Iterator design sample in Swift

This time I will deal with the iterator design sample. The sample is closely used within the Swift customary library, there are protocols that will provide you with help if you have to create an iterator, however truthfully: I’ve by no means carried out this sample immediately. 😅

The reality is that most likely in 99% of the use circumstances you will by no means must cope with this sample, as a result of there may be superb help for iterators built-in immediately into Swift. All the time use sequences, arrays, dictionaries as an alternative of immediately implementing this sample, nevertheless it’s good to understand how issues are working underneath the hood, is not it? 🙃

What’s the iterator desin sample?

Because the identify suggests, the sample allows you to iterate over a group of components. 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 quick the iterator offers you an interface that may 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 {

    non-public let values: [String]
    non-public var index: Int?

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

    non-public 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 {

    non-public var dataSource: [String]

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

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

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

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

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

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 knowledge construction by making a customized iterator object. You simply must retailer the present index and return your subsequent component in response to that every time the following operate will get referred to as. 😛

import Basis

struct Emojis: Sequence {
    let animals: [String]

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

struct EmojiIterator: IteratorProtocol {

    non-public let values: [String]
    non-public var index: Int?

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

    non-public 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 knowledge construction and supply a generic iterable interface. Think about what would occur when you have been about to begin utilizing a dictionary as an alternative 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. Components within them might be accessed by way of subscript in addition they outline each a startIndex and an endIndex, plus particular person components of a group might be accessed a number of occasions. Sounds good? 👍

Generally it may be helpful to create a customized assortment kind. For instance if you would like to eradicate non-compulsory values. Think about a categorized favourite mechanism, for each class you’d have an array of favorites, so that you’d must cope with empty and non-existing circumstances. With a customized assortment you might conceal that further code inside your customized knowledge construction and supply a clear interface for the remainder of your app. 😍

class Favorites {

    typealias FavoriteType = [String: [String]]

    non-public(set) var record: FavoriteType

    public static let shared = Favorites()

    non-public init() {
        self.record = FavoriteType()
    }
}


extension Favorites: Assortment {

    typealias Index = FavoriteType.Index
    typealias Factor = FavoriteType.Factor

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

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

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

extension Favorites {

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

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

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

        if values.isEmpty {
            self.record.removeValue(forKey: class)
        }
        else {
            self.record[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.record

I do know, it is a actually dumb instance, nevertheless it demonstrates why collections are extra superior in comparison with pure sequences. Additionally within the hyperlinks under there are nice demos of effectively written collections. Be at liberty to study extra about these tremendous protocols and customized knowledge sorts hidden (not so deep) contained in the Swift customary library. 🤐