Android Spinner Setting Selection with 2-Way Binding

Question

I am struggling to get some functionality to work with Android spinners when configured with 2-way databinding. I would like to set the initial value of the spinner via the 2-way databinding on android:selectedItemPosition. The spinner entries are initialised by the ViewModel and are populated correctly, hence databinding appears to be working correctly.

The problem is with the 2-way binding of selectedItemPosition. The variable is initialised to 5 by the ViewModel but the spinner's selected item remains at 0 (the first item). When debugging it appears that the value of the ObservableInt is initially 5 (as set) but is reset to zero during the second phase of executeBindings.

Any help would be appreciated.

test_spinner_activity.xml

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

    <data>
        <variable name="viewModel"
                  type="com.aapp.viewmodel.TestSpinnerViewModel"/>
    </data>
    <LinearLayout android:layout_width="match_parent"
                  android:layout_height="wrap_content">
       <android.support.v7.widget.AppCompatSpinner
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            android:id="@+id/sTimeHourSpinner"
            android:selectedItemPosition="@={viewModel.startHourIdx}"
            android:entries="@{viewModel.startTimeHourSelections}"/>
    </LinearLayout>
</layout>

TestSpinnerViewModel.java

public class TestSpinnerViewModel {
    public final ObservableArrayList<String> startTimeHourSelections = new ObservableArrayList<>();
    public final ObservableInt startHourIdx = new ObservableInt();

    public TestSpinnerViewModel(Context context) {
        this.mContext = context;

        for (int i=0; i < 24; i++) {
            int hour = i;
            startTimeHourSelections.add(df.format(hour));
        }
        startHourIdx.set(5);
    }
}

TestSpinnerActivity.java

public class TestSpinnerActivity extends AppCompatActivity {
    private TestSpinnerActivityBinding binding;
    private TestSpinnerViewModel mTestSpinnerViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        binding = DataBindingUtil.bind(findViewById(R.id.test_spinner));
        mTestSpinnerViewModel = new TestSpinnerViewModel(this);
        binding.setViewModel(mTestSpinnerViewModel);
    }

I am using Android Studio 2.2.2 and Databinding is enabled.

Answer 1

thank you for your suggestions. But I found the answer to my own question. It turns out that the reason that the android:selectedItemPosition=@={viewModel.startHourIdx} variable was being reset from the initialised value of 5 to 0 is because of the declaration order of the selectedItemPosition and entries attributes. In my example they were declared in that specific order and the auto-generated binding code produces initialisation in that same order.

Hence, even though the selectedItemPosition was set correctly the initialisation of the entries causes instantiation of the an ArrayAdapter which resets the selectedItemPosition to 0.

Hence, the fix is to swap the two attribute declarations in the layout file.

<data>
    <variable name="viewModel"
              type="com.aapp.viewmodel.TestSpinnerViewModel"/>
</data>
<LinearLayout android:layout_width="match_parent"
              android:layout_height="wrap_content">
   <android.support.v7.widget.AppCompatSpinner
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:id="@+id/sTimeHourSpinner"
        android:entries="@{viewModel.startTimeHourSelections}"
        android:selectedItemPosition="@={viewModel.startHourIdx}"/>
</LinearLayout>

Answer 2

I recently created a demo app on GitHub to show how to achieve 2-way databinding on spinners utilising bindingAdapter and InverseBindingAdapter mechanism.

In this app, I am not binding the "android:selectedItemPosition" attribute but binding the selected item itself (utilising ObservableField class) of the spinner as shown in the snippet below. Because it's a two way binding, by assigning an initial value to the bound ObservableField (i.e., the selected item) during spinner adapter setup, along with a special handling within the bindingAdapter of the spinner, the spinner initial selection can be achieved.

Feel free to check the demo app here for more details.

acivity_main.xml

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

    <data>
        <variable
            name="bindingPlanet"
            type="au.com.chrisli.spinnertwowaydatabindingdemo.BindingPlanet"/>
        <variable
            name="spinAdapterPlanet"
            type="android.widget.ArrayAdapter"/>
    </data>

    <RelativeLayout
        android:id="@+id/activity_main"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        ...>

        <android.support.v7.widget.AppCompatSpinner
            android:id="@+id/spin"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_centerInParent="true"
            style="@style/Base.Widget.AppCompat.Spinner.Underlined"
            bind:selectedPlanet="@={bindingPlanet.obvSelectedPlanet_}"
            app:adapter="@{spinAdapterPlanet}"/>

        ...(not relevant content omitted for simplicity)
    </RelativeLayout>

</layout>

Special handling within binding adapter in BindingPlanet.java

public final ObservableField<Planet> obvSelectedPlanet_ = new ObservableField<>(); //for simplicity, we use a public variable here

private static class SpinPlanetOnItemSelectedListener implements AdapterView.OnItemSelectedListener {

    private Planet initialSelectedPlanet_;
    private InverseBindingListener inverseBindingListener_;

    public SpinPlanetOnItemSelectedListener(Planet initialSelectedPlanet, InverseBindingListener inverseBindingListener) {
        initialSelectedPlanet_ = initialSelectedPlanet;
        inverseBindingListener_ = inverseBindingListener;
    }

    @Override
    public void onItemSelected(AdapterView<?> adapterView, View view, int i, long l) {
        if (initialSelectedPlanet_ != null) {
            //Adapter is not ready yet but there is already a bound data,
            //hence we need to set a flag so we can implement a special handling inside the OnItemSelectedListener
            //for the initial selected item
            Integer positionInAdapter = getPlanetPositionInAdapter((ArrayAdapter<Planet>) adapterView.getAdapter(), initialSelectedPlanet_);
            if (positionInAdapter != null) {
                adapterView.setSelection(positionInAdapter); //set spinner selection as there is a match
            }
            initialSelectedPlanet_ = null; //set to null as the initialization is done
        } else {
            if (inverseBindingListener_ != null) {
                inverseBindingListener_.onChange();
            }
        }
    }

    @Override
    public void onNothingSelected(AdapterView<?> adapterView) {}
}

@BindingAdapter(value = {"bind:selectedPlanet", "bind:selectedPlanetAttrChanged"}, requireAll = false)
public static void bindPlanetSelected(final AppCompatSpinner spinner, Planet planetSetByViewModel,
                                      final InverseBindingListener inverseBindingListener) {

    Planet initialSelectedPlanet = null;
    if (spinner.getAdapter() == null && planetSetByViewModel != null) {
        //Adapter is not ready yet but there is already a bound data,
        //hence we need to set a flag in order to implement a special handling inside the OnItemSelectedListener
        //for the initial selected item, otherwise the first item will be selected by the framework
        initialSelectedPlanet = planetSetByViewModel;
    }

    spinner.setOnItemSelectedListener(new SpinPlanetOnItemSelectedListener(initialSelectedPlanet, inverseBindingListener));

    //only proceed further if the newly selected planet is not equal to the already selected item in the spinner
    if (planetSetByViewModel != null && !planetSetByViewModel.equals(spinner.getSelectedItem())) {
        //find the item in the adapter
        Integer positionInAdapter = getPlanetPositionInAdapter((ArrayAdapter<Planet>) spinner.getAdapter(), planetSetByViewModel);
        if (positionInAdapter != null) {
            spinner.setSelection(positionInAdapter); //set spinner selection as there is a match
        }
    }
}

@InverseBindingAdapter(attribute = "bind:selectedPlanet", event = "bind:selectedPlanetAttrChanged")
public static Planet captureSelectedPlanet(AppCompatSpinner spinner) {
    return (Planet) spinner.getSelectedItem();
}