Simple Image Downloading/Caching Service

ImageCachingService.swift

//
//  ImageCachingService.swift
//  Exuberant
//
//  Created by TJ Barber on 12/20/17.
//  Copyright © 2017 Thomas J. Barber. All rights reserved.
//

import UIKit

enum ImageCachingError: Error {
    case cannotEncodeString
    case documentsDirectoryDoesNotExist
    case newLocationDoesNotExist
    case couldNotMoveFile
    case couldNotOpenImageFile
}

class ImageCachingService: NSObject {
    static let sharedInstance = ImageCachingService()
    let sessionConfiguration = URLSessionConfiguration.default
    let cache = NSCache<NSString, UIImage>()
    let fileManager = FileManager.default
    
    func downloadAndCacheImage(from url: URL, completion: @escaping (UIImage?, Error?) -> Void) {
        guard let encodedAbsoluteString = encodeString(url.absoluteString) else {
            completion(nil, ImageCachingError.cannotEncodeString)
            return
        }
        
        let request = URLRequest(url: url, cachePolicy: .returnCacheDataElseLoad, timeoutInterval: 60.0)
        let session = URLSession(configuration: sessionConfiguration)
        let downloadTask = session.downloadTask(with: request) { location, response, error in
            if let error = error {
                completion(nil, error)
                return
            }
            guard let location = location else { completion(nil, ImageCachingError.newLocationDoesNotExist); return }
            guard let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first else { completion(nil, ImageCachingError.documentsDirectoryDoesNotExist); return }
            let filePath = documentsDirectory.appendingPathComponent(encodedAbsoluteString)
            
            do {
                try self.fileManager.moveItem(at: location, to: filePath)
                guard let imageData = self.fileManager.contents(atPath: filePath.absoluteString) else { completion(nil, ImageCachingError.couldNotOpenImageFile); return }
                guard let image = UIImage(data: imageData) else { completion(nil, ImageCachingError.couldNotOpenImageFile); return }
                completion(image, nil)
            } catch (let e) {
                print(e)
                completion(nil, ImageCachingError.couldNotMoveFile)
            }
        }
        downloadTask.resume()
    }
    
    func getImageWith(url: URL, completion: @escaping (UIImage?, Error?) -> Void) {
        guard let encodedAbsoluteString = encodeString(url.absoluteString) else {
            completion(nil, ImageCachingError.cannotEncodeString)
            return
        }
        
        let urlString = NSString(string: encodedAbsoluteString)
        
        if let image = self.cache.object(forKey: urlString) {
            // We have the image in memory
            print("loading file from cache...")
            completion(image, nil)
            return
        }
        
        guard let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first else { completion(nil, ImageCachingError.documentsDirectoryDoesNotExist); return }
        let filePath = documentsDirectory.appendingPathComponent(encodedAbsoluteString)
        
        if fileManager.fileExists(atPath: filePath.path) {
            print("loading file from disk...")
            guard let imageData = self.fileManager.contents(atPath: filePath.path) else { completion(nil, ImageCachingError.couldNotOpenImageFile); return }
            guard let image = UIImage(data: imageData) else { completion(nil, ImageCachingError.couldNotOpenImageFile); return }
            self.cache.setObject(image, forKey: urlString)
            completion(image, nil)
            return
        }
        
        print("downloading file...")
        self.downloadAndCacheImage(from: url) { image, error in
            
            if let error = error {
                completion(nil, error)
                return
            }
            
            guard let image = image else { fatalError("No error and no image") }
            
            self.cache.setObject(image, forKey: urlString)
            completion(image, nil)
        }
    }
    
    func encodeString(_ string: String) -> String? {
        return string.addingPercentEncoding(withAllowedCharacters: CharacterSet.urlHostAllowed)
    }
}