Cannot observe LiveData> from ViewModel in my fragment

Question

MyFragment.kt:

viewModel.studentsTemp.observe(this, Observer {
    adapter.submitList(it)
})

MyViewModel.kt

private var _studentsTemp = MutableLiveData<MutableList<Student>>()
val studentsTemp: LiveData<MutableList<Student>> get() = _studentsTemp
init {
        _studentsTemp.value = mutableListOf<Student>()
}

Observer is only being called when the application starts i.e. when ViewModel is created i.e. when init block runs in View Model.

For every other case observer is working. Except for this case. — Ahmad Jamal Mughal, Apr 07 '20 at 18:40
in fragments you should use `viewLifecycleOwner` instead of `this` — IR42, Apr 07 '20 at 18:43
@IR42 alright thanks. Could you please also explain why we use `binding.lifecycleowner = this` in Fragments when using ViewModels? I'm new to this. — Ahmad Jamal Mughal, Apr 07 '20 at 19:03

score 7 · Accepted Answer · answered Apr 07 '20 at 19:04

7

You have a MutableList in your MutableLiveData. Note that if you add or remove items from your MutableList this will NOT trigger the observer. To trigger the observer you have to update the LiveData variable.

So this will not trigger the observer

studentsTemp.value?.add(student)

but this will

studentsTemp.value = studentsTemp.value?.add(student) ?: mutableListOf(studen)

answered Apr 07 '20 at 19:04

Francesc

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Thank you so so so much for bringing my attention to it. I just added the line `_studentsTemp.value = _studentsTemp.value` after the add function and it triggered the observer. – Ahmad Jamal Mughal Apr 07 '20 at 19:27

Cannot observe LiveData> from ViewModel in my fragment

1 Answers1