Essenty

The most essential libraries for Kotlin Multiplatform development.

Supported targets:

  • android
  • jvm
  • js (IR and LEGACY)
  • iosArm64, iosX64
  • watchosArm32, watchosArm64, watchosX64
  • tvosArm64, tvosX64
  • macosX64
  • linuxX64

Lifecyle

When writing Kotlin Multiplatform (common) code we often need to handle lifecycle events of a screen. For example, to stop background operations when the screen is destroyed, or to reload some data when the screen is activated. Essenty provides the Lifecycle API to help with lifecycle handling in the common code. It is very similar to Android Activity lifecycle.

Setup

Groovy:

// Add the dependency, typically under the commonMain source set
implementation "com.arkivanov.essenty:lifecycle:<essenty_version>"
Groovy

Kotlin:

// Add the dependency, typically under the commonMain source set
implementation("com.arkivanov.essenty:lifecycle:<essenty_version>")
Kotlin

Lifecycle state transitions

Content

The main Lifecycle interface provides ability to observe the lifecycle state changes.

The LifecycleRegistry interface extends both the Lifecycle and the Lifecycle.Callbacks at the same time. It can be used to manually control the lifecycle, for example in tests.

The LifecycleOwner just holds the Lifecyle. It may be implemented by an arbitrary class, to provide convenient API.

Android extensions

From Android, the Lifecycle can be obtained by using special functions, can be found here.

Usage example

The lifecycle can be observed using its subscribe/unsubscribe methods:

import com.arkivanov.essenty.lifecycle.Lifecycle

class SomeLogic(lifecycle: Lifecycle) {
    init {
        lifecycle.subscribe(
            object : Lifecycle.Callbacks {
                override fun onCreate() {
                    // Handle lifecycle created
                }

                // onStart, onResume, onPause, onStop are also available

                override fun onDestroy() {
                    // Handle lifecycle destroyed
                }
            }
        )
    }
}
Kotlin

Or using the extension functions:

import com.arkivanov.essenty.lifecycle.Lifecycle
import com.arkivanov.essenty.lifecycle.doOnCreate
import com.arkivanov.essenty.lifecycle.doOnDestroy
import com.arkivanov.essenty.lifecycle.subscribe

class SomeLogic(lifecycle: Lifecycle) {
    init {
        lifecycle.subscribe(
            onCreate = { /* Handle lifecycle created */ },
            // onStart, onResume, onPause, onStop are also available
            onDestroy = { /* Handle lifecycle destroyed */ }
        )

        lifecycle.doOnCreate {
            // Handle lifecycle created
        }

        // doOnStart, doOnResume, doOnPause, doOnStop are also available

        lifecycle.doOnDestroy {
            // Handle lifecycle destroyed
        }
    }
}
Kotlin

Parcelable and Parcelize

Essenty brings both Android Parcelable interface and the @Parcelize annotation from kotlin-parcelize compiler plugin to Kotlin Multiplatform, so they both can be used in common code. This is typically used for state/data preservation over Android configuration changes, when writing common code targeting Android.

Setup

Groovy:

plugins {
    id "kotlin-parcelize" // Apply the plugin for Android
}

// Add the dependency, typically under the commonMain source set
implementation "com.arkivanov.essenty:parcelable:<essenty_version>"
Groovy

Kotlin:

plugins {
    id("kotlin-parcelize") // Apply the plugin for Android
}

// Add the dependency, typically under the commonMain source set
implementation("com.arkivanov.essenty:parcelable:<essenty_version>")
Kotlin

Usage example

Once the dependency is added and the plugin is applied, we can use it as follows:

import com.arkivanov.essenty.parcelable.Parcelable
import com.arkivanov.essenty.parcelable.Parcelize

@Parcelize
data class User(
    val id: Long,
    val name: String
) : Parcelable
Kotlin

When compiled for Android, the Parcelable implementation will be generated automatically. When compiled for other targets, it will be just a regular class without any extra generated code.

Parcelize for Darwin/Apple targets

Currently there is no extra code generated when compiled for Darwin/Apple targets. However I made a proof of concept: kotlin-parcelize-darwin compiler plugin. It is not used yet by Essenty, and the applicabilty is being considered. Please raise a Discussion if you are interested.

StateKeeper

When writing common code targetting Android, it might be required to preserve some data over Android configuration changes or process death. For this purpose, Essenty provides the StateKeeper API, which is inspired by the AndroidX SavedStateHandle. The StateKeeper API relies on the Parcelable interface provided by the parcelable module described above.

