data class Article(val id: Long,
                   val title: String,
                   val body: String,
                   val createdAt: Instant)

// A DAO interface for use with Room
@Dao
interface ArticleDao {
    @Query("SELECT * FROM article WHERE id = :id LIMIT 1")
    fun loadArticleById(id: Long): Flowable<Article>

    @Update
    fun insertArticles(articles: List<Article>)
}

// A web service interface for use with Retrofit
interface ArticleWebService {
    @GET("/articles")
    fun fetchArticles(): Single<List<Article>>
}

@Singleton
class ArticleRepository
@Inject constructor(private val articleDao: ArticleDao,
                    private val articleWebService: ArticleWebService) {

    fun article(articleId: Long): Flowable<Article> {
        return articleDao.loadArticleById(articleId)
    }

    fun syncArticles(isForcedUpdate: Boolean): Completable {
        // If the current data has not yet expired, no need to fetch anything
        if (isArticleDataStillValid(isForcedUpdate))
            return Completable.complete()

        // Fetch fresh remote data and persist it in the local data store
        return articleWebService.fetchArticles()
                .map { articles -> articleDao.insertArticles(articles) }
                .toCompletable()
                .subscribeOn(Schedulers.io())
    }

    ...
}

class ArticleViewModel
@Inject constructor(private val articleRepository: ArticleRepository) {

    private var article: Article?

    var articleId: Long? = null


    fun onActive() {
        assert(articleId != null, { "Article id  must be configured " })

        articleRepository.article(articleId!!)
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe({ article -> run {
                                   this.article = article
                                   this.notifyChange()
                               }
                           })
    }

    var articleId: Long? = null
    lateinit var disposable: Disposable

    fun onActive() {
        assert(articleId != null, { "Article id  must be configured " })

        disposable = articleRepository.article(articleId!!)
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe({ article -> run {
                                   this.article = article
                                   this.notifyChange()
                               }
                           })
    }

    fun onInactive() {
        disposable.dispose()
    }
    
    ...
}

    val title: String?
        get() = article?.title + " by " + author.name

    val body: String?
        get() = article?.body

    val publishDate: String?
        get() = article?.let { DATE_FORMATTER.format(it.createdAt) } 

    fun onPublishDateClicked() {
        TODO()
    }
   
    ...
}

class ArticleViewModel
@Inject constructor(private val articleRepository: ArticleRepository) {

    private var article: Article?
    private var author: Author?

<layout xmlns:android="http://schemas.android.com/apk/res/android">
  <data>
    <variable name="viewModel" type="com.example.ArticleViewModel" />
  </data>


























</layout>

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

    <TextView
        style="@style/header_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@{viewModel.title}" />

    <TextView
        style="@style/body_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@{viewModel.body}"/>

    <TextView
        style="@style/body_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="@{v -> viewModel.onPublishDateClicked()}"
        android:text="@{viewModel.publishDate}" />
  </LinearLayout>

class ArticleViewModel {

    val title: String?

    val body: String?

    val publishDate: String?
    
    fun onPublishDateClicked()
}

class ArticleActivity : AppCompatActivity() {

    private lateinit var viewModel: ArticleViewModel
    private lateinit var binding: ArticleScreenBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        viewModel = obtainViewModel()
        binding = DataBindingUtil.setContentView(this, R.layout.article_screen)
        binding.setVariable(BR.viewModel, viewModel)
    }

    private fun obtainViewModel(): ArticleViewModel {
        // Obtain an instance of the ViewModel associated to this view, either 
        // by creating it or by re-loading it after a configuration change.
        // Possible approaches: ViewModelProviders, Dependency Injection or
        // cache persisted via onRetainCustomNonConfigurationInstance
        ...
    }

    ...
}

class ArticleActivity : AppCompatActivity() {

    private lateinit var viewModel: ArticleViewModel
    private lateinit var binding: ArticleScreenBinding

    override fun onStart() {
        super.onStart()
        viewModel.onActive()
    }

    override fun onStop() {
        super.onStop()
        viewModel.onInactive()
    }

    override fun onDestroy() {
        super.onDestroy()
        viewModel.onDestroy()
    }
    ...
}

class StringResources
@Inject constructor(@param:Named("Application") private val context: Context) {

    fun getString(@StringRes resId: Int): String =
            context.getString(resId)

    fun getString(@StringRes resId: Int, vararg formatArgs: Any): String =
            context.getString(resId, *formatArgs)
}

class ArticleViewModel
@Inject constructor(private val stringResources: StringResources){
    
    val author: String
        get() = stringResources.getString(R.string.author, article.author)

    @get:StringRes
    val extras: Int
        get() = if (article.isReadOnly) R.string.option_a else R.string.option_b

}

sealed class ViewCommand {
    class NavToAuthorScreen(private val authorId: Long) : ViewCommand()
    class NavUp() : ViewCommand()
    class ShowPasswordDialog() : ViewCommand()
}

class ArticleViewModel {

    val commands: Subject<ViewCommand> = PublishSubject.create()

    fun onAuthorClicked() {
        commands.onNext(ViewCommand.NavToAuthorScreen(article.author))
    }
}

class ArticleActivity : AppCompatActivity() {
    
    override fun onStart() {
        super.onStart()
        ...
        viewModel.commands.subscribe(this::handleCommand)
        // TODO: Disposable cleanup omitted for brevity
    }
}