MediatorLiveData


LiveData subclass which may observe other LiveData objects and react on OnChanged events from them.

This class correctly propagates its active/inactive states down to source LiveData objects.

Consider the following scenario: we have 2 instances of LiveData, let's name them liveData1 and liveData2, and we want to merge their emissions in one MediatorLiveData object: liveDataMerger. Then, liveData1 and liveData2 will become sources for the liveDataMerger and every time onChanged callback is called for either of them, we set a new value in liveDataMerger.

LiveData<Integer> liveData1 = ...;
LiveData<Integer> liveData2 = ...;

MediatorLiveData<Integer> liveDataMerger = new MediatorLiveData<>();
liveDataMerger.addSource(liveData1, value -> liveDataMerger.setValue(value));
liveDataMerger.addSource(liveData2, value -> liveDataMerger.setValue(value));

Let's consider that we only want 10 values emitted by liveData1, to be merged in the liveDataMerger. Then, after 10 values, we can stop listening to liveData1 and remove it as a source.

liveDataMerger.addSource(liveData1, new Observer<Integer>() {
     private int count = 1;

     @Override public void onChanged(@Nullable Integer s) {
         count++;
         liveDataMerger.setValue(s);
         if (count > 10) {
             liveDataMerger.removeSource(liveData1);
         }
     }
});
Parameters
<T>

The type of data hold by this instance

Summary

Public constructors

Creates a MediatorLiveData with no value assigned to it.

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MediatorLiveData(value: T!)

Creates a MediatorLiveData initialized with the given value.

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Public functions

Unit
@MainThread
<S> addSource(source: LiveData<S!>, onChanged: Observer<S!>)

Starts to listen to the given source LiveData, onChanged observer will be called when source value was changed.

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Unit
@MainThread
<S> removeSource(toRemote: LiveData<S!>)

Stops to listen the given LiveData.

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Protected functions

Unit

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

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Unit

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

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Inherited functions

From androidx.lifecycle.LiveData
T?

Returns the current value.

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Boolean

Returns true if this LiveData has active observers.

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Boolean

Returns true if this LiveData has observers.

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Boolean

Returns whether an explicit value has been set on this LiveData.

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Unit
@MainThread
observe(owner: LifecycleOwner, observer: Observer<T!>)

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

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Unit

Adds the given observer to the observers list.

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Unit

Removes the given observer from the observers list.

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Unit

Removes all observers that are tied to the given LifecycleOwner.

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From androidx.lifecycle.MutableLiveData
Unit
postValue(value: T!)

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

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Unit
setValue(value: T!)

Sets the value.

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Public constructors

MediatorLiveData

MediatorLiveData()

Creates a MediatorLiveData with no value assigned to it.

MediatorLiveData

MediatorLiveData(value: T!)

Creates a MediatorLiveData initialized with the given value.

Parameters
value: T!

initial value

Public functions

addSource

@MainThread
fun <S> addSource(source: LiveData<S!>, onChanged: Observer<S!>): Unit

Starts to listen to the given source LiveData, onChanged observer will be called when source value was changed.

onChanged callback will be called only when this MediatorLiveData is active.

If the given LiveData is already added as a source but with a different Observer, IllegalArgumentException will be thrown.

Parameters
<S>

The type of data hold by source LiveData

source: LiveData<S!>

the LiveData to listen to

onChanged: Observer<S!>

The observer that will receive the events

removeSource

@MainThread
fun <S> removeSource(toRemote: LiveData<S!>): Unit

Stops to listen the given LiveData.

Parameters
<S>

the type of data hold by source LiveData

toRemote: LiveData<S!>

LiveData to stop to listen

Protected functions

onActive

@CallSuper
protected fun onActive(): Unit

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

This callback can be used to know that this LiveData is being used thus should be kept up to date.

onInactive

@CallSuper
protected fun onInactive(): Unit

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

This does not mean that there are no observers left, there may still be observers but their lifecycle states aren't STARTED or RESUMED (like an Activity in the back stack).

You can check if there are observers via hasObservers.