notes.txt

****** To know the state of User (authenticated or not) ***********

-->> Use Conditional Navigation : 
	https://developer.android.com/guide/navigation/navigation-conditional

1. Create a class to hold the User state
   with LiveData of Firebase

  class FirebaseUserLiveData : LiveData<FirebaseUser>() {....}

2. Create a Enum class to hold the states:
	enum class AuthenticationState {
    		AUTHENTICATED, UNAUTHENTICATED, INVALID_AUTHENTICATION
	}
	
3. In LoginViewModel create LiveData to save the state of User

val authenticationState = FirebaseUserLiveData()
        .map { user ->
            if (user != null) {
                AuthenticationState.AUTHENTICATED
            }else {
                AuthenticationState.UNAUTHENTICATED
            }
        }

4. In LoginFragment access the state with
	
   viewModel.authenticationState.observe(viewLifecycleOwner,
	Observer{ state ->
		when(state) {
			AuthenticationState.AUTHENTICATED -> {
                        // remove this destine (LoginFragment) from the stack
			// so we go back to MainFragment (ProfileFragment).
				navController.popBackStack()
                    	}
			....
		}	
    	}
   )	

5. 

**** When use this funtion .map in LoginViewModel**** 

 val authenticationState = FirebaseUserLiveData().map {...}

IDE told me:
"Cannot inline bytecode built with JVM target 1.8 into bytecode
 that is being built with JVM target 1.X".

Solution for this is just add to build.gradle(app)

compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }

kotlinOptions {
        jvmTarget = JavaVersion.VERSION_1_8.toString()
    }