Storing data persistently on iOS is something what is needed quite often. In this post, we are going to look into how to build a persistent data store and how to store image data.
Initialising the persistent data store
Persistent data store is an object managing a folder on disk. It allows writing and reading data asynchronously.
Firstly, we need to create a folder where to store all the files. As every instance of the data store should manage its own folder, we will add an argument name to the initialiser. Then we can create a folder in user’s documents folder with that name. As writing and reading data is an expensive operation, we are going to offload the work to a concurrent DispatchQueue. Concurrent dispatch queue allows us to read multiple files at the same time (more about it a bit later).
| final class PersistentDataStore { | |
| let name: String | |
| private let dataStoreURL: URL | |
| private let queue: DispatchQueue | |
| init(name: String) throws { | |
| self.name = name | |
| queue = DispatchQueue(label: "com.augmentedcode.persistentdatastore", qos: .userInitiated, attributes: .concurrent, autoreleaseFrequency: .workItem) | |
| let documentsURL = try FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false) | |
| dataStoreURL = documentsURL.appendingPathComponent(name, isDirectory: true) | |
| try FileManager.default.createDirectory(at: dataStoreURL, withIntermediateDirectories: true, attributes: nil) | |
| } | |
| } |
Storing data asynchronously
Method for storing data on disk consists of closure, identifier and completion handler. This allows us to create a closure what transforms object to data. For example, it could transform UIImage to Data. Secondly, this transformation, possibly slow operation, can be offloaded to the same thread writing the data into a file. Using closure gives us a flexible API what we can extend with convenience methods.
| typealias Identifier = String | |
| enum Result { | |
| case failed(Error) | |
| case noData | |
| case success(Identifier) | |
| } | |
| func storeData(_ dataProvider: @escaping () -> (Data?), identifier: Identifier = UUID().uuidString, completionHandler block: @escaping (Result) -> ()) { | |
| queue.async(flags: .barrier) { | |
| let url = self.url(forIdentifier: identifier) | |
| guard let data = dataProvider(), !data.isEmpty else { | |
| DispatchQueue.main.async { | |
| block(.noData) | |
| } | |
| return | |
| } | |
| do { | |
| try data.write(to: url, options: .atomic) | |
| DispatchQueue.main.async { | |
| block(.success(identifier)) | |
| } | |
| } | |
| catch { | |
| DispatchQueue.main.async { | |
| block(.failed(error)) | |
| } | |
| } | |
| } | |
| } | |
| // Example (adding data to data store with unique identifier): | |
| persistentStore.storeData({ () -> (Data?) in | |
| return image.jpegData(compressionQuality: 1.0) | |
| }) { (result) in | |
| switch result { | |
| case .success(let identifier): | |
| print("Stored data successfully with identifier \(identifier).") | |
| case .noData: | |
| print("No data to store.") | |
| case .failed(let error): | |
| print("Failed storing data with error \(error)") | |
| } | |
| } |
Identifier is internally used as a filename and default implementation creates unique identifier. Therefore, when data store consumer would like to replace the current file, it can supply an identifier, otherwise new file is created.
Completion handler contains a
Result enum type. Result enum consists of three cases: success, transformation failure and data writing failure. Success’ associated value is identifier, failure contains error object and transformation failure is equal to noData.Important to note here is that the work item has barrier specified. Barrier means that when
DispatchQueue starts to handle the work item, it will wait until all the previous work items have finished running. Meaning, we will never try to update a file on disk when some other request is busy reading it.
Loading data asynchronously
Load data is generic method allowing the data transformation closure to return a specific type (e.g. transforming Data to UIImage). Shortly, load data reads file from disk and transforms it into a different type. As transformation can be a lengthy task, it is yet again running on the background thread and will not cause any hiccups in the UI.
| func loadData<T>(forIdentifier identifier: Identifier, dataTransformer: @escaping (Data) -> (T?), completionHandler block: @escaping (T?) -> ()) { | |
| queue.async { | |
| let url = self.url(forIdentifier: identifier) | |
| guard FileManager.default.fileExists(atPath: url.path) else { | |
| DispatchQueue.main.async { | |
| block(nil) | |
| } | |
| return | |
| } | |
| do { | |
| let data = try Data(contentsOf: url, options: .mappedIfSafe) | |
| let object = dataTransformer(data) | |
| DispatchQueue.main.async { | |
| block(object) | |
| } | |
| } | |
| catch { | |
| print("Failed reading data at URL \(url).") | |
| DispatchQueue.main.async { | |
| block(nil) | |
| } | |
| } | |
| } | |
| } | |
| // Example | |
| persistentStore.loadData(forIdentifier: "my_identifier", dataTransformer: { UIImage(data: $0) }) { (image) in | |
| guard let image = image else { | |
| print("Failed loading image.") | |
| return | |
| } | |
| print(image) | |
| } |
Removing data asynchronously
Removing a single file or all of the files is pretty straight-forward. As we are modifying files on disk, we will use barrier again and then FileManager’s removeItem(at:) together with contentsOfDirectory(at:includingPropertiesForKeys:options:).
| func removeData(forIdentifier identifier: Identifier) { | |
| queue.async(flags: .barrier) { | |
| let url = self.url(forIdentifier: identifier) | |
| guard FileManager.default.fileExists(atPath: url.path) else { return } | |
| do { | |
| try FileManager.default.removeItem(at: url) | |
| } | |
| catch { | |
| print("Failed removing file at URL \(url) with error \(error).") | |
| } | |
| } | |
| } | |
| func removeAll() { | |
| queue.async(flags: .barrier) { | |
| do { | |
| let urls = try FileManager.default.contentsOfDirectory(at: self.dataStoreURL, includingPropertiesForKeys: nil, options: []) | |
| try urls.forEach({ try FileManager.default.removeItem(at: $0) }) | |
| } | |
| catch { | |
| print("Failed removing all files with error \(error).") | |
| } | |
| } | |
| } |
Extension for storing images
It is easy to extend the PersistentDataStore with convenience methods for storing a specific type of data. This allows us to hide the technical details of transforming image to data and vice-versa. Moreover, calling the method gets easier to read as data transformation closure is not visible anymore.
| extension PersistentDataStore { | |
| func loadImage(forIdentifier identifier: Identifier, completionHandler block: @escaping (UIImage?) -> (Void)) { | |
| loadData(forIdentifier: identifier, dataTransformer: { UIImage(data: $0) }, completionHandler: block) | |
| } | |
| func storeImage(_ image: UIImage, identifier: String = UUID().uuidString, completionHandler handler: @escaping (Result) -> ()) { | |
| storeData({ image.jpegData(compressionQuality: 1.0) }, identifier: identifier, completionHandler: handler) | |
| } | |
| } | |
| // Examples: | |
| persistentStore.storeImage(image) { (result) in | |
| print(result) | |
| } | |
| persistentStore.loadImage(forIdentifier: "my_identifier") { (image) -> (Void) in | |
| guard let image = image else { | |
| print("Failed loading image.") | |
| return | |
| } | |
| print(image) | |
| } |
Summary
We created a persistent data store what is performant and has a flexible API. API can be extended easily to support any other data transformation. In addition, it uses thread-safe techniques for making sure data never gets corrupted.
Playground
PersistentDataStore (GitHub) Xcode 10, Swift 4.2
References
DispatchQueues (Apple)
dispatch_barrier_async (Apple)
Augmented Code 