The purpose of Architecture Components is to provide guidance on app architecture, with libraries for common tasks like lifecycle management and data persistence.

Architecture components help you structure your app in a way that is robust, testable, and maintainable with less boilerplate code.

This is the Kotlin version of the codelab. The Java version here.

What are the recommended Architecture Components?

Here is a short introduction to the Architecture Components and how they work together. Note that this codelab focuses on a subset of the components, namely LiveData, ViewModel and Room. Each component is explained more as you use it.

This diagram shows a basic form of this architecture:

Entity: When working with Room, this is an annotated class that describes a database table.

SQLite database: On the device, data is stored in an SQLite database. The Room persistence library creates and maintains this database for you.

DAO: Data access object. A mapping of SQL queries to functions. When you use a DAO, you call the methods, and Room takes care of the rest.

Room database: Database layer on top of SQLite database that takes care of mundane tasks that you used to handle with an SQLiteOpenHelper. Database holder that serves as an access point to the underlying SQLite database. The Room database uses the DAO to issue queries to the SQLite database.

Repository: A class that you create. You use the Repository for managing multiple data sources.

ViewModel: Provides data to the UI. Acts as a communication center between the Repository and the UI. Hides where the data originates from the UI. ViewModel instances survive Activity/Fragment recreation.

LiveData: A data holder class that can be observed. Always holds/caches latest version of data. Notifies its observers when the data has changed. LiveData is lifecycle aware. UI components just observe relevant data and don't stop or resume observation. LiveData automatically manages all of this since it's aware of the relevant lifecycle status changes while observing.

What you will build

You will build an app that uses Android Architecture Components and implements the architecture from Guide to App Architecture for these components. The sample app stores a list of words in a Room database and displays it in a RecyclerView. The app is bare bones but sufficiently complex that you can use it as a template to build upon.

In this codelab you build an app that does the following:

RoomWordSample architecture overview

The following diagram shows all the pieces of the app. Each of the enclosing boxes (except for the SQLite database) represents a class that you will create.

What you'll learn

There are a lot of steps to using the Architecture Components and implementing the recommended architecture. The most important thing is to create a mental model of what is going on, and understand how the pieces fit together and how the data flows. As you work through this codelab, don't just copy and paste the code, but try to start building that inner understanding.

What you'll need

You need to be familiar with Kotlin, object-oriented design concepts, and Android Development Fundamentals. In particular:

If you are not familiar with Kotlin, a version of this codelab is provided in Java here.

This codelab is focused on Android Architecture Components. Off-topic concepts and code are provided for you to simply copy and paste.

This codelab provides all the code you need to build the complete app.

Open Android Studio and create an app as follows:

You have to add the component libraries to your gradle files.

In your build.gradle (Module: app) make the following changes:

Apply the kapt Kotlin plugin by adding it after the other plugins defined on the top of your build.gradle (Module: app) file.

apply plugin: 'kotlin-kapt'

Add the following code to the dependencies block.

// Room components
implementation "android.arch.persistence.room:runtime:$rootProject.roomVersion"
kapt "android.arch.persistence.room:compiler:$rootProject.roomVersion"
androidTestImplementation "android.arch.persistence.room:testing:$rootProject.roomVersion"

// Lifecycle components
implementation "android.arch.lifecycle:extensions:$rootProject.archLifecycleVersion"
kapt "android.arch.lifecycle:compiler:$rootProject.archLifecycleVersion"

// Coroutines
api "org.jetbrains.kotlinx:kotlinx-coroutines-core:$rootProject.coroutines"
api "org.jetbrains.kotlinx:kotlinx-coroutines-android:$rootProject.coroutines"

Enable the coroutines support by adding the following code at the end of your build.gradle (Module: app) file.

kotlin {
    experimental {
        coroutines "enable"
    }
}

In your build.gradle (Project: RoomWordsSample) file, add the version numbers to the end of the file, as given in the code below.

ext {
   roomVersion = '1.1.1'
   archLifecycleVersion = '1.1.1'
   coroutines = '0.26.1'
}

The data for this app is words, and each word is an Entity. Create a data class called Word that describes a word Entity. You need public values for the columns, because that's how Room knows to instantiate your objects.

