Retained view model skeleton for using with data binding on android. Useful when you have rxjava stuff in your view model and you want it to survive configuration changes.

MyActivity.java

public class MyActivity extends AppCompatActivity {
    private MyViewModel viewModel;

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

        viewModel = RetainedFragment.getViewModel(
                getFragmentManager(),
                MyViewModel.class.getName(),
                () -> new MyViewModel("Some data"));
        viewModel.bind(this);

        MyActivityBinding binding = DataBindingUtil.setContentView(this, R.layout.my_activity);
        binding.setViewModel(viewModel);
    }
}

RetainedFragment.java

package org.sugr.example.viewmodel;

import android.app.Fragment;
import android.app.FragmentManager;
import android.content.SharedPreferences;
import android.os.Bundle;

public class RetainedFragment<VM extends RetainedViewModel<T>, T> extends Fragment {
    private VM viewModel;

    // Interface for creating a view model with a lambda
    public interface Factory<VM extends RetainedViewModel<T>, T> {
        VM create();
    }

    @Override public void onCreate(Bundle state) {
        super.onCreate(state);
        setRetainInstance(true);
    }

    @Override public void onDestroy() {
        super.onDestroy();

        if (viewModel != null) {
            viewModel.destroy();
        }
    }

    public void setViewModel(VM viewModel) {
        this.viewModel = viewModel;
    }

    public VM getViewModel() {
        return viewModel;
    }

    @SuppressWarnings("unchecked")
    public static <VM extends RetainedViewModel<T>, T> VM getViewModel(FragmentManager fm,
                                                                       String tag,
                                                                       Factory<VM, T> factory) {

        RetainedFragment<VM, T> fragment =
                (RetainedFragment<VM, T>) fm.findFragmentByTag(tag);

        if (fragment == null) {
            fragment = new RetainedFragment<>();
            fragment.setViewModel(factory.create());

            // TODO: commit -> commitNow (support v24)
            fm.beginTransaction().add(fragment, tag).commit();
        }

        return fragment.getViewModel();
    }
}

RetainedViewModel.java

package org.sugr.example.viewmodel;

import android.support.annotation.Nullable;

import rx.Observable;
import rx.subjects.PublishSubject;

public class RetainedViewModel<T> {
    @Nullable  protected T consumer;

    private PublishSubject<Void> unbindSubject = PublishSubject.create();
    private PublishSubject<Void> destroySubject = PublishSubject.create();

    @Override public void bind(T consumer) {
        this.consumer = consumer;
    }

    @Override public void unbind() {
        unbindSubject.onNext(null);

        consumer = null;
    }

    @Override public void destroy() {
        destroySubject.onNext(null);
    }

    // Useful for stopping observable emission when an unbind happens
    public <O> Observable.Transformer<O, O> takeUntilUnbind() {
        return o -> o.takeUntil(unbindSubject);
    }

    // Similar to takeUntilUnbind, but for destroy
    public <O> Observable.Transformer<O, O> takeUntilDestroy() {
        return o -> o.takeUntil(destroySubject);
    }
}

ViewUtil.java

package org.sugr.example.ui.view.util;

import android.app.Activity;
import android.content.Context;
import android.content.ContextWrapper;
import android.support.annotation.Nullable;
import android.view.View;

public class ViewUtil {
    // Get the activity of an attached view
    public static Activity getActivity(View view) {
        Context context = view.getContext();
        while (context instanceof ContextWrapper) {
            // Non-compat context wrappers can be activities
            if (context instanceof Activity) {
                break;
            }

            context = ((ContextWrapper) context).getBaseContext();
        }

        if (context instanceof Activity) {
            return (Activity) context;
        }

        throw new RuntimeException("An attached view will always have an activity context");
    }

    // Various checks to see if the view is actually part of the regular lifecycle, and not just part of a transition
    public static boolean isAlive(View view) {
        // Do nothing if the view is initialized by android studio for its preview feature.
        // The tag is needed by the binding, and may be null when the view is attached
        // for animation purposes, such as when leaving the activity.
        return !view.isInEditMode() && view.getTag() != null;
    }
}