Room : LiveData from Dao will trigger Observer.onChanged on every Update, even if the LiveData value has no change

Question

I found that the LiveData returned by Dao will call its observer whenever the row is updated in DB, even if the LiveData value is obviously not changed.

Consider a s

Answer 1

Avoid false positive notifications for observable queries
Let’s say that you want to get a user based on the user id in an observable query:

@Query(“SELECT * FROM Users WHERE userId = :id)
fun getUserById(id: String): LiveData

You’ll get a new emission of the User object whenever that user updates. But you will also get the same object when other changes (deletes, updates or inserts) occur on the Users table that has nothing to do with the User you’re interested in, resulting in false-positive notifications. Even more, if your query involves multiple tables, you’ll get a new emission whenever something changed in any of them.

If your query returns a LiveData, you can use a MediatorLiveData that only allows distinct object emissions from a source.

fun  LiveData.getDistinct(): LiveData {
    val distinctLiveData = MediatorLiveData()
    distinctLiveData.addSource(this, object : Observer {
        private var initialized = false
        private var lastObj: T? = null
        override fun onChanged(obj: T?) {
            if (!initialized) {
                initialized = true
                lastObj = obj
                distinctLiveData.postValue(lastObj)
            } else if ((obj == null && lastObj != null) 
                       || obj != lastObj) {
                lastObj = obj
                distinctLiveData.postValue(lastObj)
            }
        }
    })
    return distinctLiveData
}

In your DAOs, make method that returns the distinct LiveData public and the method that queries the database protected.

@Dao
 abstract class UserDao : BaseDao() {
   @Query(“SELECT * FROM Users WHERE userid = :id”)
   protected abstract fun getUserById(id: String): LiveData
   fun getDistinctUserById(id: String): 
         LiveData = getUserById(id).getDistinct()
}

See more of the code here and also in Java.