LiveData prevent receive the last value when start observing

Solution 1:

I`m using this EventWraper class from Google Samples inside MutableLiveData

/**
 * Used as a wrapper for data that is exposed via a LiveData that represents an event.
 */
public class Event<T> {

    private T mContent;

    private boolean hasBeenHandled = false;


    public Event( T content) {
        if (content == null) {
            throw new IllegalArgumentException("null values in Event are not allowed.");
        }
        mContent = content;
    }
    
    @Nullable
    public T getContentIfNotHandled() {
        if (hasBeenHandled) {
            return null;
        } else {
            hasBeenHandled = true;
            return mContent;
        }
    }
    
    public boolean hasBeenHandled() {
        return hasBeenHandled;
    }
}

In ViewModel :

 /** expose Save LiveData Event */
 public void newSaveEvent() {
    saveEvent.setValue(new Event<>(true));
 }

 private final MutableLiveData<Event<Boolean>> saveEvent = new MutableLiveData<>();

 public LiveData<Event<Boolean>> onSaveEvent() {
    return saveEvent;
 }

In Activity/Fragment

mViewModel
    .onSaveEvent()
    .observe(
        getViewLifecycleOwner(),
        booleanEvent -> {
          if (booleanEvent != null)
            final Boolean shouldSave = booleanEvent.getContentIfNotHandled();
            if (shouldSave != null && shouldSave) saveData();
          }
        });

Solution 2:

Faced the same problem, and I created some simple kotlin extention functions which can solve the problem easily.

Usage as below:

val liveData = MutableLiveData<String>()
liveData.value = "Hello"

val freshResult = mutableListOf<String>()
val normalResult = mutableListOf<String>()

liveData.observeForeverFreshly(Observer {
    freshResult.add(it)
})

liveData.observeForever(Observer {
    normalResult.add(it)
})

liveData.value = "World"

assertEquals(listOf("World"), freshResult)
assertEquals(listOf("Hello", "World"), normalResult)

Basic source code is explained as bllow.

For some more detail (to support some special situations for example MediatorLiveData returned from Transformations.map), you can view it in github : livedata-ext

FreshLiveData.kt

fun <T> LiveData<T>.observeFreshly(owner: LifecycleOwner, observer: Observer<in T>) { 
    // extention fuction to get LiveData's version, will explain in below.
    val sinceVersion = this.version()
    this.observe(owner, FreshObserver<T>(observer, this, sinceVersion))
}

fun <T> LiveData<T>.observeForeverFreshly(observer: Observer<in T>, skipPendingValue: Boolean = true) {
    val sinceVersion = this.version()
    this.observeForever(FreshObserver<T>(observer, this, sinceVersion))
}

// Removes the observer which has been previously observed by [observeFreshly] or [observeForeverFreshly].
fun <T> LiveData<T>.removeObserverFreshly(observer: Observer<in T>) {
    this.removeObserver(FreshObserver<T>(observer, this, 0))
}

class FreshObserver<T>(
    private val delegate: Observer<in T>,
    private val liveData: LiveData<*>,
    private val sinceVersion: Int
) : Observer<T> {

    override fun onChanged(t: T) {
        if (liveData.version() > sinceVersion) {
            delegate.onChanged(t)
        }
    }

    override fun equals(other: Any?): Boolean {
        if (this === other) return true
        if (javaClass != other?.javaClass) return false
        if (delegate != (other as FreshObserver<*>).delegate) return false
        return true
    }

    override fun hashCode(): Int {
        return delegate.hashCode()
    }
}

Becasue we need to access LiveData's pcakage visibile methond getVersion() for comparasion, so create a class in package android.arch.lifecycle or androidx.lifecycle (AndroidX):

LiveDataHiddenApi.kt

package androidx.lifecycle

fun LiveData<*>.version(): Int {
    return this.getVersion()
}