MutableLiveData

MutableLiveData is a lifecycle-aware data holder class that is observable within Android apps. It is a mutable subclass of LiveData, part of Android Jetpack's Lifecycle and LiveData components. MutableLiveData holds a value of a generic type T and notifies active observers whenever the value changes.

Mutability is provided by the methods setValue and postValue. setValue(T) updates the value on the main thread and notifies observers immediately. postValue(T) can be called from a background thread; the value will be updated on the main thread at the next opportunity, and observers will be notified then. If multiple posts occur in quick succession, the last value is delivered.

Observers register with observe, typically from an Activity or Fragment, using a lifecycle owner so updates are delivered only while the UI is active. LiveData is lifecycle-aware, so it only emits to observers when they are in an active state (STARTED or RESUMED). MutableLiveData is especially popular in the ViewModel layer, where data survives configuration changes and is exposed to the UI via an immutable LiveData wrapper to prevent external modification.

A typical usage pattern is to declare private MutableLiveData<T> in a ViewModel and expose public LiveData<T> getX() { return x; }. This enables the UI to react to changes while ensuring only the ViewModel can mutate the data. Related concepts include Transformations and MediatorLiveData, which operate on LiveData values.