Android 12 Developer Preview is here! Try it out, and give us your feedback!


abstract class LiveData<T : Any!>
   ↳ androidx.lifecycle.LiveData

LiveData is a data holder class that can be observed within a given lifecycle. This means that an Observer can be added in a pair with a LifecycleOwner, and this observer will be notified about modifications of the wrapped data only if the paired LifecycleOwner is in active state. LifecycleOwner is considered as active, if its state is Lifecycle.State#STARTED or Lifecycle.State#RESUMED. An observer added via observeForever(Observer) is considered as always active and thus will be always notified about modifications. For those observers, you should manually call removeObserver(Observer).

An observer added with a Lifecycle will be automatically removed if the corresponding Lifecycle moves to Lifecycle.State#DESTROYED state. This is especially useful for activities and fragments where they can safely observe LiveData and not worry about leaks: they will be instantly unsubscribed when they are destroyed.

In addition, LiveData has LiveData#onActive() and LiveData#onInactive() methods to get notified when number of active Observers change between 0 and 1. This allows LiveData to release any heavy resources when it does not have any Observers that are actively observing.

This class is designed to hold individual data fields of ViewModel, but can also be used for sharing data between different modules in your application in a decoupled fashion.


Public constructors
<init>(value: T)

Creates a LiveData initialized with the given value.

Creates a LiveData with no value assigned to it.

Public methods
open T?

Returns the current value.

open Boolean

Returns true if this LiveData has active observers.

open Boolean

Returns true if this LiveData has observers.

open Unit
observe(@NonNull owner: LifecycleOwner, @NonNull observer: Observer<in T>)

Adds the given observer to the observers list within the lifespan of the given owner.

open Unit
observeForever(@NonNull observer: Observer<in T>)

Adds the given observer to the observers list.

open Unit
removeObserver(@NonNull observer: Observer<in T>)

Removes the given observer from the observers list.

open Unit

Removes all observers that are tied to the given LifecycleOwner.

Protected methods
open Unit

Called when the number of active observers change from 0 to 1.

open Unit

Called when the number of active observers change from 1 to 0.

open Unit
postValue(value: T)

Posts a task to a main thread to set the given value.

open Unit
setValue(value: T)

Sets the value.

Extension functions
From androidx.lifecycle

Creates a Flow containing values dispatched by originating LiveData: at the start a flow collector receives the latest value held by LiveData and then observes LiveData updates.


Creates a new LiveData object does not emit a value until the source this LiveData value has been changed.

LiveData<X>.map(crossinline transform: (X) -> Y)

Returns a LiveData mapped from this LiveData by applying transform to each value set on this LiveData.

LiveData<T>.observe(owner: LifecycleOwner, crossinline onChanged: (T) -> Unit)

Adds the given onChanged lambda as an observer within the lifespan of the given owner and returns a reference to observer.