Iterator design sample in Swift

This time I’ll concentrate on the iterator design sample. The sample is closely used within the Swift customary library, there are protocols that gives you assist if you want to create an iterator, however actually: I’ve by no means carried out this sample straight. 😅

The reality is that most likely in 99% of the use instances you will by no means should cope with this sample, as a result of there may be superb assist for iterators built-in straight into Swift. At all times use sequences, arrays, dictionaries as a substitute of straight implementing this sample, but it surely’s good to know the way issues are working beneath the hood, is not it? 🙃

What’s the iterator desin sample?

Because the title suggests, the sample allows you to iterate over a group of parts. Right here is the definition from the gang of 4 e-book:

Present a approach to entry the weather of an mixture object sequentially with out exposing its underlying illustration.

Lengthy story brief the iterator provides you an interface that can allow you to iterate over collections no matter how they’re carried out within the background. Here’s a fast instance of the idea above utilizing a string iterator.

import Basis

protocol StringIterator {
    func subsequent() -> String?
}

class ArrayStringIterator: StringIterator {

    personal let values: [String]
    personal var index: Int?

    init(_ values: [String]) {
        self.values = values
    }

    personal func nextIndex(for index: Int?) -> Int? {
        if let index = index, index < self.values.depend - 1 {
            return index + 1
        }
        if index == nil, !self.values.isEmpty {
            return 0
        }
        return nil
    }

    func subsequent() -> String? {
        if let index = self.nextIndex(for: self.index) {
            self.index = index
            return self.values[index]
        }
        return nil
    }
}


protocol Iterable {
    func makeIterator() -> StringIterator
}

class DataArray: Iterable {

    personal var dataSource: [String]

    init() {
        self.dataSource = ["🐶", "🐔", "🐵", "🦁", "🐯", "🐭", "🐱", "🐮", "🐷"]
    }

    func makeIterator() -> StringIterator {
        return ArrayStringIterator(self.dataSource)
    }
}

let information = DataArray()
let iterator = information.makeIterator()

whereas let subsequent = iterator.subsequent() {
    print(subsequent)
}

As you may see there are two principal protocols and a extremely easy implementation for each of them. Our DataArray class now acts like an actual array, the underlying parts might be iterated by utilizing a loop. Let’s ditch the idea and re-implement the instance from above by utilizing actual Swift customary library parts. 😉

Customized sequences in Swift

Swift has a built-in sequence protocol that will help you creating iterators. Implementing your personal sequence in Swift is all about hiding your underlying information construction by making a customized iterator object. You simply should retailer the present index and return your subsequent ingredient in response to that every time the following perform will get known as. 😛

import Basis

struct Emojis: Sequence {
    let animals: [String]

    func makeIterator() -> EmojiIterator {
        return EmojiIterator(self.animals)
    }
}

struct EmojiIterator: IteratorProtocol {

    personal let values: [String]
    personal var index: Int?

    init(_ values: [String]) {
        self.values = values
    }

    personal func nextIndex(for index: Int?) -> Int? {
        if let index = index, index < self.values.depend - 1 {
            return index + 1
        }
        if index == nil, !self.values.isEmpty {
            return 0
        }
        return nil
    }

    mutating func subsequent() -> String? {
        if let index = self.nextIndex(for: self.index) {
            self.index = index
            return self.values[index]
        }
        return nil
    }
}

let emojis = Emojis(animals: ["🐶", "🐔", "🐵", "🦁", "🐯", "🐭", "🐱", "🐮", "🐷"])
for emoji in emojis {
    print(emoji)
}

So the Sequence protocol is a generic counterpart of our customized iterable protocol used within the first instance. The IteratorProtocol is considerably just like the string iterator protocol used earlier than, however extra Swiftish and naturally extra generic.

So, that is nice. Lastly you know the way to create a customized sequence. Which is nice if you would like to cover your information construction and supply a generic iterable interface. Think about what would occur in case you have been about to start out utilizing a dictionary as a substitute of an array for storing named emojis with out an iterator that wraps them. 🤔

Now the factor is that there’s another tremendous helpful factor within the Swift customary library that I might like to speak about. That is proper, one abstraction stage up and right here we’re:

Customized collections in Swift

Collections are one step past sequences. Parts inside them might be accessed through subscript additionally they outline each a startIndex and an endIndex, plus particular person parts of a group might be accessed a number of occasions. Sounds good? 👍

Generally it may be helpful to create a customized assortment sort. For instance if you would like to remove non-compulsory values. Think about a categorized favourite mechanism, for each class you’d have an array of favorites, so that you’d should cope with empty and non-existing instances. With a customized assortment you possibly can cover that further code inside your customized information construction and supply a clear interface for the remainder of your app. 😍

class Favorites {

    typealias FavoriteType = [String: [String]]

    personal(set) var checklist: FavoriteType

    public static let shared = Favorites()

    personal init() {
        self.checklist = FavoriteType()
    }
}


extension Favorites: Assortment {

    typealias Index = FavoriteType.Index
    typealias Component = FavoriteType.Component

    var startIndex: Index {
        return self.checklist.startIndex
    }
    var endIndex: Index {
        return self.checklist.endIndex
    }

    subscript(index: Index) -> Iterator.Component {
        return self.checklist[index]
    }

    func index(after i: Index) -> Index {
        return self.checklist.index(after: i)
    }
}

extension Favorites {

    subscript(index: String) -> [String] {
        return self.checklist[index] ?? []
    }

    func add(_ worth: String, class: String) {
        if var values = self.checklist[category] {
            guard !values.comprises(worth) else {
                return
            }
            values.append(worth)
            self.checklist[category] = values
        }
        else {
            self.checklist[category] = [value]
        }
    }

    func take away(_ worth: String, class: String) {
        guard var values = self.checklist[category] else {
            return
        }
        values = values.filter { $0 == worth }

        if values.isEmpty {
            self.checklist.removeValue(forKey: class)
        }
        else {
            self.checklist[category] = values
        }
    }
}


Favorites.shared.add("apple", class: "fruits")
Favorites.shared.add("pear", class: "fruits")
Favorites.shared.add("apple", class: "fruits")

Favorites.shared["fruits"]

Favorites.shared.take away("apple", class: "fruits")
Favorites.shared.take away("pear", class: "fruits")
Favorites.shared.checklist

I do know, this can be a actually dumb instance, but it surely demonstrates why collections are extra superior in comparison with pure sequences. Additionally within the hyperlinks beneath there are nice demos of nicely written collections. Be happy to be taught extra about these tremendous protocols and customized information varieties hidden (not so deep) contained in the Swift customary library. 🤐