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Krate is a SharedPreferences wrapper library that uses delegated properties for convenient access to SharedPreferences values.

Here's what its basic usage looks like, extending the provided SimpleKrate class:

class UserSettings(context: Context) : SimpleKrate(context) {

    var notificationsEnabled by booleanPref("notifications_enabled", false)
    var loginCount by intPref("login_count", 0)
    var nickname by stringPref("nickname", "")


val settings = UserSettings(context)
settings.loginCount = 10
Log.d("LOGIN_COUNT", "Count: ${settings.loginCount}")


You can include Krate in your project from the mavenCentral() repository, like so:

implementation 'hu.autsoft:krate:0.5.0'

Optionals vs defaults

Each stored property can be declared with or without a default value. Here's how the two options differ:

Optional values:

A property declared with the one-argument delegate function will have a nullable type. It will have a null value if no value has been set for this property yet, and its current value can be erased from SharedPreferences completely by setting it to null.

var username: String? by stringPref("username")

Default values:

A property declared with the two-argument delegate function takes its default value as the second argument, and it will have a non-nullable type. Reading from this property will return either the value it was last set to or the default value. Setting this property will update the value stored in SharedPreferences. Note that there's no way to remove these values from SharedPreferences (although you could set it explicitly to the default value).

var username: String by stringPref("username", defaultValue = "admin")

Custom Krate implementations

You can usually get away with extending SimpleKrate, as it does allow you to pass in a custom name for the SharedPreferences to be used to store your values in its constructor as an optional parameter. (If you pass in no name parameter to its constructor, it will default to using the instance returned by PreferenceManager.getDefaultSharedPreferences(context).)

However, you can also implement the Krate interface directly if you want to manage the SharedPreferences instance yourself for whatever reason - all this interface requires is a property that holds a SharedPreferences instance. With that, you can use the delegate functions the same way as shown above:

class ExampleCustomKrate(context: Context) : Krate {

    override val sharedPreferences: SharedPreferences

    init {
        sharedPreferences = context.applicationContext.getSharedPreferences("custom_krate_prefs", Context.MODE_PRIVATE)

    var exampleBoolean by booleanPref("exampleBoolean", false)

For simple applications, your Activity or Fragment can easily serve as a Krate implementation:

class MainActivity : AppCompatActivity(), Krate {

    override val sharedPreferences: SharedPreferences by lazy {
        getPreferences(Context.MODE_PRIVATE) // Could also fetch a named or default SharedPrefs
    var username by stringPref("username", "")


Third party implementations

You can create the SharedPreferences instance to implement Krate's interface by using third party SharedPreferences implementations as well. For example, EncryptedSharedPreferences or Harmony are such implementations.

Here's how you'd use EncryptedSharedPreferences with Krate (see this source file for the full code):

class EncryptedKrate(applicationContext: Context) : Krate {
    override val sharedPreferences: SharedPreferences

    init {
        /* ... */
        sharedPreferences = EncryptedSharedPreferences.create(applicationContext, ...)

    val myStringValue: String by stringPref("my_string_value", "")


You can add validation rules to your Krate properties by providing an additional lambda parameter, isValid:

var percentage: Int by intPref(
        key = "percentage",
        defaultValue = 0,
        isValid = { it in 0..100 }

If this validation fails, an IllegalArgumentException will be thrown.


Krate, by default, supports the types that SharedPreferences supports. These are Boolean, Float, Int, Long, String and Set<String>. You may of course want to store additional types in Krate. For some common types, addon libraries are available, as described below.

If you don't find support for the type you're looking for, implementing your own delegate in your own project based on the code of existing delegates should be quite simple, this is very much a supported use case. If you think your type might be commonly used, you can also open an issue to ask for an addon library to provide for that type.

Gson support

The krate-gson artifact provides you a gsonPref delegate which can store any arbitrary type, as long as GSON can serialize and deserialize it. This addon, like the base library, is available from mavenCentral():

implementation 'hu.autsoft:krate-gson:0.5.0'

Its basic usage is the same as with any of the base library's delegates:

class GsonKrate(context: Context) : SimpleKrate(context) {
    var user: User? by gsonPref("user")
    var savedArticles: List<Article>? by gsonPref("articles")

By default, the Gson instance created by a simple Gson() constructor call is used. If you want to provide your own Gson instance that you've configured, you can set the gson extension property on your Krate. Any gsonPref delegates within this Krate will use this instance for serialization and deserialization.

class CustomGsonKrate(context: Context) : SimpleKrate(context) {
    init {
        gson = GsonBuilder().create()

Moshi support

Similarly to [krate-gson], the following artifacts add support for using Moshi to serialize values to SharedPreferences via Krate.

Since Moshi supports both reflection-based serialization and code generation, there are multiple Krate artifacts for different use cases. You should always include only one of the dependencies below, otherwise you'll end up with a dexing error.

The usage of the Krate integration is the same for both setups:

class MoshiKrate(context: Context) : SimpleKrate(context) {
    var user: User? by moshiPref("user")
    var savedArticles: List<Article>? by moshiPref("articles")

If you want to provide your own Moshi instance that you've configured with your own custom adapters, you can set the moshi extension property on your Krate. Any moshiPref delegates within this Krate will use this instance for serialization and deserialization.

class CustomMoshiKrate(context: Context) : SimpleKrate(context) {
    init {
        moshi = Moshi.Builder().build()

Only codegen

If you only want to use Moshi adapters that you generate via Moshi's codegen facilities, you can use the following Krate artifact in your project to make use of these adapters:

implementation 'hu.autsoft:krate-moshi-codegen:0.5.0'

This will give you a default Moshi instance created by a call to Moshi.Builder().build(). This instance will find and use any of the adapters generated by Moshi's codegen automatically.

Only reflection, or mixed use of reflection and codegen

If you rely on reflection for your Moshi serialization, and therefore need a KotlinJsonAdapterFactory included in your Moshi instance, use the following Krate Moshi dependency:

implementation 'hu.autsoft:krate-moshi-reflect:0.5.0'

The default Moshi instance from this dependency will include the aforementioned factory, and be able to serialize any Kotlin class. Note that this approach relies on the kotlin-reflect library, which is a large dependency.

You may choose to use Moshi's codegen for some classes in your project, and serialize the ones with no adapters generated with the default approach via reflection. For this mixed use case, you should probably choose this dependency.


Copyright 2020 AutSoft

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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distributed under the License is distributed on an "AS IS" BASIS,
See the License for the specific language governing permissions and
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