An observable CustomStringConvertible, RandomAccess-, RangeReplaceableCollection in 50 lines of swift

SmartCollection.swift

//  Created by Damiaan on 23/01/2019.
//  Copyright © 2019 Devian. All rights reserved.

class SmartCollection<Element> {
	class Reference {
		private (set) var pointee: Element
		
		init(to element: Element) {
			pointee = element
		}
		
		fileprivate subscript<Value>(smartPath: WritableKeyPath<Element, Value>) -> Value {
			get { return pointee[keyPath: smartPath] }
			set {
				pointee[keyPath: smartPath] = newValue
			}
		}
	}
	
	private (set) var references = [Reference]()
	
	typealias FieldChangeSubscription = ((_ index: Int, _ changedKeyPath: PartialKeyPath<Element>) -> Void)
	typealias ReplaceSubscription = ((_ subrange: Range<Int>, _ newElements: Array<Element>) -> Void)
	
	var preFieldChangeSubscriptions = [FieldChangeSubscription]()
	var postFieldChangeSubscriptions = [FieldChangeSubscription]()
	var preReplaceSubscriptions = [ReplaceSubscription]()
	var postReplaceSubscriptions = [ReplaceSubscription]()
	
	required init() {}
	
	subscript<Value>(_ index: Int, keyPath: WritableKeyPath<Element, Value>) -> Value {
		get { return references[index][keyPath] }
		set {
			let notify: (FieldChangeSubscription)->() = { $0(index, keyPath) }
			preFieldChangeSubscriptions.forEach(notify)
			references[index][keyPath] = newValue
			postFieldChangeSubscriptions.forEach(notify)
		}
	}
}

extension SmartCollection: RandomAccessCollection {
	var startIndex: Int { return references.startIndex }
	var endIndex: Int { return references.endIndex}
	
	subscript(_ index: Int) -> Element {
		return references[index].pointee
	}
}

extension SmartCollection: RangeReplaceableCollection {
	func replaceSubrange<C, R>(_ subrange: R, with newElements: C) where C : Collection, R : RangeExpression, Element == C.Element, Int == R.Bound {
		
		let notify: (ReplaceSubscription)->() = { $0(subrange.relative(to: self.references), Array(newElements)) }
		
		preReplaceSubscriptions.forEach(notify)
		references.replaceSubrange(subrange, with: newElements.map(Reference.init))
		postReplaceSubscriptions.forEach(notify)
	}
}

extension SmartCollection: CustomStringConvertible where Element: CustomStringConvertible {
	var description: String {
		return "SmartCollection " + references.map {$0.pointee.description}.description
	}
}

extension SmartCollection.Reference: CustomStringConvertible where Element: CustomStringConvertible {
	var description: String {
		return "Reference to \(pointee)"
	}
}