There are several ways to save data onto a mobile device: you can persist data to a file, or you can use a local database, such as SQLite, or you can use SharedPreferences (on Android) or NSUserDefaults (on iOS).
shared_preferences should not be used for critical data as data stored there is not encrypted, and writes are not always guaranteed.
When using Flutter, you can take advantage of the shared_preferences library: it wraps both NSUserDefaults and SharedPreferences so that you can store simple data seamlessly in both iOS and Android without dealing with the specifics of the two operating systems.
Data is always persisted to disk asynchronously when you use shared_preferences.
SharedPreferences is an easy way to persist key-value data on disk. You can only store primitive data: int, double, bool, String, and stringList...