Introduction to Closures In Swift

What are Closures?

Closures are enclosed in curly braces {} without func keyword and func name. They are defined by function type ()-> ()

{ (params) -> returnType in
  statements
}

in keyword separates closure header with its body.

Function VS Closures

Define and Call

That’s how we define function and closures:

func callAFunc() { }

var callAClosure = { () -> () in }

Closure is been stored in a variable so that it can be called when required.

Calling functions and closures:

callAFunc()

callAClosure()
 

Both are identical.

Function to Closure

Let’s convert the below function to closure.

func multiplyNumbers(a: CGFloat, b: CGFloat) -> String {
	return “Result is: \(a * b)”
}

Steps:

• Removing curly braces

func multiplyNumbers(a: CGFloat, b: CGFloat)  -> String 
	return “Result is: \(a * b)”
 

• Add in keyword between argument and body

func multiplyNumbers(a: CGFloat, b: CGFloat)  -> String in
	return “Result is: \(a * b)”

• Removing func name and keyword

(a: CGFloat, b: CGFloat)  -> String in
	return “Result is: \(a * b)”
 

• Put all in curly braces

{ (a: CGFloat, b: CGFloat)  -> String in
	return “Result is: \(a * b)”
     }

It’s done. Now we can assign this closure to a variable and call it whenever required.

var multiply = { (a: CGFloat, b: CGFloat)  -> String in
	return “Result is: \(a * b)”
}

multiply(126.43,256.566)

Thanks