swift - extensions
Tags: swift
Published: 2022-01-16 06:56:09

Extensions add new functionality to an existing class, structure, enumeration, or protocol type. This includes the ability to extend types for which you don’t have access to the original source code (known as retroactive modeling). Extensions are similar to categories in Objective-C. (Unlike Objective-C categories, Swift extensions don’t have names.

### Extensions in Swift can:

- Add computed instance properties and computed type properties
- Define instance methods and type methods
- Provide new initializers
- Define subscripts
- Define and use new nested types
- Make an existing type conform to a protocol

> Extensions can add new functionality to a type, but they can’t override existing functionality.

## Extension Syntax
Declare extensions with the extension keyword:
```swift
extension SomeType {
    // new functionality to add to SomeType goes here
}
```
## computed instance properties

Extensions can add computed instance properties and computed type properties to existing types. This example adds five computed instance properties to Swift’s built-in Double type, to provide basic support for working with distance units:

```swift
extension Double {
    var km: Double { return self * 1_000.0 }
    var m: Double { return self }
    var cm: Double { return self / 100.0 }
    var mm: Double { return self / 1_000.0 }
    var ft: Double { return self / 3.28084 }
}
let oneInch = 25.4.mm
print("One inch is \(oneInch) meters")
// Prints "One inch is 0.0254 meters"
let threeFeet = 3.ft
print("Three feet is \(threeFeet) meters")
// Prints "Three feet is 0.914399970739201 meters"
```

## Initializers
Extensions can add new initializers to existing types. This enables you to extend other types to accept your own custom types as initializer parameters, or to provide additional initialization options that were not included as part of the type’s original implementation.

The example below defines a custom Rect structure to represent a geometric rectangle. The example also defines two supporting structures called Size and Point, both of which provide default values of 0.0 for all of their properties:

```swift
struct Size {
    var width = 0.0, height = 0.0
}
struct Point {
    var x = 0.0, y = 0.0
}
struct Rect {
    var origin = Point()
    var size = Size()
}
```
Because the Rect structure provides default values for all of its properties, it receives a default initializer and a memberwise initializer automatically, as described in Default Initializers. These initializers can be used to create new Rect instances:

```swift
let defaultRect = Rect()
let memberwiseRect = Rect(origin: Point(x: 2.0, y: 2.0),
   size: Size(width: 5.0, height: 5.0))
```

You can extend the Rect structure to provide an additional initializer that takes a specific center point and size:

```swift
extension Rect {
    init(center: Point, size: Size) {
        let originX = center.x - (size.width / 2)
        let originY = center.y - (size.height / 2)
        self.init(origin: Point(x: originX, y: originY), size: size)
    }
}


let centerRect = Rect(center: Point(x: 4.0, y: 4.0),
                      size: Size(width: 3.0, height: 3.0))
// centerRect's origin is (2.5, 2.5) and its size is (3.0, 3.0)

```

## Methods
Extensions can add new instance methods and type methods to existing types. The following example adds a new instance method called repetitions to the Int type:

```swift
extension Int {
    func repetitions(task: () -> Void) {
        for _ in 0..<self {
            task()
        }
    }
}
```
The repetitions(task:) method takes a single argument of type () -> Void, which indicates a function that has no parameters and doesn’t return a value.

After defining this extension, you can call the repetitions(task:) method on any integer to perform a task that many number of times:

```swift
3.repetitions {
    print("Hello!")
}
// Hello!
// Hello!
// Hello!
```

## Subscripts
Extensions can add new subscripts to an existing type. This example adds an integer subscript to Swift’s built-in Int type. This subscript [n] returns the decimal digit n places in from the right of the number:

123456789[0] returns 9
123456789[1] returns 8
…and so on:

```swift
extension Int {
    subscript(digitIndex: Int) -> Int {
        var decimalBase = 1
        for _ in 0..<digitIndex {
            decimalBase *= 10
        }
        return (self / decimalBase) % 10
    }
}
746381295[0]
// returns 5
746381295[1]
// returns 9
746381295[2]
// returns 2
746381295[8]
// returns 7
```