Convert one-way data binding to two-way using Android Architecture Components

Question

I am refactoring my Android app for an University project to use Architecture Components and I am having a hard time implementing two-way data binding on a SwitchCompat. The app has got a simple user interface with a TextView displaying the status of location updates and the aforementioned SwitchCompat, which toggles on and off location updates.
For now I am using one-way data binding on the SwitchCompat's checked attribute, but would like to use two-way databinding.
The current implementation, using a Model-View-ViewModel architecture is the following:
MainViewModel.java:

public class MainViewModel extends ViewModel {

    private LiveData<Resource<Location>> mLocationResource;

    public MainViewModel() {
        mLocationResource = Repository.getInstance().getLocationResource();
    }

    public LiveData<Resource<Location>> getLocationResource() {
        return mLocationResource;
    }

    public void onCheckedChanged (Context context, boolean isChecked) {
        if (isChecked) {
            Repository.getInstance().requestLocationUpdates(context);
        } else {
            Repository.getInstance().removeLocationUpdates(context);
        }
    }
}

Resource<Location> (saw the idea here) is a class holding nullable data (Location) and a non null state the TextView can handle:
State.java

public enum State {
    LOADING,
    UPDATE,
    UNKNOWN,
    STOPPED
}

And now the android:onCheckedChanged implementation in fragment_main.xml:

android:onCheckedChanged="@{(buttonView, isChecked) -> viewModel.onCheckedChanged(context, isChecked)}"

And finally the custom binding adapter to convert from state to boolean checked state:

@BindingAdapter({"android:checked"})
public static void setChecked(CompoundButton view, Resource<Location> locationResource) {
    State state = locationResource.getState();
    boolean checked;
    if (state == State.STOPPED) {
        checked = false;
    } else {
        checked = true;
    }
    if (view.isChecked() != checked) {
        view.setChecked(checked);
    }
}

and the implementation of the android:checked attribute in fragment_main.xml:

android:checked="@{viewModel.getLocationResource}"

As the Android Developers guide I linked above said, how can I do all the work inside android:checked instead of having both android:checked and android:onCheckedChanged (one-way databinding to two-way data binding)?
Also, please let me know if you think the architecture/logic of my app can be improved :)

Answer 1

Here is how I would do it (sorry for the Kotlin code):

First I would refactor the Resource<T> class and make the state variable a MutableLiveData<State> object:

enum class State {
    LOADING,
    UPDATE,
    UNKNOWN,
    STOPPED
}

class Resource<T>() {
    var state = MutableLiveData<State>().apply { 
        value = State.STOPPED //Setting the initial value to State.STOPPED
    }
}

Then I would create the following ViewModel:

class MainViewModel: ViewModel() {

     val locationResource = Resource<Location>()

}

In the databinding layout I would write the following:

<layout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto">

    <data>

        <variable
            name="viewModel"
            type="MainViewModel" />

    </data>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <androidx.appcompat.widget.SwitchCompat
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:resourceState="@={viewModel.locationResource.state}" />

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{String.valueOf(viewModel.locationResource.state)}" />

    </LinearLayout>

</layout>

Note the two-way databinding expression @= at the SwitchCompat view.

And now to the BindingAdapter and InverseBindingAdapter:

@BindingAdapter("resourceState")
fun setResourceState(compoundButton: CompoundButton, resourceState: State) {
    compoundButton.isChecked = when (resourceState) {
        // You can decide yourself how the mapping should look like:
        State.LOADING -> true 
        State.UPDATE -> true
        State.UNKNOWN -> true
        State.STOPPED -> false
    }
}

@InverseBindingAdapter(attribute = "resourceState", event = "resourceStateAttrChanged")
fun getResourceStateAttrChanged(compoundButton: CompoundButton): State =
    // You can decide yourself how the mapping should look like:
    if (compoundButton.isChecked) State.UPDATE else State.STOPPED

@BindingAdapter("resourceStateAttrChanged")
fun setResourceStateAttrChanged(
    compoundButton: CompoundButton,
    attrChange: InverseBindingListener
) {
    compoundButton.setOnCheckedChangeListener { _, isChecked -> 
        attrChange.onChange()

        // Not the best place to put this, but it will work for now:
        if (isChecked) {
            Repository.getInstance().requestLocationUpdates(context);
        } else {
            Repository.getInstance().removeLocationUpdates(context);
        }
    }
}

And thats it. Now:

• Whenever locationResource.state changes to State.STOPPED, the SwitchCompat button will go to the unchecked state.
• Whenever locationResource.state changes from State.STOPPED to another state, the SwitchCompat button will go to the checked state.
• Whenever the SwitchCompat button is tapped and changes to the checked state then the value of locationResource.state will change to State.UPDATE.
• Whenever the SwitchCompat button is tapped and changes to the unchecked state then the value of locationResource.state will change to State.STOPPED.

Feel free to ask me any questions if something is not clear.

Answer 2

At the end, I gave up trying to convert from one-way data binding to two-way data binding, but I managed to simplify a little bit the android:checked attribute:
I replaced value

"@{viewModel.getLocationResource}"

with

"@{viewModel.locationResource.state != State.STOPPED}"

and completely removed the android:checked @BindingAdapter.