Introduction: A Kotlin Multiplatform library for mobile apps to save settings as key-value pairs
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This is a Kotlin library for Multiplatform apps, so that common code can persist key-value data. It stores things using SharedPreferences on Android and NSUserDefaults on iOS.

Adding to your project

First, add the multiplatform-settings bintray url to the repositories block of any module using it.

repositories {
    maven { url '' }

Then, simply add the dependency to your common source-set dependencies

implementation "com.russhwolf:multiplatform-settings:0.3.2"

See also the sample project, which uses this structure.


The Settings interface has implementations on the Android, iOS (arm64 and x64), macOS (x64), JVM, and JS platforms. (Note that the JVM and JS implementations are currently marked as experimental.)

The Android implementation is AndroidSettings, which wraps SharedPreferences.

val delegate: SharedPreferences = ...
val settings: Settings = AndroidSettings(delegate)

On iOS or macOS, AppleSettings wraps NSUserDefaults.

val delegate: NSUserDefaults = ...
val settings: Settings = AppleSettings(delegate)

Once the Settings instance is created, you can store values by calling the various putXXX() methods, or their operator shortcuts

settings.putInt("key", 3)
settings["key"] = 3

You can retrieve stored values via the getXXX() methods or their operator shortcuts. If a key is not present, then the supplied default will be returned instead.

val a: Int = settings.getInt("key")
val b: Int = settings.getInt("key", defaultValue = -1) 
val c: Int = settings["key", -1]

The getXXX() and putXXX() operation for a given key can be wrapped using a property delegate. This has the advantage of ensuring that the key is always accessed with a consistent type.

val a: Int by"key")
val b: Int by"key", defaultValue = -1)

Nullable delegates exists so that absence of a key can be indicated by null instead of a default value

val a: Int? by settings.nullableInt("key")

The key parameter can be omitted for delegates, and the property name will be reflectively used instead.

val a: Int by // internally, key is "a"

Existence of a key can be queried

val a: Boolean = settings.hasKey("key")
val b: Boolean = "key" in settings

Values can also be removed by key

settings -= "key"  

Finally, all values in a Settings instance can be removed


For the Android, iOS, and macOS platforms, a Factory class also exists, so that multiple named Settings instances can coexist with the names being controlled from common code.

On Android, this factory needs a Context parameter

val context: Context = ...
val factory: Settings.Factory = AndroidSettings.Factory(context)

On iOS and macOS, the factory can be instantiated without passing any parameter

val factory: Settings.Factory = AppleSettings.Factory()


A testing dependency is available to aid in testing code that interacts with this library.

implementation "com.russhwolf:multiplatform-settings-test:0.3.2"

This includes a MockSettings implementation of the Settings interface, which is backed by an in-memory MutableMap on all platforms.

Experimental API

Experimental Platforms

A pure-JVM implementation exists which wraps the Java Properties API. Its experimental status is marked with the @ExperimentalJvm annotation

val delegate: Properties = ...
val settings: Settings = JvmSettings(delegate)

A JS implementation exists which wraps the Storage API. Its experimental status is marked with the @ExperimentalJvm annotation

val delegate: Storage = ...
val settings: Settings = JsSettings(delegate)

val settings: Settings = JsSettings() // use localStorage by default


Update listeners are available using an experimental API, only on Android, iOS, and macOS. These platforms are marked with the ObservableSettings interface, which includes addListener() and removeListener() methods.

val settingsListener: SettingsListener = settings.addListener(key) { ... }

The SettingsListener returned from the call should be used to signal when you're done listening:


This current listener implementation is not designed with any sort of thread-safety so it's recommended to only interact with these APIs from the main thread of your application.

The listener APIs make use of the Kotlin @Experimental annotation. All usages must be marked with @ExperimentalListener or @UseExperimental(ExperimentalListener::class).

Project Structure

The library logic lives in the commonMain, androidMain, and iosMain sources. The common source holds the Settings interface which exposes apis for persisting values of the Int, Long, String, Float, Double, and Boolean types. The common source also holds property delegate wrappers and other operator functions for cleaner syntax and usage. The android and ios sources then hold implementations, delegating to SharedPreferences or NSUserDefaults. The macOS platform reads from the same sources as iOS. The experimental JVM and JS implementations reside in the jvmMain and jsMain sources, respectively

Some unit tests are defined which can be run via ./gradlew test. These use Robolectric on Android to mock out the android-specific behavior, and use the ios simulator to run the ios tests. The macOS tests run natively on macOS hosts. The experimental JS implementation is configured to run tests with Mocha, and the experimental JVM implementation runs standard junit tests.

There is also a sample project to demonstrate usage, which is configured as a separate IDEA/gradle project in the sample directory. It includes a shared module with common, android, and ios sources, to demo a shared logic layer consuming the library. The app-android module consumes shared and provides an Android UI. The app-ios directory holds an Xcode project which builds an iOS app in the usual way, consuming a framework produced by shared. The sample project does not currently have implementations on macOS, JVM, or JS.

The shared module includes some simple unit tests in common code to demonstrate manually mocking out the Settings interface when testing code that interacts with it.


Copyright 2018-2019 Russell Wolf

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
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