If you type the annotations yourself instead of pasting , Android Studio will auto-import the annotation classes. You can find a complete list of annotations in the Room package summary reference. The compiler checks the SQL and generates queries from convenience annotations for common queries, such as Insert. Room has Kotlin coroutines support. This allows your queries to be annotated with the suspend modifier and then called from a coroutine or from another suspension function.
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 when it changes, you can react. To observe data changes you will use Flow from kotlinx-coroutines. Use a return value of type Flow in your method description, and Room generates all necessary code to update the Flow when the database is updated.
But more on these components once we get to implement them. Your Room database class must be abstract and extend RoomDatabase. Usually, you only need one instance of a Room database for the whole app. 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.
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.
Create a Kotlin class file called WordRepository and paste the following code into it:. The ViewModel 's role is to provide data to the UI and survive configuration changes. You can also use a ViewModel to share data between fragments.
The ViewModel is part of the lifecycle library. 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.
LiveData is an observable data holder - you can get notified every time the data changes. Unlike Flow, LiveData is lifecycle aware, meaning that it will respect the lifecycle of other components like Activity or Fragment. LiveData automatically stops or resumes observation depending on the lifecycle of the component that listens for changes. This makes LiveData the perfect component to be used for for changeable data that the UI will use or display.
This ensures that every time the data changes in the database, your UI is automatically updated. In Kotlin, all coroutines run inside a CoroutineScope. A scope controls the lifetime of coroutines through its job. When you cancel the job of a scope, it cancels all coroutines started in that scope. The AndroidX lifecycle-viewmodel-ktx library adds a viewModelScope as an extension function of the ViewModel class, enabling you to work with scopes.
Create a Kotlin class file for WordViewModel and add this code to it:. By using viewModels and ViewModelProvider. Factory ,the framework will take care of the lifecycle of the ViewModel. It will survive configuration changes and even if the Activity is recreated, you'll always get the right instance of the WordViewModel class.
This codelab assumes that you are familiar with creating layouts in XML, so we are just providing you with the code. Make your application theme material by setting the AppTheme parent to Theme.
Now your layout should look like this:. 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. ViewHolder , and ListAdapter work. In the onCreate method after setContentView :. Run your app to make sure everything works. There are no items, because you have not hooked up the data yet.
You want to have only one instance of the database and of the repository in your app. An easy way to achieve this is by creating them as members of the Application class. Then they will just be retrieved from the Application whenever they're needed, rather than constructed every time. Create a new class called WordsApplication that extends Application. Here's the code:. Now that you created the Application class, update the AndroidManifest file and set WordsApplication as application android:name.
At the moment, there is no data in the database. You will add data in two ways: Add some data when the database is created, and add an Activity for adding words.
To delete all content and repopulate the database whenever the app is created, you'll create a RoomDatabase. Callback and override onCreate. To launch a coroutine you need a CoroutineScope. Update the getDatabase method of the WordRoomDatabase class, to also get a coroutine scope as parameter:. Populating the database isn't related to a UI lifecycle, therefore you shouldn't use a CoroutineScope like viewModelScope.
It's related to the app's lifecycle. Callback , that also gets a CoroutineScope as constructor parameter. Then, you override the onCreate method to populate the database.
Here is the code for creating the callback within the WordRoomDatabase class:. Finally, add the callback to the database build sequence right before calling.
Create a new empty Android Activity with the Empty Activity template:. 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 setWords method to update the adapter's cached data and refresh the displayed list.
In MainActivity , create the ViewModel :. This is constructed based on the repository retrieved from the WordsApplication. The onChanged method the default method for our Lambda fires when the observed data changes and the activity is in the foreground:.
You want to open the NewWordActivity when tapping on the FAB and, once you are back in the MainActivity , to either insert the new word in the database or show a Toast. Now run your app! Now that you have a working app, let's recap what you've built. Here is the app structure again:. The automatic update is possible because you are using LiveData. In the MainActivity , there is an Observer that observes the words LiveData from the database and is notified when they change.
When there is a change, the observer's onChange method is executed and updates mWords in the WordListAdapter. The data can be observed because it is LiveData.
It provides methods for accessing the data layer, and it returns LiveData so that MainActivity can set up the observer relationship. Views and Activities and Fragments only interact with the data through the ViewModel. As such, it doesn't matter where the data comes from. In this case, the data comes from a Repository.
The ViewModel does not need to know what that Repository interacts with. It just needs to know how to interact with the Repository , which is through the methods exposed by the Repository.
The Repository manages one or more data sources. In the WordListSample app, that backend is a Room database. Room is a wrapper around and implements a SQLite database.
