Learn about closures and how to write closures in swift

What are closures?

As per apple documentation, closures are self contained blocks of functionality that can be passed around code. In swift these are similar to what we called blocks in objective c and lambda in other languages. The definition is quiet complex to understand, so in simple words we can define closure as a chuck of code that can perform certain functionality or calculations and then pass the control to the class from where its getting called or invoked, like a callback.
Example of closures are completionHandlers we often used while making an API request. So in this tutorial we will learn how to write closures in swift, before we start learning how to write closures, we must knew how to write function (i hope you are aware of syntax to create a function), if you are novice and learning swift then below is the code snippet for the same

	func welcomeGuest(nameOfGuest:String) -> String {
	return "We welcome \(nameOfGuest)"
	}

	/*
	1. func = keyword for tellig compiler that we are going to create a function
	2. welcomeGuest = name for our function
	3. () = if your function is acceoting no paramaeter then it should be empty
	4. (nameOfGuest:String) = function accepting one parameter of type string
	5. -> String = if your function returning some value, in this case String

	Example of function with empty parameter and returning nothing
	func welcomeGuest() {
	return "We welcome you"
	}

	Example of function with string parameter and returning string value
	func welcomeGuest(nameOfGuest:String) -> String {
	return "We welcome \(nameOfGuest)"
	}
	*/

view raw functionsyntax.swift hosted with ❤ by GitHub

Difference between function and closure

1. Closures don’t have func keyword
2. Closures don’t have name
3. Closures have in keyword (function don’t have)

Writing closure

At this point we knows the difference between a function and closure, so let us make our function a closure.

Step 1: Remove ‘func’ keyword from our function and name ‘welcomeGuest’ also removed. We satisfied two conditions of closure that it doesn’t contain ‘func’ keyword and name. Our closure will look like

	(nameOfGuest:String) -> String {
	return "We welcome \(nameOfGuest)"
	}

view raw Writingclosure-step1.swift hosted with ❤ by GitHub

Step 2: Third point tells us that closure has ‘in‘ keyword, so we need to place ‘in’ keyword in place of opening curly brace of and move that open curly brace to start of the closure. Our code will look like

	{ (nameOfGuest:String) -> String in
	return "We welcome \(nameOfGuest)"
	}

view raw Writingclosure-step2.swift hosted with ❤ by GitHub

Step 3: At this point compiler is showing you an error “expected declaration”, so in order to remove error, we need to assign the closure to a variable, as shown in below code snippet


    
      
        
  
    
    

        


  


  
        
          
          let closureWelcome = { (nameOfGuest:String) -> String in
        
        
          
                  return "We welcome \(nameOfGuest)"
        
        
          
          }
        
  



    

  


      
      
        view raw
        
          Writingclosure-step3.swift
        
        hosted with ❤ by GitHub

Step 4: We are done with closures as we successfully write a closure that returning a string value. In order to call our closure inside viewDidLoad function as shown below

	override func viewDidLoad() {
	super.viewDidLoad()

	// Do any additional setup after loading the view.
	print("Calling closure == \( closureWelcome("iosTutorialJunction"))")
	}

view raw Writingclosure-step4.swift hosted with ❤ by GitHub

Where to go from here:

In this tutorial, we learned how to write closures in swift, We learned about difference between closure and function and how to write closure in swift and use them in our code. If you have question, please feel free to ask.

What are closures?

Difference between function and closure

Writing closure

Where to go from here:

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