KStorage

KStorage is a lightweight Kotlin Multiplatform library that provides a unified and type-safe interface for key-value storage across different platforms. It simplifies the process of storing and retrieving data by abstracting the underlying storage mechanisms.

✨ Features

• Multiplatform Support: Works across Android, iOS, JVM, JS, Linux, macOS, Windows, tvOS, watchOS and WasmJS.
• Type-Safe API: Ensures compile-time type checking for stored values.
• Thread-Safe by Default: Every Krate is protected by a built-in platform-specific lock — no manual synchronization needed.
• Reactive: First-class StateFlow and Flow support for observing value changes.
• Lightweight: Minimal overhead with a focus on simplicity and performance.
• Extensible: Easily integrate with any storage backend — SharedPreferences, DataStore, ROOM, files, or your own.

🚀 Installation

Version catalogs

[versions]
klibs-kstorage = "<latest-version>"

[libraries]
klibs-kstorage = { module = "ru.astrainteractive.klibs:kstorage", version.ref = "klibs-kstorage" }

Gradle

implementation("ru.astrainteractive.klibs:kstorage:<version>")
// or version catalogs
implementation(libs.klibs.kstorage)

🛠️ Basic Usage

Blocking API

Use MutableKrate when your storage is fast and synchronous (e.g. in-memory maps, SharedPreferences):

class SettingsApi(private val settings: MutableMap<String, Int>) {

  // Basic mutable krate — reads/writes are synchronous
  val volumeKrate: MutableKrate<Int> = DefaultMutableKrate(
    factory = { 50 },
    loader = { settings["volume"] },
    saver = { value -> settings["volume"] = value }
  )
}

The factory provides the default value when the loader returns null. That's it — three lambdas and you have a fully thread-safe, type-safe storage accessor.

CachedKrate — Keep a Value in Memory

Wrap any Krate to avoid repeated loads:

val cachedVolume: CachedMutableKrate<Int> = volumeKrate.asCachedMutableKrate()

// Access the in-memory cached value directly
val current = cachedVolume.cachedValue

StateFlowKrate — Reactive Observation

Need to observe changes in your UI? Wrap into a StateFlowMutableKrate:

val reactiveVolume: StateFlowMutableKrate<Int> = volumeKrate.asStateFlowMutableKrate()

// Collect in your ViewModel / Compose / SwiftUI
reactiveVolume.cachedStateFlow.collect { volume ->
    println("Volume changed: $volume")
}

Property Delegation

Don't want to call .cachedValue every time? Use Kotlin's by delegation:

val volume by reactiveVolume          // delegates to cachedStateFlow.value
val cached by cachedVolume            // delegates to cachedValue

Suspend API

Use SuspendMutableKrate for I/O-bound storage (databases, files, network):

class SuspendSettingsApi(private val settings: MutableMap<String, Int>) {

    val volumeKrate: SuspendMutableKrate<Int> = DefaultSuspendMutableKrate(
        factory = { 50 },
        loader = { settings["volume"] },
        saver = { value -> settings["volume"] = value }
    )
}

The API mirrors the blocking version, but every operation is suspend:

// Save a value
volumeKrate.save(75)

// Transform and save
volumeKrate.save { current -> current + 10 }

// Transform, save, and get the new value
val newVolume = volumeKrate.saveAndGet { current -> current + 10 }

// Reset to factory default
volumeKrate.reset()

StateFlowSuspendMutableKrate — Reactive + Suspend

Combine suspend operations with StateFlow observation:

val reactiveVolume: StateFlowSuspendMutableKrate<Int> = DefaultStateFlowSuspendMutableKrate(
    factory = { 50 },
    loader = { settings["volume"] },
    saver = { value -> settings["volume"] = value }
)

// Observe reactively
reactiveVolume.cachedStateFlow.collect { volume ->
    println("Volume: $volume")
}

// Write asynchronously
reactiveVolume.save(100)

Flow API

Use FlowMutableKrate to integrate with Jetpack DataStore or any Flow-based backend:

class FlowSettingsApi {
    private val key = intPreferencesKey("volume")
    private val dataStore: DataStore<Preferences> = /* ... */

    val volumeKrate: FlowMutableKrate<Int> = DefaultFlowMutableKrate(
        factory = { 50 },
        loader = { dataStore.data.map { it[key] } },
        saver = { value -> dataStore.edit { it[key] = value } }
    )

    // Convert to StateFlow when needed
    val stateFlow: StateFlow<Int> = volumeKrate.stateFlow(viewModelScope)
}

You can even convert a FlowMutableKrate into a StateFlowSuspendMutableKrate:

val reactiveKrate = volumeKrate.asStateFlowSuspendMutableKrate(viewModelScope)

🔒 Thread Safety & Locks

Every Krate is thread-safe by default. You don't need to add any synchronization yourself.

