Introduction to Closures in Swift 4
You’ve seen it. Let’s talk about it.
Apple, what are closures?
Closures are self-contained blocks of functionality that can be passed around and used in your code.
— Apple
Don’t worry if that make no sense. At first, it probably sounds like:
Closures are self-contained bunch of words passed around like a damn basketball. In case you don’t know, there happen to be a basketball court in the Playground. Go check it out.
— Apple
PURPOSE
Introduce closures on what, how, and where.
WHAT ARE CLOSURES
Closures take one of three forms:
- Global functions
- Nested functions
- Closures expression
The first two are special cases of closures. If you don’t know how functions work, I have an article for you here:
When we talk about closures in Swift, we often refer to closure expressions. If there’s one thing to help you remember what closures are, this is it:
Closures are headless functions.
Closures are functions without the func keyword and the function name. They are also known as anonymous functions. They can’t be called by themselves like a function because they have no names. They are used in conjunction.
CLOSURES SYNTAX
Closures are encouraged due to its simplicity in syntax.
FUNCTION VS CLOSURE
Here are the key differences to remember between a function and a closure:
FUNCTION
- has a name
- has func keyword
- has no in keyword
CLOSURE
- has no name
- has no func keyword
- has in keyword
DEFINE
Let’s see how we define a function versus a closure.
func giveAFunc() { }var giveNoFunc = { () -> () in }
Note: Our closure begins with { and ends with }. We store the closure inside of a variable so we can call it.
CALL
Let’s call a function, then call a closure.
giveAFunc()giveNoFunc()
The way we call it is identical. Now that we have seen how closures compare to functions. Let’s see how we can transform from a function to a closure.
FUNCTION TO CLOSURE
Let’s transform sayHello to a closure.
//Soon to become a closure
func sayHello(name: String) -> String {
return "Hello \(name)"
}STEPS TO BECOME A CLOSURE
- Remove curly braces
func sayHello(name: String) -> String
return "Hello \(name)"2. Add in keyword between argument list and function body
func sayHello(name: String) -> String in
return "Hello \(name)"3. Remove func keyword and function name
(name: String) -> String in
return "Hello \(name)"4. Surround with curly braces
{ (name: String) -> String in
return "Hello \(name)"
}That’s it. Done.
You can then assign the closure to a variable and call it like a function.
var sayHello = { (name: String) -> String in
return "Hello \(name)"
}sayHello("Jayven")
WHERE ARE CLOSURES
A topic that people often leave out when talking about closures is where do we actually find closures? Well here is the answer. Everywhere. Closures are deeply rooted within the Swift language. Most common use cases include completion blocks/callbacks, higher order functions, and syntactic sugars.
WRAP UP
We have introduced closures. What they are. What they look like. How they are formed. And where they are. Congratulations for making it this far.
Introduction, introduced.
LAST REMARKS
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