Collection that lazily initializes and caches its elements. Initially requires only the indexes and initializes elements as needed (e.g. UITableView data)

LazyCachedCollection.swift

//
//  LazyCachedCollection.swift
//
//  Created by John Estropia on 2017/09/27.
//  Copyright © 2017 John Estropia. All rights reserved.
//

import Foundation

class LazyCachedCollection<Element>: RandomAccessCollection {
    
    init(_ indices: CountableRange<Index>, _ initializer: @escaping (Index) -> Element) {
        
        self.startIndex = indices.lowerBound
        self.endIndex = indices.upperBound
        
        var cache: [(Index, Element)] = []
        self.lazyEnumeration = indices.enumerated().lazy.map { (offset, index) -> (index: Index, element: Element) in
            
            if offset < cache.endIndex {
                
                return cache[offset]
            }
            let value = initializer(index)
            let tuple: (index: Index, element: Element) = (index: index, element: value)
            cache.append(tuple)
            return tuple
        }
    }
    
    
    // MARK: Sequence
    
    typealias Iterator = AnyIterator<Element>
    
    func makeIterator() -> Iterator {
        
        var indexIterator = self.lazyEnumeration.makeIterator()
        return AnyIterator { indexIterator.next()?.element }
    }
    
    
    // MARK: Collection
    
    typealias Index = Int
    
    let startIndex: Index
    let endIndex: Index
    
    subscript(position: Index) -> Iterator.Element {
        
        return self.lazyEnumeration.first(where: { (index, _) in position == index })!.1
    }
    
    func index(after i: Index) -> Index {
        
        return self.indices.index(after: i)
    }
    
    
    // MARK: Private
    
    private let lazyEnumeration: LazyMapSequence<EnumeratedSequence<CountableRange<Index>>, (index: Index, element: Element)>
}

Usage:

let collection = LazyCachedCollection<String>(0 ..< 6) { (index) -> String in
    print("cache added: \(index)")
    return "Item\(index)"
}
print("= \(collection[3]) // Initializes 4 items")
print("= \(collection[2]) // Uses cached values")
print("= \(collection[5]) // Initializes 2 more items")

Output

cache added: 0
cache added: 1
cache added: 2
cache added: 3
= Item3 // Initializes 4 items
= Item2 // Uses cached values
cache added: 4
cache added: 5
= Item5 // Initializes 2 more items