Closure in Swift 5

Learn by examples what is Closure and how to use it in your code

credits pixabay.com

What is Closure?

Closures are self-contained blocks of functionality that can be passed around and used in your code. Closures in Swift are similar to blocks in C and Objective-C and to lambdas in other programming languages.

swift.org

• Closure is a function and “func” keyword
• Closure is a block of code that you can assign to a variable.
• In Swift Variables “var” and Constants “let” store information, Functions “func” execute tasks and Closures have a function code in form of a variable and can be passed around.

What are the types of the Closure?

• Closure without niether parameters nor return value.
• Closure that accepts parameters.
• Closure returns value.
• Trailing Closure

Closure without parameters or return value

var myClosure = {
    print("Hello I'm a Closure")
}
myClosure()         
=> Hello I'm a Closure

Closure that acceptes parameters

var closure: (String) -> Void = { str in
    print(str)
}
var closure: (Int, Int) -> Void = { first, second in
    print(first + second)
}

Since closures parameters don’t have names you can refer to them with $ + parameters number

var closure: (Int, Int) -> Void = {
    print($0 + $1)
}
closure(3, 2)          
=> 5

Closure that returns values

var closure: () -> String = {
    return "Hello Closure"
}
var value = closure()
print(value)  
=> Hello Closure

closures can also accept parameters and return value

var addTwoNumbers: (Int, Int) -> Int = {
    return $0 + $1
}
addTwoNumbers(4, 5)   
=> 9

Trailing Closure

If the last parameter to a function is closure that’s called trailing closure

func callMe(closure: () -> Void) {
    print("will call you")
    closure()
    print("did call you")
}
callMe {
    print("calling you...")
}
=> will call you
=> calling you...
=> did call you

Another example of the trailing closure with parameter

func callMe(name: String, closure: (String) -> Void) {
    print("\(name) will call you")
    closure(name)
    print("\(name) did call you")
}
callMe(name: "Smith") { name in
    print(name, "is calling you")
}
=> Smith will call you
=> Smith is calling you
=> Smith did call you

Trailing closure with parameter and returns value

func callMe(name: String, closure: (String) -> String) -> String {
    return closure(name)
}
var whoIsCalling = callMe(name: "Smith") { (name) -> String in
    return name
}
print(whoIsCalling)
=> Smith is calling.