Closures are often used as function arguments. But sometimes there are situations where local closures can be very handy.



Hint: This post has been updated to Swift 3, iOS 10 and Xcode 8

Imagine you have a view controller that has two GUI modes:

enum GUIMode { case Mode1 case Mode2 } 1 2 3 4 enum GUIMode { case Mode1 case Mode2 }

For each GUI mode you want to set some properties for three labels:

var guiMode: GUIMode = .Mode1 { didSet { switch guiMode { case .Mode1: label1.text = "1" label1.textColor = UIColor.red label1.font = UIFont(name: "HelveticaNeue", size: 10) label2.text = "2" label2.textColor = UIColor.blue label2.font = UIFont(name: "HelveticaNeue", size: 12) label3.text = "3" label3.textColor = UIColor.yellow label3.font = UIFont(name: "HelveticaNeue", size: 11) case .Mode2: label1.text = "4" label1.textColor = UIColor.yellow label1.font = UIFont(name: "HelveticaNeue", size: 11) label2.text = "5" label2.textColor = UIColor.blue label2.font = UIFont(name: "HelveticaNeue", size: 9) label3.text = "6" label3.textColor = UIColor.brown label3.font = UIFont(name: "HelveticaNeue", size: 10) } } } 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 var guiMode : GUIMode = . Mode1 { didSet { switch guiMode { case . Mode1 : label1 . text = "1" label1 . textColor = UIColor . red label1 . font = UIFont ( name : "HelveticaNeue" , size : 10 ) label2 . text = "2" label2 . textColor = UIColor . blue label2 . font = UIFont ( name : "HelveticaNeue" , size : 12 ) label3 . text = "3" label3 . textColor = UIColor . yellow label3 . font = UIFont ( name : "HelveticaNeue" , size : 11 ) case . Mode2 : label1 . text = "4" label1 . textColor = UIColor . yellow label1 . font = UIFont ( name : "HelveticaNeue" , size : 11 ) label2 . text = "5" label2 . textColor = UIColor . blue label2 . font = UIFont ( name : "HelveticaNeue" , size : 9 ) label3 . text = "6" label3 . textColor = UIColor . brown label3 . font = UIFont ( name : "HelveticaNeue" , size : 10 ) } } }

This is a lot of code. You could create a function that sets the properties for a label, but it is very unlikely that you will use that function ever again. So in this case it is a nice solution to define a local closure within the function:

var guiMode: GUIMode = .Mode1 { didSet { let styleLabel: (_ label:UILabel,_ text:String,_ color:UIColor,_ size:CGFloat) -> () = { (label,text,color,size) in label.text = text label.textColor = color label.font = UIFont(name: "HelveticaNeue", size:size) } switch guiMode { case .Mode1: styleLabel(label1, "1", UIColor.red, 10) styleLabel(label2, "2", UIColor.blue, 12) styleLabel(label3, "3", UIColor.yellow, 11) case .Mode2: styleLabel(label1, "4", UIColor.yellow, 11) styleLabel(label2, "5", UIColor.black, 9) styleLabel(label3, "6", UIColor.brown, 10) } } } 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 var guiMode : GUIMode = . Mode1 { didSet { let styleLabel : ( _ label : UILabel , _ text : String , _ color : UIColor , _ size : CGFloat ) -> ( ) = { ( label , text , color , size ) in label . text = text label . textColor = color label . font = UIFont ( name : "HelveticaNeue" , size : size ) } switch guiMode { case . Mode1 : styleLabel ( label1 , "1" , UIColor . red , 10 ) styleLabel ( label2 , "2" , UIColor . blue , 12 ) styleLabel ( label3 , "3" , UIColor . yellow , 11 ) case . Mode2 : styleLabel ( label1 , "4" , UIColor . yellow , 11 ) styleLabel ( label2 , "5" , UIColor . black , 9 ) styleLabel ( label3 , "6" , UIColor . brown , 10 ) } } }

It is short and it is nice.

References

Image: @ Sergey Nivens / shutterstock.com