Instance retaining¶

Sometimes it might be necessary to preserve state or data in a component when it gets destroyed. This commonly used in Android when configuration changes occur. The ComponentContext interface extends the InstanceKeeperOwner interface, which provides the InstanceKeeper - a multiplatform abstraction for instances retaining. It is provided by Essenty library (from the same author).

The decompose module adds Essenty's instance-keeper module as api dependency, so you don't need to explicitly add it to your project. Please familiarise yourself with Essenty library, especially with the InstanceKeeper.

Usage example¶

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

class SomeComponent(
    componentContext: ComponentContext
) : ComponentContext by componentContext {

    private val someLogic = instanceKeeper.getOrCreate(::SomeLogic)

    /*
     * Instances of this class will be retained.
     * ⚠️ Pay attention to not leak any dependencies.
     */
    private class SomeLogic : InstanceKeeper.Instance {
        override fun onDestroy() {
            // Clean-up
        }
    }
}

Retained components (since v2.1.0-alpha-03)¶

Although discouraged, it is still possible to have all components retained over configuration changes on Android. On the one hand, this makes InstanceKeeper no longer required. But on the other hand, this prevents from supplying dependencies that capture the hosting Activity or Fragment.

Warning

Pay attention when supplying dependencies to a retained component to avoid leaking the hosting Activity or Fragment.

Warning

The retainedComponent function must only be called once during the lifetime of the host Activity or Fragment, typically in onCreate. Calling it a second time will result in a crash.

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import com.arkivanov.decompose.retainedComponent

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        val root =
            retainedComponent { componentContext ->
                DefaultRootComponent(componentContext)
            }
    }
}