Android LiveData prevent receive the last value on observe

Question

Is it possible to prevent LiveData receive the last value when start observing? I am considering to use LiveData as events.

For example eve

Answer 1

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

Usage as below:

val liveData = MutableLiveData()
liveData.value = "Hello"

val freshResult = mutableListOf()
val normalResult = mutableListOf()

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  LiveData.observeFreshly(owner: LifecycleOwner, observer: Observer) { 
    // extention fuction to get LiveData's version, will explain in below.
    val sinceVersion = this.version()
    this.observe(owner, FreshObserver(observer, this, sinceVersion))
}

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

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

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

    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()
}