On multithreaded platforms, each Krate is backed by a platform-specific recursive lock:

Platform	Lock Implementation
JVM / Android	ReentrantLock
iOS / macOS / tvOS / watchOS	NSRecursiveLock
Linux / MinGW	Recursive pthread_mutex

On JS / WasmJS there is no real locking — the runtime is single-threaded, so the Lock implementation is a lightweight no-op wrapper that simply delegates to the block directly.

How It Works

• Every Krate internally implements LockOwner, which holds a Lock instance.
• All read and write operations are automatically wrapped in withLock {} (blocking) or withSuspendLock {} (suspend).
• Nested wrappers share the same lock — when you call .asCachedMutableKrate() or .asStateFlowMutableKrate(), the wrapper reuses the lock from the original Krate via LockOwner.Reusable. This prevents deadlocks and keeps everything consistent.

val krate: MutableKrate<Int> = DefaultMutableKrate(
    factory = { 0 },
    loader = { settings["key"] },
    saver = { value -> settings["key"] = value }
)

// The cached wrapper shares the same lock — no deadlocks, no double-locking
val cached = krate.asCachedMutableKrate()
val stateFlow = krate.asStateFlowMutableKrate()

Concurrent Access

You can safely read and write from multiple threads or coroutines without any additional synchronization:

// Safe to call from any thread
coroutineScope {
    repeat(100) {
        launch(Dispatchers.Default) {
            krate.save { current -> current + 1 }
        }
    }
}
// krate.getValue() == 100 ✅

✂️ Null-to-Non-Null with .withDefault

Have a nullable Krate but need a non-null one somewhere? Use .withDefault:

val nullableKrate: MutableKrate<Int?> = DefaultMutableKrate(
    factory = { null },
    loader = { settings["key"] },
    saver = { value ->
        if (value == null) settings.remove("key")
        else settings["key"] = value
    }
)

// Wrap it — getValue() will never return null
val nonNullKrate: MutableKrate<Int> = nullableKrate.withDefault { 42 }

.withDefault is available on every Krate type — Krate, MutableKrate, SuspendKrate, SuspendMutableKrate, FlowKrate, FlowMutableKrate, StateFlowKrate, StateFlowSuspendKrate, and StateFlowSuspendMutableKrate.

🧩 In-Memory Krates

Need a quick in-memory store for testing or caching?

// Blocking
val counter: MutableKrate<Int> = InMemoryMutableKrate { 0 }

// Suspend
val asyncCounter: SuspendMutableKrate<Int> = InMemorySuspendMutableKrate { 0 }

🧪 Advanced Usage

🏷️ Class-Based Krate

Encapsulate a Krate as a dedicated class using delegation:

class IntKrate(
    key: String,
    settings: MutableMap<String, Int>
) : MutableKrate<Int?> by DefaultMutableKrate(
    factory = { null },
    loader = { settings[key] },
    saver = { value -> settings[key] = value }
)

🧱 Custom Data Types

Store any type by mapping to/from the underlying storage format:

data class UserSettings(val theme: String, val fontSize: Int)

class UserSettingsKrate(
    jsonStore: MutableMap<String, String>,
    json: Json
) : MutableKrate<UserSettings> by DefaultMutableKrate(
    factory = { UserSettings(theme = "light", fontSize = 14) },
    loader = {
        jsonStore["user_settings"]?.let { json.decodeFromString(it) }
    },
    saver = { value ->
        jsonStore["user_settings"] = json.encodeToString(value)
    }
)

🔄 Dynamic Keys

Create Krates on-the-fly for different entities:

fun scoreKrateForUser(userId: String): MutableKrate<Int> {
    return DefaultMutableKrate(
        factory = { 0 },
        loader = { settings["score_$userId"] },
        saver = { value -> settings["score_$userId"] = value }
    )
}

🔗 Wrapping & Composing Krates

The real power of KStorage is the ability to layer behaviors:

val krate: MutableKrate<Int?> = DefaultMutableKrate(
  factory = { null },
  loader = { settings["key"] },
  saver = { value -> settings["key"] = value }
)

// 1. Make it non-null
val nonNull: MutableKrate<Int> = krate.withDefault { 0 }

// 2. Add in-memory caching
val cached: CachedMutableKrate<Int> = nonNull.asCachedMutableKrate()

// 3. Make it reactive
val reactive: StateFlowMutableKrate<Int> = nonNull.asStateFlowMutableKrate()

All wrappers share the same underlying lock, so the entire chain is thread-safe.

🌍 Platform Support

KStorage supports a wide range of Kotlin Multiplatform targets:

• JVM / Android
• iOS (x64, arm64, simulator)
• macOS (x64, arm64)
• tvOS (x64, arm64, simulator)
• watchOS (x64, arm64, simulator)
• Linux (x64)
• Windows (mingwX64)
• JS (IR)
• WasmJS