Jetpack DataStore sample with Kotlin Seirialization

A sample Android app to demonstrate Jetpack DataStore with Kotlin Serialization instead of Protocol Buffers.

Typed DataStore doesn't require protocol buffers

Although Android Developers documents explain that we need to define data scheme by usign protocol buffers, it's actually not required. We can use any data serialization method, e.g. Kotlin Serialization or Moshi, with Proto DataStore (or sometimes it's called Typed DataStore).

DataStore is a library to serialize data object to binary and store it on storage. Data conversion between data object and binary is abstracted by Serializer interface. App can provide arbitrary implementation of Serializer when instantiating DataStore.

Android Developers explains how to implement Serializer with protocol buffers. However, Serializer and DataStore doesn't depend on protocol buffers. We can implement Serializer interface with another serialization method and can utilize Typed DataStore without protocol buffers.

How to implement Serializer with Kotlin Serialization

MyDataSerializer.kt is an example implementation of Serializer with Kotlin Serialization.

class MyDataSerializer(
    private val stringFormat: StringFormat = Json {
        ignoreUnknownKeys = true
        encodeDefaults = true
    },
) : Serializer<MyData> {
    override val defaultValue: MyData
        get() = MyData()

    override fun readFrom(input: InputStream): MyData {
        try {
            val bytes = input.readBytes()
            val string = bytes.decodeToString()
            return stringFormat.decodeFromString(string)
        } catch (e: SerializationException) {
            throw CorruptionException("Cannot read stored data", e)
        }
    }

    override fun writeTo(t: MyData, output: OutputStream) {
        val string = stringFormat.encodeToString(t)
        val bytes = string.encodeToByteArray()
        output.write(bytes)
    }
}

Here is the definition of the corresponding data class MyData.

@Serializable
data class MyData(
    val myBooleanValue: Boolean = false,
    val myStringValue: String? = null,
    val myEnumValue: MyEnum = MyEnum.FOO,
) {
    @Serializable
    enum class MyEnum {
        @SerialName("FOO")
        FOO,

        @SerialName("BAR")
        BAR,

        @SerialName("BAZ")
        BAZ,
    }
}

How to use DataStore with MyDataSerializer

App can create DataStore with MyDataSerializer and use it to persist data on storage.

    private val dataStore = context.createDataStore(
        fileName = "my_data.json",
        serializer = MyDataSerializer(),
    )

See Android Developers guide about how to read from and write to DataStore.

The DataStore will generate and store JSON file files/datastore/my_data.json on app's storage like this.

{"myBooleanValue":true,"myStringValue":"Sample string","myEnumValue":"BAR"}