Room does a lot of work for you that you used to have to do yourself. Because the result returned from the query is observed LiveData , every time the data in Room changes, the Observer interface's onChanged method is executed and the UI updated. If you haven't already, you can take a look at the solution code for the codelab. You can look at the github repository or download the code here:.
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.
Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4. For details, see the Google Developers Site Policies. Before you begin The Android Architecture Components collection provides guidance on app architecture, with libraries for common tasks like lifecycle management and data persistence. Prerequisites You need to be familiar with Kotlin, object-oriented design concepts, and Android development fundamentals, in particular: RecyclerView and adapters SQLite database and the SQLite query language Basic coroutines If you're not familiar with coroutines, you can start with Using Kotlin Coroutines in your Android App.
What you'll do You'll learn how to design and construct an app using the architecture components Room, ViewModel, and LiveData. Your app will: implement the recommended architecture using Android Architecture Components.
When the user enters a word, that word is added to the database and displayed in the RecyclerView list. Here's a preview: What you'll need Android Studio 4. An Android device or emulator. This codelab provides all the code you need to build the complete app.
Using the Architecture Components Here is a short diagram to introduce you to the Architecture Components and how they work together. RoomWordSample architecture overview The following diagram shows how all of the pieces of the app should interact. On the next screen, name the app RoomWordSample, and click Finish. Update Gradle files Next, you'll have to add the component libraries to your Gradle files.
Open build. Apply the kapt annotation processor Kotlin plugin by adding it in the plugins section defined on the top of your build. In your build. Create an Entity The data for this app is words, and you will need a simple table to hold those values: Room allows you to create tables via an Entity. They take him up on the offer and move in, much to Lucy's initial horror. George plays tennis with the Honeychurches on a Sunday when Cecil is at his most intolerable.
After the game, Cecil reads from a book by Miss Lavish, a woman who also stayed with Lucy and Charlotte at the pension in Florence. The novel records a kiss among violets, and Lucy realizes that Charlotte let the secret out. In a moment alone, George kisses her again. Lucy tells him to leave, but George insists that Cecil is not the right man for her, characterizing Cecil as controlling and appreciative of things rather than people.
Lucy sees Cecil in a new light, and breaks off her engagement that night. However, Lucy will not believe that she loves George; she wants to stay unmarried and travel to Greece with some elderly women she met in Italy, the Miss Alans.
She meets old Mr. Emerson by chance, who insists that she loves George and should marry him, because it is what her soul truly wants. Lucy realizes he is right, and though she must fly against convention, she marries George, and the book ends with the happy couple staying together in the Florence pension again, in a room with a view. The book depicts Lucy's struggles as she emerges as her own woman, growing from indecision to fulfillment.
She struggles between strict, old-fashioned Victorian values and newer, more liberal mores. In this struggle Lucy's own idea of what is true evolves and matures. Her trip to Italy opens her sheltered eyes to ideas and people unlike those she has known growing up in the English countryside. She also notices how freely Italian classes seem to mix, and realizes that the social boundaries she has always regarded as fixed are actually arbitrary.
Her experience with the Emersons shows her that there can be beauty in the things that are considered improper, and Charlotte's betrayal shows her that propriety is not always the best judge of what is true. Having more clearly found herself in Italy, Lucy's real test lies at home, where she must confront her old familiar surroundings.
She is still uncertain, however, and confused about what to think about her new experiences. That she missteps and becomes engaged to the pretentious and domineering Cecil shows her susceptibility to the pressures of society. As her bold piano playing suggests, she is cut out for a more daring life, if only she could cut herself away from the restricting social boundaries that engulf her.
The Emersons, as free-thinking, modern, truth-loving people, are her deliverers from the grips of society. It is this freedom that allows her to see beyond the dictates of propriety that forbid her marriage to the lower-class George and, therefore, to follow her heart. George is troubled by existential worries in Italy. He doesn't understand how life can be truly joyful and worthwhile when it is always shadowed by enigma, symbolized by the question mark that he hangs on the wall of his hotel.
Lucy, though cautious, is loving by nature and enjoys life even when it challenges her understanding. The two are united by a shared appreciation for beauty, which might be captured in their love of views: Lucy adores the view of the Arno through the pension window, while George's first memory is of himself and his parents gazing at a view.
Each possesses what the other needs: George finds simple joys staying with the Honeychurch family, while Lucy finds the courage to recognize her own individuality through her contact with the Emersons. However, its strength lies in its vivid cast of characters, humorous dialogue, and comedic play upon the manners of the day, and in Forster's engaging, sympathetic exploration of Lucy's character. Ace your assignments with our guide to A Room with a View!
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