BaseViewModel is an abstraction used to do the ViewModel able to implement the Observable behavior.

BaseViewModel.kt

import android.app.Application
import androidx.databinding.Bindable
import androidx.databinding.Observable
import androidx.databinding.PropertyChangeRegistry
import androidx.lifecycle.AndroidViewModel

open class BaseViewModel(application: Application) : AndroidViewModel(application), Observable {

    @Transient
    private var mCallbacks: PropertyChangeRegistry? = null

    override fun addOnPropertyChangedCallback(callback: Observable.OnPropertyChangedCallback) {
        synchronized(this) {
            if (mCallbacks == null) {
                mCallbacks = PropertyChangeRegistry()
            }
        }
        mCallbacks?.add(callback)
    }

    override fun removeOnPropertyChangedCallback(callback: Observable.OnPropertyChangedCallback) {
        synchronized(this) {
            if (mCallbacks == null) {
                return
            }
        }
        mCallbacks?.remove(callback)
    }

    /**
     * Notifies listeners that a specific property has changed. The getter for the property
     * that changes should be marked with [Bindable] to generate a field in
     * `BR` to be used as `fieldId`.
     *
     * @param fieldId The generated BR id for the Bindable field.
     */
    fun notifyPropertyChanged(fieldId: Int) {
        synchronized(this) {
            if (mCallbacks == null) {
                return
            }
        }
        mCallbacks?.notifyCallbacks(this, fieldId, null)
    }

}

Reference:

• https://www.programcreek.com/java-api-examples/?code=openid/OpenYOLO-Android/OpenYOLO-Android-master/demoapps/passwordlogin/java/org/openyolo/demoapp/passwordlogin/MainActivity.java#