Locally persisting data
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Persisting data locally consists of keeping app data after the app has been killed and the variables removed from memory. The persisted data offers many advantages. We can use it to show initial data when the app starts and waits for the first batch data to be fetched from the server. It can also be used to allow for offline app usage.
iOS isolates app data from other apps
For security reasons, each app is isolated from the rest of the apps. This is called sandboxing. This article shows the different ways that allow two or more apps to share their data
There are many ways to persist data in SwiftUI that we cover below.
UserDefaults
It is a very simple key-value storage that persists data in a file. The API surface is very small and the developer does not need to manage the persisted file. This makes this technique very efficient for simple storage use cases. You can find a short guide here.
Here is sample code that shows how to persist and load data.
UserDefaults.standard.set(self.tapCount, forKey: "Tap")
@State private var tapCount = UserDefaults.standard.integer(forKey: "Tap")
Codable saved in a file
A more advanced and powerful technique is to manually load and persist a Codable into a file. This technique is useful if you want to store complex objects (such as the state or model) in a JSON file. There are two steps in this process, the first one consists of decoding / encoding the object from / into JSON using JSONDecoder().decode and JSONEncoder().encode. The second step consists of loading / saving the encoded data and we can think of two ways to achieve this. The first one consists of user defaults' dataForKey: to load the data and setObject:ForKey to persist it. Another one consists of creating and managing a file by the developer using file APIs such as fileHandle.availableData to load the data from a file and data.write to save it.
Sophisticated data persistence libraries
For storing data in a database or similar fashion, SQLite is available as a low level library. It is not recommended to use it unless there is a strong performance concern. Instead, it is recommended to use libraries specialized in data persistence. Some can be assimilated to an ORM library (Object Relational Mapper). The remainder of this section describes some of them.
Please be careful about the pricing of cloud storage
Sophisticated databases generally provide cloud storage to provide a complete offer. If you're interested in storing data in the cloud, please take some time to read the pricing page to avoid any bad surprises when your app runs in production.
Core Data
Core Data is the official library to "Persist or cache data on a single device, or sync data to multiple devices with CloudKit". It existed since iOS 3 and Apple continuously updates it to keep it relevant. It also has the reputation of having a steep learning curve, but it remains famous among developers.
It works similarly as an ORM where classes are mapped into tables. Xcode provides a graphical editor that allows to specify the tables, the relations and generate the necessary code (in Swift or Objective-C).
Even though Core Date existed before SwiftUI, Apple made sure that both of them can be used together. This article shows how to use Core Data in a SwiftUI project.
Realm
Realm is a high level alternative to SQLite. It can be seen as alternative to Core Data as they seem to provide a similar list of features. Most notably, the possibility to store data locally or in the cloud. The points where Realm wins is that the library seems simpler to learn and to use and that it is also available in Android.
Firebase datastore (or any other cloud based storage)
As opposed to Realm and Core Data, which are local first databases, Firebase datastore is a cloud first database. This means that Firebase Datastore requires an internet connection to store and load the data. However, the library is simple to use and supports real time updates.
TIP
Firebase datastore is part of a bigger suite of service called Firebase. For example, we can Firebase App Distribution in Firebase, which is a service that allows to deploy and distribute apps without going the burden of using TestFlight.
PW: complete the official iOS persisting data tutorial
This PW shows how to save a Codable in a manually managed file using JSON encoder and filesystem APIs.
https://developer.apple.com/tutorials/app-dev-training/persisting-data