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Preferences

Property syntax for Android's SharedPreferences/DataStore or iOS/macOS NSUserDefaults.

Supported platforms: macOS (x64), iOS (arm32, arm64 & x64), Android.

This library uses Kotlin's property delegation to make using SharedPreferences and DataStore<Preferences> as easy as accessing a property on an object, and provides an NSUserDefaults backed implementation for macOS and iOS.

On Android, it relies on the appCtx module of this library to allow usage in object, and can support storage on device encrypted storage for devices supporting Direct Boot. See the source code for more information.

Setup

If you want to use this dependency without using one of the fun packs, you can use Splitties.preferences, provided you have refreshVersions added to the project.

For reference, the maven coordinates of this module are com.louiscad.splitties:splitties-preferences.

Table of contents

Defining the preferences properties in an object

Define your preferences in an object that extends splitties.preferences.Preferences, like in the example below:

import splitties.preferences.Preferences

object GamePreferences : Preferences("gameState") {
  var magicNumber by intPref("magicNumber", defaultValue = 0)
  var currentLevel by intPref("currentLevel", 1)
  var bossesFought by intPref("bossBattleVictories", 0)
  var lastTimePlayed by longPref("lastSessionTime", 0L)

  val pseudoFlow: Flow<String>
  var pseudo by stringPref("playerPseudo", "Player 1").also {
    pseudoFlow = it.valueFlow()
  }

  val favoriteCharacterFlow: Flow<String?>
  var favoriteCharacter by stringOrNullPref("favoriteCharacter").also {
    favoriteCharacterFlow = it.valueFlow()
  }
}

Then just use the properties:

fun setResponseOfTheUltimateQuestionOfLifeTheUniverseAndEverything() {
    GamePreferences.magicNumber = 42
}

fun doSomeMagic() {
    toast("Magic: ${GamePreferences.magicNumber}!")
}

fun resetProgress() {
    GamePreferences.edit { // Batch edit
        currentLevel = 1
        bossesFought = 0
    }
}

The supported types are:

  • Boolean
  • Int
  • Float
  • Long
  • String
  • String?
  • Set<String>
  • Set<String>?

For default SharedPreferences, make an object that extends DefaultPreferences instead of Preferences.

Note that for better encapsulation, you might want to keep the mutable delegated properties private in some cases, and expose functions and flows instead.

Why object and not class?

Unless you use coroutines (read more about this in next section just below), a class instead of an object is not recommended because it would mean you can instantiate it multiple times, while the underlying preferences xml file is cached for the rest of your app's process lifetime once loaded, so in a class you'd be allocating the delegates more times than needed, leading to an additional, unneeded, small pressure on the garbage collector.

However, you may make an abstract subclass of Preferences for specific use cases where adding logic to base Preferences or sharing some properties may be desirable. (If you do, please open an issue to tell us about this use case. It may become an example shown here.)

Loading the preferences without blocking the main thread

Note that this feature is currently only supported on Android. Feel free to open an issue if you want it on other platforms.

The object approach described above has several advantages, one of the most significant being ease of use anywhere in your app, but that also means you can easily access it from the main thread, and the first time you access the object, the underlying xml file where the preferences are stored is loaded, which may block the main thread for longer that you would want, possibly dropping a few frames.

With coroutines, it's easy to offload something on another thread, and this split embraces this capability.

Let's see a modified version of the GamePreferences described above, before passing in review each change.

import splitties.preferences.SuspendPrefsAccessor
import splitties.preferences.Preferences

class GamePreferences private constructor() : Preferences("gameState") {
    companion object : SuspendPrefsAccessor<GamePreferences>(::GamePreferences)

    var magicNumber by intPref("magicNumber", defaultValue = 0)
    var currentLevel by intPref("currentLevel", 1)
    var bossesFought by intPref("bossBattleVictories", 0)
    var lastTimePlayed by longPref("lastSessionTime", 0L)
    
    val pseudoFlow: Flow<String>
    var pseudo by stringPref("playerPseudo", "Player 1").also {
      pseudoFlow = it.valueFlow()
    }
    
    val favoriteCharacterFlow: Flow<String?>
    var favoriteCharacter by stringOrNullPref("favoriteCharacter").also {
      favoriteCharacterFlow = it.valueFlow()
    }
}

Here are all the changes:

  1. We moved from object to class.
  2. We added a private constructor().
  3. We added a companion object that extends the SuspendPrefsAccessor abstract class and calls its constructor with a reference to the constructor.

With this change, we can no longer access the GamePreferences singleton directly from anywhere… unless we are in a coroutine!

From any suspend function, you just have to call GamePreferences() like you were calling a constructor, but in reality, it is a function call that suspends while loading the preferences for the first time in process life in Dispatchers.IO.

If the preferences have already been loaded, it immediately returns the now instantiated singleton.

If you have non suspending functions that would need to access the preferences, you have two options: pass your Preferences subclass as a parameter, or make it a suspend function.

Using AndroidX DataStore instead of SharedPreferences under the hood

Android's SharedPreferences has one performance drawback: it can perform disk I/O on the main thread on its own when the onStop callback of an Activity is called. The only way to ensure your app will not be affected by the potential ANRs (Application Not Responding) it can cause is to replace it with something else.

The AndroidX team made DataStore just for that, but its API is not compatible, which would require significant code changes in host projects.

If you use this (Splitties Preferences) however, all the hard work has been done for you.

There are just 3 quick steps to perform to use DataStore instead of SharedPreferences:

  1. Make sure you use SuspendPrefsAccessor, as described in the section above (not doing so will crash on first access).
  2. Extend DataStorePreferences instead of Preferences.
  3. Opt-in by adding @OptIn(DataStorePreferencesPreview::class) to your class.

Once you're done, test your app to make sure it still works fine.

Data migration is automatically done if you're moving from SharedPreferences and keep the same name. If you were using DefaultPreferences, you can pass null to name.

Be sure that there's only one class that uses the same name, or DataStore will throw.

Finally, note that direct boot (aka. deviceProtectedStorage) is not supported yet, but is being considered. Add a 👍 on this issue to raise the priority.