Observer in Fragment works but not in Activity

Question

I created an Observer in a Fragment which works perfectly (it fires a toast when an Int increases), but when I try to move this code into the Activity, the observer doesn't seem to connect and it does not update when the LiveData changes.

Fragment (this works!):

    override fun onActivityCreated(savedInstanceState: Bundle?) {
    super.onActivityCreated(savedInstanceState)

    loginViewModel = ViewModelProviders.of(this).get(LoginViewModel::class.java)

    loginViewModel.getLoginAttemptCount().observe(this, Observer { count ->
        if (count > 0) makeToast("Authentication failed")
    })
}

Activity (when I put the observer in the Activity it doesn't!):

    override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.login_activity)

    loginViewModel = ViewModelProviders.of(this).get(LoginViewModel::class.java)

    loginViewModel.getLoginAttemptCount().observe(this, Observer { count ->
        if (count > 0) makeToast("Authentication failed")
    })
}

ViewModel (both call same function in VM):

    fun getLoginAttemptCount(): MutableLiveData<Int> {
    Log.d(TAG, "getLoginAttemptCount()")
    return firestoreRepository.getLoginAttemptCount()  }

Repo (called from VM):

        fun getLoginAttemptCount(): MutableLiveData<Int>{
            Log.d(TAG, "getLoginAttemptCount()")
            return loginAttempt
    }

loginAttempt.value is increased everytime there is a login attempt and I have verified this works in Logcat..

For info, makeToast is simply a function to create a justified Toast (text and position):

private fun makeToast(message: String) {

    val centeredText: Spannable = SpannableString(message)
    centeredText.setSpan(
        AlignmentSpan.Standard(Layout.Alignment.ALIGN_CENTER),
        0, message.length - 1,
        Spannable.SPAN_INCLUSIVE_INCLUSIVE
    )

    val toast = Toast.makeText(this, centeredText, Toast.LENGTH_LONG)
    toast.setGravity(Gravity.CENTER,0,0)
    toast.show()
    Log.d(TAG, "Toast message: $message")
}

I'm assuming it is to do with the lifeCycleOwner but I am at a loss!

have you tried, posting some data manually to `AttemptCount` inside your activity to see if it's triggered or not? — Anuj Sharma, Jan 24 '20 at 14:26
I am logging each part of the process and the LiveData is changing in the Repository - the observer is just not hooking up to the LiveData when the Activity is created (as I say, it works fine with same code in the Fragment).. — Scamparelli, Jan 24 '20 at 14:46

score 3 · Accepted Answer · answered Jan 27 '20 at 19:52

loginViewModel = ViewModelProviders.of(this).get(LoginViewModel::class.java)

In Fragment you are using above line to create loginviewmodel passing the context of fragment to viewmodel so,the first thing android does is that it check ,if it contain's any other viewmodel associated with this fragment, if it contains it will not create new Viewmodel it will return the old one if it does not contain it create a new one.Viewmodel are created using key value pair. So in your case you are creating total two viewmodel each of fragment and activity you are changing the live data of fragment but you are trying to observe it in activity using activity viewmodel. If you want to acheive that you need to create shared viewmodel among activity and fragment.How to create shared viewmodel

Perfect thank you Roshan. While researching Shared Viewmodels have also found the following blog post which includes some code that helped me with understanding: https://medium.com/mindorks/how-to-communicate-between-fragments-and-activity-using-viewmodel-ca733233a51c — Scamparelli, Jan 27 '20 at 20:55

Observer in Fragment works but not in Activity

1 Answers1