java-Android ViewModel 的 LiveData 没有更新 UI

我有以下片段：

todays_training_fragment.xml

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <data>
        <import type="android.view.View" />
        <variable
            name="viewmodel"
            type="com.myapp.viewmodel.CurrentTrainingSessionViewModel" />
    </data>

    <LinearLayout
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".fragments.CurrentProgramFragment"
        android:id="@+id/todays_training_fragment"
        >

        <TextView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="@{String.valueOf(viewmodel.activeProgramId)}" />
    </LinearLayout>
</layout>

TodaysTrainingFragment.java

public class TodaysTrainingFragment extends Fragment {
    CurrentTrainingSessionViewModel viewModel;
    private TodaysTrainingFragmentBinding binding;


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        binding = DataBindingUtil
                .inflate(inflater, R.layout.todays_training_fragment, container, false);
        binding.setLifecycleOwner(this);

        viewModel =
                ViewModelProviders
                        .of(this)
                        .get(CurrentTrainingSessionViewModel.class);

        return binding.getRoot();
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        binding.setViewmodel(viewModel);

        binding.getViewmodel().setActiveProgramId(12342); // <-- I'm expecting the layout to show this value
        binding.setLifecycleOwner(this);
    }
}

CurrentTrainingSessionViewModel.java

public class CurrentTrainingSessionViewModel extends ViewModel {
    private MutableLiveData activeProgramId = new MutableLiveData<>(0L);



    public long getActiveProgramId() {
        return activeProgramId.getValue();
    }

    public void setActiveProgramId(long id) {
        activeProgramId.postValue(id);
    }

}

我希望我的布局在文本视图中显示0，因为它是activeProgramId的初始值，然后在随后更改时将显示的值更新为12342。但是，该值保持为0，不会更新。我做错了什么？