A short introduction to closures in practical usage.

• Understand what a closure is
• Identify closures used commonly in UIKit
• Write closures yourself
• Create system that makes paractical use of a closure

Hands on.

Closures are self-contained blocks of functionality. Sometimes closures are called blocks, lambdas, or anonymous functions in other languages.

If you are familiar with functions than you should understand closures with little effort. In Swift functions are a special case of closure. By learning closures you will expand your ability to use functions into new and useful territory. You will also develop a stronger understanding of the built frameworks.

In a nustshell closures are just the functions that you are already familiar with the following added features:

• Closures can be passed as values
• Closures can be assigned to variables
• Closures capture (or enclose) variables from the surrounding context

Question:

• Assuming everyone is familiar with functions explain a function?
• Who is familar with closures?
• Given the features above suggest some use cases for Closures?

Closures Example:

Playground: Closures1.playground

Exercise: Math functions. Student directed.

Closures are values and as such they have a type. The Type of a closures is it's signature. The signature is the parameters it accepts, and the value it returns.

var a = {(Int) -> Int in /* do stuff */} // This clsoure is type: (Int) -> (Int)

You could declare a var of this type that could be assigned the closure above:

var f: (Int) -> (Int)

Closures can capture variables that exist outside their local scope. You could say a closure encloses these values. Which might explain the name?

Example Files: Closures2.playground

Important! closures capture the value of parameters passed to a function that creates a closure.

Exercise: Iterator! Make an iterator. This will be a function that takes an array as a parameter, and returns a closure that will iterate the contents of the array. See the notes in the playground for details. Students work in pairs to solve this problem.

Post exercise discussion: Everyone pair up and decide what is most important and what is most confusing about capturing values.

Activity: Closures in the Wild. Download and open the project below. Find the closures in the sample project. How many did you find?

Project: ClosuresExample1

The closures used in this project might be referred to as "callbacks". A callback is a block of code passed to another part of your program to execute at a more convenient time.

Closures are values. Take a look at the callback in presentViewController(). A quick look at the signature of a function.

Add a new UIAlertAction. The handler parameter is a closure! Write the code block to set the background color to red when this button is tapped. (Don't forget to add your alert action to the alert!)

Further challenges:

• When closing the alert show the user name in the label
• Make the new action show a view or label that slides up from the bottom of the screen.
• Make the new action display second alert dialog with three text fields.

• What do the closures do here?
• Does presentViewController() use a closure?
• Take a close look at: addTextFieldWithConfigurationHandler() what does the closure do here?
• Can you compare closures to the delegate pattern?

Exercise:

Write the function types for the closures in the example, do this with a partner.

• UIAlertAction( ... handler:) (the handler param)
• presentViewController(... completion:) (The completion param)
• addTextFieldWithConfigurationHandler()

After you've written the type look up the documentation for each of these functions and compare.

Closures are useful when you want to add functionality to something, or need the flexibility to decide functionality as convenient.

In the case of presentViewController "completion" might be referred to as a "callback" since it's execution is asynchronous. The animation must complete before the aniamtion is finished.

With UIAlertAction handler is a block of code that may or may not be executed at some point in the future. We call this use a handler since the closure is how we are handling user interaction with a button.

Discussion

• Why does UIKit use closures for UIAlertAction and presentViewController()?
• Why does addTextFieldWithConfigurationHandler() use a closure?
• Why do you have to use self in closures?

The MSButtonNode class is another use case for closures. The selectedHandler is a class property the value of which is a function that takes no parameters and returns nothing.

Project file: Closure example 2

Exercise: Write a closure to make the button do something.

Challenge: Make an alternative to the ButtonNode class call it: ToggleButtonNode. Toggle button will display two states like a switch or a check box. It will change states with each tap and execute one of two closures alternating.

In the previous example you made use of UIKit objects and methods that make use of closures. In this example you will add a closure to an existing class.

The Joystick class is used with SpriteKit. It creates a virtual joystick that appears on the screen. Your goal is to add a new property to this class that will take two parameters: x and y. Joystick should invoke this closure passing it's xValue, and yValue. Use the closure to move the box.

Project files: Closure Example 3

Closures are really useful. They are used extensively in the UIKit framework. A function is a closure. Think of closures as blocks of fucntional code that can be assigned, passed, and returned.

Exercise:

• List the features of closures?
• Contrast the difference between using a closure and the delegate pattern?

Here are some terms used when talking about closures

A function/closure assigned to variable. This is sometimes called a "function literal"

var a = {(Int) -> Int in /* do stuff */}

A higher order function is a function that takes another function as a parameter.

A closure passed to an object. Usually executed Asynchronously.

A closure executed by another object, usually in response to an action, like a tap, swipe, or other event/action.

Another name for a Closure (In swift Functions are a special case of closure)

Another name for closure.