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SavedState VectorViewModel

A subclass of VectorViewModel which provides easier state persistence across process deaths, by providing access to a SavedStateHandle from ViewModel SavedState library in Android Jetpack.

Making state persistable

The easiest way to persist your UI state is to simply save the entire state object using the saved state handle. However, you state class needs to implementing Parcelable for this, which is a tedious and error prone process.

Luckily, Kotlin comes with an Android Extension which helps use make our classes Parcelable with just a single annotation.

import kotlinx.android.parcel.Parcelize

@Parcelize
data class UserState(
  val userId: Long = -1,
  val user: User? = null,
  val isUserPremium: Boolean = false
): Parcelable

Make sure that you have turned on the experimental flag in your build.gradle file to be able to access this feature:

androidExtensions {
    experimental = true
}

Persisting State

The SavedStateVectorViewModel class has a setStateAndPersist method which is the same as the regular setState method, except that it also persists the new state.

fun greetUser() = setStateAndPersist {
  copy(greeting = "Hello!")
}

If you want to exclude some properties in your state object from being persisted, you must annotate them with @Transient.

@Parcelize
data class UserState(
  val userId: Long = -1,
  @Transient val user: User? = null,
  val isUserPremium: Boolean = false
): Parcelable

This method by default tries to persist your entire state object using KEY_SAVED_STATE key defined in this class. If you need to customize this behaviour, you should override the persistState method.

override fun persistState() = withState { state ->
  // Your custom implementation
}