Jun 10 '14 at 06:05
4

@jtbandes It's working. I've created with two arguments as `typealias CompletionBlock = ( completionName : NSString, flag : Int) -> ()` – Mani Jun 10 '14 at 06:09
@zumzum It separates the parameter name "`reason`" from the method body "`println(reason)`" – jtbandes Jul 01 '14 at 17:35

score 14 · Answer 2 · answered Oct 27 '15 at 19:26

Here is awesome blog about swift closure.

Here are some examples:

As a variable:

var closureName: (inputTypes) -> (outputType)

As an optional variable:

var closureName: ((inputTypes) -> (outputType))?

As a type alias:

typealias closureType = (inputTypes) -> (outputType)

2 Answers2