androidandroid-livedatamutablelivedata

How to emit distinct values from MutableLiveData?


I observed that MutableLiveData triggers onChanged of an observer even if the same object instance is provided to its setValue method.

//Fragment#onCreateView - scenario1
val newValue = "newValue"
mutableLiveData.setValue(newValue) //triggers observer
mutableLiveData.setValue(newValue) //triggers observer

//Fragment#onCreateView - scenario2
val newValue = "newValue"
mutableLiveData.postValue(newValue) //triggers observer
mutableLiveData.postValue(newValue) //does not trigger observer

Is there a way to avoid an observer be notified twice if the same or an equivalent instance is provided to setValue()/postValue()

I tried extending MutableLiveData but that did not work. I could be missing something here

class DistinctLiveData<T> : MutableLiveData<T>() {

    private var cached: T? = null

    @Synchronized override fun setValue(value: T) {
        if(value != cached) {
            cached = value
            super.setValue(value)
        }
    }

    @Synchronized override fun postValue(value: T) {
        if(value != cached) {
            cached = value
            super.postValue(value)
        }
    }
}

Solution

  • You can use the following magic trick to consume "items being the same":

    fun <T> LiveData<T>.distinctUntilChanged(): LiveData<T> = MediatorLiveData<T>().also { mediator ->
        mediator.addSource(this, object : Observer<T> {
            private var isInitialized = false
            private var previousValue: T? = null
    
            override fun onChanged(newValue: T?) {
                val wasInitialized = isInitialized
                if (!isInitialized) {
                    isInitialized = true
                }
                if(!wasInitialized || newValue != previousValue) {
                    previousValue = newValue
                    mediator.postValue(newValue)
                }
            }
        })
    }
    

    If you want to check referential equality, it's !==.


    But it has since been added to Transformations.distinctUntilChanged.