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  • Writer's pictureQuinne Farenwald

Building Blocks of Functionality: Understanding Closures in Swift

If you're diving into the world of Swift programming, you'll inevitably encounter the concept of closures. Closures are powerful and flexible blocks of code that can be passed around and used in your programs. In this blog post, we'll start from the basics of high-level closure syntax and gradually build up to more complex examples, all while keeping it beginner-friendly.


High-Level Closure Syntax

At its core, a closure in Swift is a self-contained block of functionality that can be passed around and used in your code. They are similar to functions, but closures can capture and store references to any constants and variables from the surrounding context in which they are defined. Let's start by exploring the high-level syntax of closures.


A closure is a value type. You can represent a closure type using any of the following syntax. The '()' represents the functionality held in a closure. These parenthesis can accept parameters to feed the internal functionality. After the '->' represents the return type.


() -> (): This represents a closure that takes no parameters and returns no value (equivalent to a function with an empty parentheses parameter and no return type).

let emptyClosure: () -> () = { 
   print("This is an empty closure.") 
}

() -> {}: This is also a valid syntax and is functionally equivalent to () -> (). The curly braces {} denote an empty code block.

let anotherEmptyClosure: () -> {} = { 
   print("Another empty closure.") 
}

() -> Void: This syntax is commonly used in Swift to represent a closure that takes no parameters and returns Void, which is equivalent to saying it returns nothing.

let voidClosure: () -> Void = { print("This is a closure with a Void return type.") }

All three representations are valid, and you can choose the one that you find most readable or consistent with your code style. In practice, () -> Void is often preferred for clarity and consistency, as it explicitly indicates that the closure returns nothing.


In Swift, closures can be written in a few different ways. Here are basic syntax examples:


Example 1: Closures That Take No Parameters and Returns Nothing

// Define closure as variable 
let greetClosure: () -> Void = { 
   print("Hello, Swift!") 
}

// Call closure 
greetClosure() 

// Prints "Hello, Swift!"

Example 2: Closures That Take Parameters and Return Something

Parameters are placed in the '()' of the type and just after the open brace. The 'in' is the key word after the parameter separates the parameters and return type from the body of the closure.

// Define closure that takes one parameter 
let greetClosure: (name: String) -> String = { name in 
   return "Hello, \(name)" 
} 

// Call closure 
let greeting = greetClosure("Quinne")

print(greeting) 
// Prints "Hello, Quinne!"

Note how when calling the closure, the parameters are not named like they can be with functions. This is true of all closures.


Example 3: Closures That Take Optional Parameters and Return Something

// Define closure that takes an optional parameter 
let greetClosure: (name1: String, name2: String?) -> String = { name1, name2 in 
   if let secondName = name2 { 
      return "Hello, \(name1) and \(secondName)" 
   } else { 
      return "Hello, \(name1)" 
   } 
} 

// Call closure with 2 parameters 
let greeting1 = greetClosure("Quinne", "Emily") 

print(greeting1) 
// Prints "Hello, Quinne and Emily!" 

// Call closure with only 1 required parameter 
let greeting2 = greetClosure("Quinne", nil) 

print(greeting2) 
// Prints "Hello, Quinne!"

Closure Shorthand Syntax

Closures can use a shorthand syntax when it comes to parameter names. Instead of naming the parameters like 'name1: String', you can simply put the type. Then within the closure, just after the opening brace, the parameters are named there.

let greetClosure: (String, String) -> String = { name1, name2 in
   return "Hello, \(name1) and \(name2)" 
} 

let greeting = greetClosure("Quinne", "Emily") 

print(greeting)

In Swift, the type inference system is quite powerful, and it can often deduce the types of parameters and return values, allowing you to omit explicit type annotations.

let greetClosure = { (name1, name2) -> String in 
   return "Hello, \(name1) and \(name2)" 
}

 And even this. Here's the version without explicit type annotations all together:

let greetClosure = { name1, name2 in 
   return "Hello, \(name1) and \(name2)" 
}

Choose whatever is most readable for you, your team, and the developers to come.


Common Closure Uses in the Wild

Closures in Swift are versatile and can be used in various scenarios. Here are some typical use cases for closures along with examples:


Custom Operations: You can define closures to encapsulate specific operations or behaviors for reuse, like in our earlier above.

let greetClosure: (String) -> String = { name in
   return "Hello, \(name)!" 
} 
let greeting = greetClosure("Alice")

Sorting Collections: Closures are commonly used for sorting arrays or other collections based on custom criteria.

let numbers = [5, 2, 8, 1, 7] 
let sortedNumbers = numbers.sorted { $0 < $1 }

Asynchronous Operations: Closures are often used in asynchronous programming, such as handling the completion of network requests.

fetchDataFromServer { result in 
   switch result { 
   case .success(let data): 
      print("Data fetched successfully: \(data)") 
   case .failure(let error): 
      print("Error fetching data: \(error)") 
   } 
}

Map, Filter, and Reduce: Closures are powerful when working with higher-order functions like map, filter, and reduce.

let numbers = [1, 2, 3, 4, 5] 
let squaredNumbers = numbers.map { $0 * $0 } 
let evenNumbers = numbers.filter { $0 % 2 == 0 } 
let sum = numbers.reduce(0, +)

Event Handling: Closures can be used for handling UI or user interaction events.

button.tapAction = { print("Button tapped!") }

Delegation: Closures can act as a form of lightweight delegation, allowing one object to notify another about specific events.

class DataManager { 
   var onDataUpdate: (() -> Void)? 

   func fetchData() { 
      // Fetch data from somewhere 
      onDataUpdate?() 
   } 
}

Delayed Execution: Closures can be used to delay the execution of code using functions like DispatchQueue.asyncAfter.

DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) { 
   print("This code will be executed after 2 seconds.") 
}

Animation Completion: Closures are often used in animations to define actions that should be performed upon completion.

UIView.animate(withDuration: 0.5, animations: { // Animation code }, completion: { finished in 
   if finished { print("Animation completed!") } 
})

These are just a few examples, and closures can be applied in many other scenarios, providing a concise and expressive way to encapsulate functionality in Swift programming.


Wrapping Up

Closures are a fundamental aspect of Swift programming, offering a flexible and concise way to write functional code. From simple syntax to more complex examples involving captured values, closures provide a powerful tool for Swift developers. As you continue your Swift journey, experimenting with closures will enhance your understanding of their versatility and applicability in various programming scenarios.


If you're eager to explore more, I really love this helpful video on closures from Hacking With Swift here. Happy building!

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