How Do I Declare a Closure in Sw

As a variable:

var closureName: (ParameterTypes) -> ReturnType = { ... }

As an optional variable:

var closureName: ((ParameterTypes) -> ReturnType)?

As a type alias:

typealias ClosureType = (ParameterTypes) -> ReturnType

As a constant:

let closureName: ClosureType = { ... }

As a parameter to another function:

funcName(parameter: (ParameterTypes) -> ReturnType)

Note: if the passed-in closure is going to outlive the scope of the method, e.g. if you are saving it to a property, it needs to be annotated with @escaping.

As an argument to a function call:

funcName({ (ParameterTypes) -> ReturnType in statements })

As a function parameter:

array.sorted(by: { (item1: Int, item2: Int) -> Bool in return item1 < item2 })

As a function parameter with implied types:

array.sorted(by: { (item1, item2) -> Bool in return item1 < item2 })

As a function parameter with implied return type:

array.sorted(by: { (item1, item2) in return item1 < item2 })

As the last function parameter:

array.sorted { (item1, item2) in return item1 < item2 }

As the last parameter, using shorthand argument names:

array.sorted { return $0 < $1 }

As the last parameter, with an implied return value:

array.sorted { $0 < $1 }

As the last parameter, as a reference to an existing function:

array.sorted(by: <)

As a function parameter with explicit capture semantics:

array.sorted(by: { [unowned self] (item1: Int, item2: Int) -> Bool in return item1 < item2 })

As a function parameter with explicit capture semantics and inferred parameters / return type:

array.sorted(by: { [unowned self] in return $0 < $1 })

This site is not intended to be an exhaustive list of all possible uses of closures.

Unable to access this site due to the profanity in the URL? http://goshdarnclosuresyntax.com is a more work-friendly mirror.