How Do I Declare a Closure in Swift?

How Do I Declare a Closure in Swift?

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 an argument to a function call:

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

As a function parameter:

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

As a function parameter with implied types:

array.sort({ (item1, item2) -> Bool in return item1 < item2 })

As a function parameter with implied return type:

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

As the last function parameter:

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

As the last parameter, using shorthand argument names:

array.sort { return $0 < $1 }

As the last parameter, with an implied return value:

array.sort { $0 < $1 }

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

array.sort(<)

As a function parameter with explicit capture semantics:

array.sort({ [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.sort({ [unowned self] in return item1 < item2 })