Here is the code: 

data class Word(val word: String)

To make the Word class meaningful to a Room database, you need to annotate it. Annotations identify how each part of this class relates to an entry in the database. Room uses this information to generate code.

You can find a complete list of annotations in the Room package summary reference.

Update your Word class with annotations as shown in this code. If you type the annotations, Android Studio will auto-import.

@Entity(tableName = "word_table")
class Word(@PrimaryKey @ColumnInfo(name = "word") val word: String)

What is the DAO?

In the DAO (data access object), you specify SQL queries and associate them with method calls. The compiler checks the SQL and generates queries from convenience annotations for common queries, such as @Insert.

The DAO must be an interface or abstract class.

By default, all queries must be executed on a separate thread.

Room uses the DAO to create a clean API for your code.

Implement the DAO

The DAO for this codelab is basic and provides queries for getting all the words, inserting a word, and deleting all the words.

  1. Create a new Interface and call it WordDao.
  2. Annotate the class with @Dao to identify it as a DAO class for Room.
  3. Declare a method to insert one word: fun insert(word: Word)
  4. Annotate the method with @Insert. You don't have to provide any SQL! (There are also @Delete and @Update annotations for deleting and updating a row, but you are not using them in this app.)
  5. Declare a method to delete all the words: fun deleteAll().
  6. There is no convenience annotation for deleting multiple entities, so annotate the method with the generic @Query.
  7. Provide the SQL query as a string parameter to @Query annotation.

@Query("DELETE FROM word_table")

  1. Create a method to get all the words and have it return a List of Words.
    fun getAllWords(): List<Word>
  2. Annotate the method with the SQL query:
    @Query("SELECT * from word_table ORDER BY word ASC")

Here is the completed code: 

@Dao
interface WordDao {

    @Query("SELECT * from word_table ORDER BY word ASC")
    fun getAllWords(): List<Word>

    @Insert
    fun insert(word: Word)

    @Query("DELETE FROM word_table")
    fun deleteAll()
}

When data changes you usually want to take some action, such as displaying the updated data in the UI. This means you have to observe the data so that when it changes, you can react. Depending on how the data is stored, this can be tricky. Observing changes to data across multiple components of your app can create explicit, rigid dependency paths between the components. This makes testing and debugging difficult, among other things.

LiveData, a lifecycle library class for data observation, solves this problem. Use a return value of type LiveData in your method description, and Room generates all necessary code to update the LiveData when the database is updated.

In WordDao, change the getAllWords() method signature so that the returned List<Word> is wrapped with LiveData.

   @Query("SELECT * from word_table ORDER BY word ASC")
   fun getAllWords(): LiveData<List<Word>>

Later in this codelab, you create an Observer of the data in the onCreate() method of MainActivity. Overriding the observer's onChanged() method makes it possible to receive notifications when the LiveData changes, in the onChanged() method. You will then update the cached data in the adapter, and the adapter will update what the user sees.

What is a Room database?

Implement the Room database

Your Room database class must be abstract and extend RoomDatabase. Usually, you only need one instance of a Room database for the whole app.

  1. Create a public abstract class that extends RoomDatabase and call it WordRoomDatabase.
    public abstract class WordRoomDatabase : RoomDatabase() {}
  2. Annotate the class to be a Room database, declare the entities that belong in the database and set the version number. Listing the entities will create tables in the database.
    @Database(entities = {Word.class}, version = 1)
  3. Define the DAOs that work with the database. Provide an abstract "getter" method for each @Dao.
    abstract fun wordDao(): WordDao

Here is the code:

@Database(entities = [Word::class], version = 1)
public abstract class WordRoomDatabase : RoomDatabase() {
   abstract fun wordDao(): WordDao

}
  1. Make the WordRoomDatabase a singleton to prevent having multiple instances of the database opened at the same time.