Setup

Groovy:

// Add the dependency, typically under the commonMain source set
implementation "com.arkivanov.essenty:state-keeper:<essenty_version>"
Groovy

Kotlin:

// Add the dependency, typically under the commonMain source set
implementation("com.arkivanov.essenty:state-keeper:<essenty_version>")
Kotlin

Content

The main StateKeeper interface provides ability to register/unregister state suppliers, and also to consume any previously saved state.

The StateKeeperDispatcher interface extens StateKeeper and allows state saving, by calling all registered state providers.

The StateKeeperOwner interface is just a holder of StateKeeper. It may be implemented by an arbitrary class, to provide convenient API.

Android extensions

From Android side, StateKeeper can be obtained by using special functions, can be found here.

Usage example

import com.arkivanov.essenty.parcelable.Parcelable
import com.arkivanov.essenty.parcelable.Parcelize
import com.arkivanov.essenty.statekeeper.StateKeeper
import com.arkivanov.essenty.statekeeper.consume

class SomeLogic(stateKeeper: StateKeeper) {
    // Use the saved State if any, otherwise create a new State
    private var state: State = stateKeeper.consume("SAVED_STATE") ?: State()

    init {
        // Register the State supplier
        stateKeeper.register("SAVED_STATE") { state }
    }

    @Parcelize
    private class State(
        val someValue: Int = 0
    ) : Parcelable
}
Kotlin

InstanceKeeper

When writing common code targetting Android, it might be required to retain objects over Android configuration changes. This use case is covered by the InstanceKeeper API, which is similar to the AndroidX ViewModel.

Setup

Groovy:

// Add the dependency, typically under the commonMain source set
implementation "com.arkivanov.essenty:instance-keeper:<essenty_version>"
Groovy

Kotlin:

// Add the dependency, typically under the commonMain source set
implementation("com.arkivanov.essenty:instance-keeper:<essenty_version>")
Kotlin

Content

The main InstanceKeeper interface is responsible for storing object instances, represented by the [InstanceKeeper.Instance] interface. Instances of the InstanceKeeper.Instance interface survive Android Configuration changes, the InstanceKeeper.Instance.onDestroy() method is called when InstanceKeeper goes out of scope (e.g. the screen is finished).

The InstanceKeeperDispatcher interface extens InstanceKeeper and adds ability to destroy all registered instances.

The InstanceKeeperOwner interface is just a holder of InstanceKeeper. It may be implemented by an arbitrary class, to provide convenient API.

Android extensions

From Android side, InstanceKeeper can be obtained by using special functions, can be found here.

Usage example

import com.arkivanov.essenty.instancekeeper.InstanceKeeper
import com.arkivanov.essenty.instancekeeper.getOrCreate

class SomeLogic(instanceKeeper: InstanceKeeper) {
    // Get the existing instance or create a new one
    private val thing: RetainedThing = instanceKeeper.getOrCreate { RetainedThing() }
}

/*
 * Survives Android configuration changes.
 * ⚠️ Pay attention to not leak any dependencies.
 */
class RetainedThing : InstanceKeeper.Instance {
    override fun onDestroy() {
        // Called when the screen is finished
    }
}
Kotlin

BackPressedDispatcher

The BackPressedDispatcher API provides ability to handle back button clicks (e.g. an Android device's back button), in common code. This API is similar to AndroidX OnBackPressedDispatcher.

Setup

Groovy:

// Add the dependency, typically under the commonMain source set
implementation "com.arkivanov.essenty:back-pressed:<essenty_version>"
Groovy

Kotlin:

// Add the dependency, typically under the commonMain source set
implementation("com.arkivanov.essenty:back-pressed:<essenty_version>")
Kotlin

Content

The BackPressedHandler interface provides ability to register/unregister back button handlers. When the device's back button is clicked, all registered handlers are called in reverse order. If a handler returns true then the event is considered as handled and the handling process stops, the remaining handlers are not called. If none of the handlers returned true then the event is considered as unhandled.

The BackPressedDispatcher interface extends BackPressedHandler and is responsible for triggering the registered handlers. The BackPressedDispatcher.onBackPressed() triggers all registered handlers in reverse order, returns true if the event is handled, and false if the event is unhandled.

Android extensions

From Android side, BackPressedDispatcher can be obtained by using special functions, can be found here.

⚠️ Due to the nature of AndroidX OnBackPressedDispatcher API, it is not possible to map it 1-1 to BackPressedHandler. Please keep in mind some possible side effects described in the corresponding KDocs.

Usage example

class SomeLogic(backPressedDispatcher: BackPressedDispatcher) {
    init {
        backPressedDispatcher.register {
            // Called when the back button is pressed
            true // Return true to consume the event, or false to allow other registered callbacks
        }
    }
}
Kotlin

Author

Twitter: @arkann1985

GitHub

https://github.com/arkivanov/Essenty