MapMemory

[][bintray] [][ci]

Simple in-memory cache conception built on Map.

---

• Installation
• Conception
• Usage
  • Reactive Style
  • Memory Scopes
  • Avoid ClassCastException
• Contributing
• License

Installation

Add the dependency:

repositories {
    jcenter()
}

dependencies {
    implementation("com.redmadrobot.mapmemory:mapmemory:1.0")
    // or if you want to work with memory in reactive style, add one of
    implementation("com.redmadrobot.mapmemory:mapmemory-rxjava2:1.0")
    implementation("com.redmadrobot.mapmemory:mapmemory-coroutines:1.0")
}

Conception

Kotlin provides delegates for access to map entries. This library exploits and improves this idea to implement in-memory storage.

There are two simple principles:

• Memory is a singleton, and it can be shared between many consumers
• Memory holds data but not knows what data it holds

Usage

class CardsInMemoryStorage(memory: MapMemory) {

    private val cards: MutableMap<String, Card> by memory.map()

    operator fun get(id: String): Card? = cards[id]
    operator fun set(id: String, card: Card) {
        cards[id] = card
    }
}

There are default accessors available:

Accessor	Default value	Description
nullable()	null	Store nullable values
map()	Empty map	Store values in map
list()	Empty list	Store values in list

You can create own accessor if need.

You, also, can use delegate without any functions:

var unsafeValue: String by memory

Be careful, there no default values, and you will get NoSuchElementException if you try read value before it was written.

Reactive Style

To use MapMemory in reactive style, replace dependency mapmemory with mapmemory-rxjava2 or mapmemory-coroutines.

You can't use both map mapmemory-rxjava2 and mapmemory-coroutines at the same time because you will get duplicates in classpath.

Both of reactive implementations contain RactiveMap. ReactiveMap works similar to MutableMap but enables you to observe data in reactive manner. It has methods getStream(key) and getAllStream() to observe one or all map values accordingly.

With reactive in-memory cache you will always have actual data on screen. Also, you can separate subscription to data and request of data to manage it easier.

RxJava 2

mapmemory-rxjava2 adds accessors for RxJava types:

Accessor	Default value	Description
behaviorSubject()	Empty subject	Store stream of values
publishSubject()	Empty subject	Store stream of values
maybe()	Maybe.empty()	Reactive analog to store "nullable"
reactiveMap()	Empty map	Store values in reactive map

Example of cache-first approach with reactive subscription:

class CardsRepository(memory: MapMemory) {

    private val cardsCache: ReactiveCache<Card> by memory.reactiveCache()

    /** Returns observable for cards in cache. */
    fun getCardsStream(): Observable<List<Card>> = cardsCache.getAllStream()

    /** Updates cards in cache. */
    fun fetchCards(): Completable {
        return Single.defer {/* get cards from network */}
            .doOnSuccess { cardsCache.replaceAll(it) }
            .ignoreResult()
    }
}

class CardsViewModel(private val repository: CardsRepository) : ViewModel() {

    init {
        // Subscribe to cache
        repository.getCardsStream()
            .subscribe {/* update cards on screen */}
    }

    fun onRefresh() {
        repository.fetchCards()
            .subscribe(
                {/* success! hide progress */},
                {/* fail. hide progress and show error */}
            )
    }
}

Coroutines

mapmemory-coroutines adds accessors for coroutines types:

Accessor	Default value	Description
stateFlow()	StateFlow with specified initialValue	Store stream of values
sharedFlow()	Empty flow	Store stream of values
reactiveMap()	Empty map	Store values in reactive map

Example of cache-first approach with reactive subscription:

class CardsRepository(private val api: CardsApi, memory: MapMemory) {

    private val cardsCache: ReactiveCache<Card> by memory.reactiveCache()

    /** Returns flow for cards in cache. */
    fun getCardsStream(): Flow<List<Card>> = cardsCache.getAllStream()

    /** Updates cards in cache. */
    suspend fun fetchCards() {
        val cards = api.getCards()
        cardsCache.replaceAll(cards.associateBy { it.id })
    }
}

class CardsViewModel(private val repository: CardsRepository) : ViewModel() {

    init {
        // Subscribe to cache
        repository.getCardsStream()
            .onEach {/* update cards on screen */}
            .launchIn(viewModelScope)
    }

    fun onRefresh() {
        viewModelScope.launch {
            repository.fetchCards()
            // success! hide progress
        }
    }
}

Memory Scopes

May be useful to create memory scopes. You can use it to control data lifetime.

/** Memory, available during a session and cleared on logout. */
@Singleton
class SessionMemory @Inject constructor() : MapMemory()

/** Memory, available during the app lifetime. */
@Singleton
class AppMemory @Inject constructor() : MapMemory()

Keep in mind that you should manually clear SessionMemory on logout.

Instead of creating subclass, you can provide MapMemory with qualifiers.

Avoid ClassCastException

It can be caused if you have declared fields with the same name but different types in several places. Note that field name used as a key to access a value in MapMemory. To avoid ClassCastException, use unique names for fields.

This snippet demonstrates how it works:

class StringsStorage(memory: MapMemory) {
    var values: MutableList<String> by memory.list()
}

class IntsStorage(memory: MapMemory) {
    var values: MutableList<Int> by memory.list() // The same name as in StringsStorage
}

val strings = StringsStorage(memory)
val ints = IntsStorage(memory)

strings.values.add("A")
ints.values.add(1)

println(memory["values"]) // [A, 1]

Contributing

Merge requests are welcome. For major changes, please open an issue first to discuss what you would like to change.

License

MIT