Here is the code:

companion object {
   @Volatile
   private var INSTANCE: WordRoomDatabase? = null

   fun getDatabase(context: Context): WordRoomDatabase {
       return INSTANCE ?: synchronized(this) {
           // Create database here
           val instance = // TODO
           INSTANCE = instance
           instance
        }
        }
   }
  1. Add the code to get a database. This code uses Room's database builder to create a RoomDatabase object in the application context from the WordRoomDatabase class and names it "word_database". 
// Create database here
val instance = Room.databaseBuilder(
      context.applicationContext,
      WordRoomDatabase::class.java, 
      "Word_database"
  ).build()

Here is the complete code for the class:

@Database(entities = arrayOf(Word::class), version = 1)
public abstract class WordRoomDatabase : RoomDatabase() {

   abstract fun wordDao(): WordDao

   companion object {
        @Volatile
        private var INSTANCE: WordRoomDatabase? = null

        fun getDatabase(context: Context): WordRoomDatabase {
            val tempInstance = INSTANCE
            if (tempInstance != null) {
                return tempInstance
            }
            synchronized(this) {
                val instance = Room.databaseBuilder(
                        context.applicationContext,
                        WordRoomDatabase::class.java, 
                        "Word_database"
                    ).build()
                INSTANCE = instance
                return instance
            }
        }
   }
}

What is a Repository?

A Repository class abstracts access to multiple data sources. The Repository is not part of the Architecture Components libraries, but is a suggested best practice for code separation and architecture. A Repository class provides a clean API for data access to the rest of the application.

Why use a Repository?

A Repository manages queries and allows you to use multiple backends. In the most common example, the Repository implements the logic for deciding whether to fetch data from a network or use results cached in a local database.

Implementing the Repository

  1. Create a public class called WordRepository.
  2. Declare the DAO as a private property in the constructor
  3. Add the list of words as a public property and initialize it. Room executes all queries on a separate thread. Observed LiveData will notify the observer when the data has changed.
class WordRepository(private val wordDao: WordDao) {

}
  1. Add the list of words as a public property and initialize it. Room executes all queries on a separate thread. Observed LiveData will notify the observer when the data has changed.
val allWords: LiveData<List<Word>> = wordDao.getAllWords()
  1. Add a wrapper for the insert() method. You must call this on a non-UI thread or your app will crash. Room ensures that you don't do any long-running operations on the main thread, blocking the UI. Add the @WorkerThread annotation, to mark that this method needs to be called from a non-UI thread. Add the suspend modifier to tell the compiler that this needs to be called from a coroutine or another suspending function. 
@WorkerThread
suspend fun insert(word: Word) {
     wordDao.insert(word)
}

Here is the complete code:

class WordRepository(private val wordDao: WordDao) {

    val allWords: LiveData<List<Word>> = wordDao.getAllWords()

    @WorkerThread
    suspend fun insert(word: Word) {
        wordDao.insert(word)
    }
}

What is a ViewModel?

The ViewModel's role is to provide data to the UI and survive configuration changes. A ViewModel acts as a communication center between the Repository and the UI. You can also use a ViewModel to share data between fragments. The ViewModel is part of the lifecycle library.

For an introductory guide to this topic, see ViewModel.

Why use a ViewModel?

A ViewModel holds your app's UI data in a lifecycle-conscious way that survives configuration changes. Separating your app's UI data from your Activity and Fragment classes lets you better follow the single responsibility principle: Your activities and fragments are responsible for drawing data to the screen, while your ViewModel can take care of holding and processing all the data needed for the UI.

In the ViewModel, use LiveData for changeable data that the UI will use or display. Using LiveData has several benefits:

Implement the ViewModel

  1. Create a class called WordViewModel that gets the Application as a parameter and extends AndroidViewModel. 
class WordViewModel(application: Application) : AndroidViewModel(application)
  1. Add a private member variable to hold a reference to the repository. 
private val repository: WordRepository
  1. Add a private LiveData member variable to cache the list of words. 
val allWords: LiveData<List<Word>>
  1. Create an init block that gets a reference to the WordDao from the WordRoomDatabase and constructs the WordRepository based on it. 
init {
   val wordsDao = WordRoomDatabase.getDatabase(application).wordDao()
   repository = WordRepository(wordsDao)
}
  1. In the init block, initialize the allWords property with the data from repository:
allWords = repository.allWords
  1. Define a parentJob, and a coroutineContext. The coroutineContext, by default, uses the parentJob and the main dispatcher to create a new instance of a CoroutineScope based on the coroutineContext.
private var parentJob = Job()
  
private val coroutineContext: CoroutineContext
   get() = parentJob + Dispatchers.Main

private val scope = CoroutineScope(coroutineContext)
  1. Override the onCleared method and cancel the parentJob. onCleared is called when the ViewModel is no longer used and will be destroyed so, now is the time to cancel any long running jobs done by the parentJob. 
override fun onCleared() {
    super.onCleared()
    parentJob.cancel()
}
  1. Create a wrapper insert() method that calls the Repository's insert() method. In this way, the implementation of insert() is completely hidden from the UI. We want the insert() method to be called away from the main thread, so we're launching a new coroutine, based on the coroutine scope defined previously. Because we're doing a database operation, we're using the IO Dispatcher.
fun insert(word: Word) = scope.launch(Dispatchers.IO) {
    repository.insert(word)
}

Here is the complete code for WordViewModel:

class WordViewModel(application: Application) : AndroidViewModel(application) {

    private var parentJob = Job()
    private val coroutineContext: CoroutineContext
        get() = parentJob + Dispatchers.Main
    private val scope = CoroutineScope(coroutineContext)

    private val repository: WordRepository
    val allWords: LiveData<List<Word>>

    init {
        val wordsDao = WordRoomDatabase.getDatabase(application).wordDao()
        repository = WordRepository(wordsDao)
        allWords = repository.allWords
    }

    fun insert(word: Word) = scope.launch(Dispatchers.IO) {
        repository.insert(word)
    }

    override fun onCleared() {
        super.onCleared()
        parentJob.cancel()
    }
}

Next, you need to add the XML layout for the list and items.

This codelab assumes that you are familiar with creating layouts in XML, so we are just providing you with the code.

Add a style for list items in values/styles.xml:

<!-- The default font for RecyclerView items is too small.
The margin is a simple delimiter between the words. -->
<style name="word_title">
   <item name="android:layout_width">match_parent</item>
   <item name="android:layout_height">26dp</item>
   <item name="android:textSize">24sp</item>
   <item name="android:textStyle">bold</item>
   <item name="android:layout_marginBottom">6dp</item>
   <item name="android:paddingLeft">8dp</item>
</style>

Add a layout/recyclerview_item.xml layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <TextView
        android:id="@+id/textView"
        style="@style/word_title"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="@android:color/holo_orange_light" />
</LinearLayout>

In layout/content_main.xml, replace the TextView with a RecyclerView:

<android.support.v7.widget.RecyclerView
   android:id="@+id/recyclerview"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:background="@android:color/darker_gray"
   tools:listitem="@layout/recyclerview_item" />

Your floating action button (FAB) should correspond to the available action. In the layout/activity_main.xml file, give the FloatingActionButton a + symbol icon:

  1. In the layout/activity_main.xml file, select File > New > Vector Asset.
  2. Click the Android robot icon in the Icon: field, then select the + ("add") asset.
  3. Change the layout file code as follows.
android:src="@drawable/ic_add_black_24dp"

You are going to display the data in a RecyclerView, which is a little nicer than just throwing the data in a TextView. This codelab assumes that you know how RecyclerView, RecyclerView.LayoutManager, RecyclerView.ViewHolder, and RecyclerView.Adapter work.

Note that the words variable in the adapter caches the data. In the next task, you add the code that updates the data automatically.

Add a class WordListAdapter that extends RecyclerView.Adapter. Here is the code. 

class WordListAdapter internal constructor(
        context: Context
) : RecyclerView.Adapter<WordListAdapter.WordViewHolder>() {

    private val inflater: LayoutInflater = LayoutInflater.from(context)
    private var words = emptyList<Word>() // Cached copy of words

    inner class WordViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        val wordItemView: TextView = itemView.findViewById(R.id.textView)
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): WordViewHolder {
        val itemView = inflater.inflate(R.layout.recyclerview_item, parent, false)
        return WordViewHolder(itemView)
    }

    override fun onBindViewHolder(holder: WordViewHolder, position: Int) {
        val current = words[position]
        holder.wordItemView.text = current.word
    }

    internal fun setWords(words: List<Word>) {
        this.words = words
        notifyDataSetChanged()
    }

    override fun getItemCount() = words.size
}

Add the RecyclerView in the onCreate() method of MainActivity.

In the onCreate() method:

   val recyclerView = findViewById<RecyclerView>(R.id.recyclerview)
   val adapter = WordListAdapter(this)
   recyclerView.adapter = adapter
   recyclerView.layoutManager = LinearLayoutManager(this)

Run your app to make sure everything works. There are no items, because you have not hooked up the data yet, so the app should display a gray background without any list items.

There is no data in the database. You will add data in two ways: Add some data when the database is opened, and add an Activity for adding words.

To delete all content and repopulate the database whenever the app is started, you create a RoomDatabase.Callback and override onOpen(). Because you cannot do Room database operations on the UI thread, onOpen() launches a coroutine on the IO Dispatcher.

To launch a coroutine we need a CoroutineScope. Update the getDatabase method of the RoomDatabase class, to also get a coroutine scope as parameter:

fun getDatabase(
       context: Context,
       scope: CoroutineScope
  ): WordRoomDatabase {
...
}

Update the database retrieval in the init block of WordViewModel, to also pass the scope

val wordsDao = WordRoomDatabase.getDatabase(application, scope).wordDao()

In the WordRoomDatabase, we create a custom implementation of the RoomDatabase.Callback(), that also gets a CoroutineScope as constructor parameter. Then, we override the onOpen method to populate the database.

Here is the code for creating the callback in the WordRoomDatabase class:

private class WordDatabaseCallback(
    private val scope: CoroutineScope
) : RoomDatabase.Callback() {
            
    override fun onOpen(db: SupportSQLiteDatabase) {
       super.onOpen(db)
       INSTANCE?.let { database ->
            scope.launch(Dispatchers.IO) {
               populateDatabase(database.wordDao())
            }
       }
   }
}

Here is the code for the function that deletes the contents of the database, then populates it with the two words "Hello" and "World". Feel free to add more words!

fun populateDatabase(wordDao: WordDao) {
     wordDao.deleteAll()

     var word = Word("Hello")
     wordDao.insert(word)
     word = Word("World!")
     wordDao.insert(word)
}

Finally, add the callback to the database build sequence right before calling .build().

.addCallback(WordDatabaseCallback(scope))

Add these string resources in values/strings.xml:

<string name="hint_word">Word...</string>
<string name="button_save">Save</string>
<string name="empty_not_saved">Word not saved because it is empty.</string>

Add this color resource in value/colors.xml:

<color name="buttonLabel">#d3d3d3</color>

Add these dimension resources in values/dimens.xml:

<dimen name="small_padding">6dp</dimen>
<dimen name="big_padding">16dp</dimen>

Use the Empty Activity template to create a new activity, NewWordActivity. Verify that the activity has been added to the Android Manifest!
<activity android:name=".NewWordActivity"></activity>

Here is the code for the activity:

class NewWordActivity : AppCompatActivity() {

    private lateinit var editWordView: EditText

    public override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_new_word)
        editWordView = findViewById(R.id.edit_word)

        val button = findViewById<Button>(R.id.button_save)
        button.setOnClickListener {
            val replyIntent = Intent()
            if (TextUtils.isEmpty(editWordView.text)) {
                setResult(Activity.RESULT_CANCELED, replyIntent)
            } else {
                val word = editWordView.text.toString()
                replyIntent.putExtra(EXTRA_REPLY, word)
                setResult(Activity.RESULT_OK, replyIntent)
            }
            finish()
        }
    }

    companion object {
        const val EXTRA_REPLY = "com.example.android.wordlistsql.REPLY"
    }
}

Update the activity_new_word.xml file in the layout folder, with the following code:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:orientation="vertical" android:layout_width="match_parent"
   android:layout_height="match_parent">

   <EditText
       android:id="@+id/edit_word"
       android:layout_width="match_parent"
       android:layout_height="wrap_content"
       android:fontFamily="sans-serif-light"
       android:hint="@string/hint_word"
       android:inputType="textAutoComplete"
       android:padding="@dimen/small_padding"
       android:layout_marginBottom="@dimen/big_padding"
       android:layout_marginTop="@dimen/big_padding"
       android:textSize="18sp" />

   <Button
       android:id="@+id/button_save"
       android:layout_width="match_parent"
       android:layout_height="wrap_content"
       android:background="@color/colorPrimary"
       android:text="@string/button_save"
       android:textColor="@color/buttonLabel" />

</LinearLayout>

The final step is to connect the UI to the database by saving new words the user enters and displaying the current contents of the word database in the RecyclerView.

To display the current contents of the database, add an observer that observes the LiveData in the ViewModel. Whenever the data changes, the onChanged() callback is invoked, which calls the adapter's setWord() method to update the adapter's cached data and refresh the displayed list.

In MainActivity, create a member variable for the ViewModel:

private lateinit var wordViewModel: WordViewModel

Use ViewModelProviders to associate your ViewModel with your Activity. When your Activity first starts, the ViewModelProviders will create the ViewModel. When the activity is destroyed, for example through a configuration change, the ViewModel persists. When the activity is re-created, the ViewModelProviders return the existing ViewModel. For more information see ViewModel.

In onCreate(), get a ViewModel from the ViewModelProvider.

wordViewModel = ViewModelProviders.of(this).get(WordViewModel::class.java)

Also in onCreate(), add an observer for the LiveData returned by getAllWords().
The onChanged() method fires when the observed data changes and the activity is in the foreground.

wordViewModel.allWords.observe(this, Observer { words ->
            // Update the cached copy of the words in the adapter.
            words?.let { adapter.setWords(it) }
})

Define a request code as a member of the MainActivity:

companion object {
        const val newWordActivityRequestCode = 1
}

In MainActivity, add the onActivityResult() code for the NewWordActivity.

If the activity returns with RESULT_OK, insert the returned word into the database by calling the insert() method of the WordViewModel.

override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
    super.onActivityResult(requestCode, resultCode, data)

    if (requestCode == newWordActivityRequestCode && resultCode == Activity.RESULT_OK) {
        data?.let {
            val word = Word(it.getStringExtra(NewWordActivity.EXTRA_REPLY))
            wordViewModel.insert(word)
        }
    } else {
        Toast.makeText(
               applicationContext,
               R.string.empty_not_saved,
                Toast.LENGTH_LONG).show()
    }
}

In MainActivity,start NewWordActivity when the user taps the FAB. Replace the code in the FAB's onClick() click handler with this code:

fab.setOnClickListener {
  val intent = Intent(this@MainActivity, NewWordActivity::class.java)
  startActivityForResult(intent, newWordActivityRequestCode)
}

RUN YOUR APP!!!

When you add a word to the database in NewWordActivity, the UI will automatically update.

Now that you have a working app, let's recap what you've built. Here is the app structure again:

The components of the app are:

(*)Views and Activities (and Fragments) only interact with the data through the ViewModel. As such, it doesn't matter where the data comes from.

[Optional] Download the solution code

Click the following link to download the solution code for this codelab:

Download source code

Unpack the downloaded zip file. This will unpack a root folder, android-room-with-a-view-kotlin, which contains the complete app.