Persist data with Room

Most production quality apps have data that needs to be saved, even after the user closes the app. For example, the app might store a playlist of songs, items on a to-do list, records of expenses and income, a catalog of constellations, or a history of personal data. For most of these cases, you use a database to store this persistent data.

Room is a persistence library that's part of Android Jetpack. Room is an abstraction layer on top of a SQLite database. SQLite uses a specialized language (SQL) to perform database operations. Instead of using SQLite directly, Room simplifies the chores of setting up, configuring, and interacting with the database. Room also provides compile-time checks of SQLite statements.

The image below shows how Room fits in with the overall architecture recommended in this course.

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Prerequisites

  • You know how to build a basic user interface (UI) for an Android app.
  • You know how to use activities, fragments, and views.
  • You know how to navigate between fragments, using Safe Args to pass data between fragments.
  • You are familiar with the Android architecture components ViewModel, LiveData, and Flow, and know how to use ViewModelProvider.Factory to instantiate the ViewModels.
  • You are familiar with concurrency fundamentals.
  • You know how to use coroutines for long-running tasks.
  • You have a basic understanding of SQL databases and the SQLite language.

What you'll learn

  • How to create and interact with the SQLite database using the Room library.
  • How to create an entity, DAO, and database classes.
  • How to use a data access object (DAO) to map Kotlin functions to SQL queries.

What you'll build

  • You'll build an Inventory app that saves inventory items into the SQLite database.

What you need

  • Starter code for the Inventory app.
  • A computer with Android Studio installed.

In this codelab, you will work with a starter app called Inventory app, and add the database layer to it using the Room library. The final version of the app displays a list items from the inventory database using a RecyclerView. The user will have options to add a new item, update an existing item, and delete an item from the inventory database (you'll complete the app's functionality in the next codelab).

Below are screenshots from the final version of the app.

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Download the starter code for this codelab

This codelab provides starter code for you to extend with features taught in this codelab. Starter code may contain code that is familiar to you from previous codelabs, and also code that is unfamiliar to you that you will learn about in later codelabs.

If you use the starter code from GitHub, note that the folder name is android-basics-kotlin-inventory-app-starter. Select this folder when you open the project in Android Studio.

To get the code for this codelab and open it in Android Studio, do the following.

Get the code

  1. Click on the provided URL. This opens the GitHub page for the project in a browser.
  2. On the GitHub page for the project, click the Code button, which brings up a dialog.

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  1. In the dialog, click the Download ZIP button to save the project to your computer. Wait for the download to complete.
  2. Locate the file on your computer (likely in the Downloads folder).
  3. Double-click the ZIP file to unpack it. This creates a new folder that contains the project files.

Open the project in Android Studio

  1. Start Android Studio.
  2. In the Welcome to Android Studio window, click Open an existing Android Studio project.

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Note: If Android Studio is already open, instead, select the File > New > Import Project menu option.

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  1. In the Import Project dialog, navigate to where the unzipped project folder is located (likely in your Downloads folder).
  2. Double-click on that project folder.
  3. Wait for Android Studio to open the project.
  4. Click the Run button 11c34fc5e516fb1c.png to build and run the app. Make sure it works as expected.
  5. Browse the project files in the Project tool window to see how the app was implemented.

Starter code overview

  1. Open the project with the starter code in Android Studio.
  2. Run the app on an Android device, or on an emulator. Make sure the emulator or connected device is running API level 26 or higher. Database Inspector works best on emulator/devices running API level 26.
  3. The app shows no inventory data. Notice the FAB to add new items to the database.
  4. Click on the FAB. The app navigates to a new screen where you can enter details for the new item.

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Problems with the starter code

  1. In the Add Item screen enter an item's details. Tap Save. The add item fragment is closed and the user is navigated back to the previous fragment. The new item is not saved and is not listed on the inventory screen. Notice that the app is incomplete and the Save button functionality is not implemented.

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In this codelab, you will add the database portion of an app that saves the inventory details in the SQLite database. You will be using the Room persistence library to interact with the SQLite database.

Code walkthrough

The starter code you downloaded has the screen layouts pre-designed for you. In this pathway, you will focus on implementing the database logic. Here is a brief walkthrough of some of the files to get you started.

main_activity.xml

The main activity that hosts all the other fragments in the app. The onCreate() method retrieves NavController from the NavHostFragment and sets up the action bar for use with the NavController.

item_list_fragment.xml

The first screen shown in the app. It mainly contains a RecyclerView and a FAB. You will implement the RecyclerView later in the pathway.

fragment_add_item.xml

This layout contains text fields for entering the details of the new inventory item to be added.

ItemListFragment.kt

This fragment contains mostly boilerplate code. In the onViewCreated() method, click listener is set on FAB to navigate to the add item fragment.

AddItemFragment.kt

This fragment is used to add new items into the database. The onCreateView() function initializes the binding variable and the onDestroyView() function hides the keyboard before destroying the fragment.

Kotlin provides an easy way to deal with data by introducing data classes. This data is accessed and possibly modified using function calls. However, in the database world, you need tables and queries to access and modify data. The following components of Room make these workflows seamless.

There are three major components in Room:

  • Data entities represent tables in your app's database. They are used to update the data stored in rows in tables, and to create new rows for insertion.
  • Data access objects (DAOs) provide methods that your app uses to retrieve, update, insert, and delete data in the database.
  • Database class holds the database and is the main access point for the underlying connection to your app's database. The database class provides your app with instances of the DAOs associated with that database.

You will implement and learn more about these components later in the codelab. The following diagram demonstrates how the components of the Room work together to interact with the database.

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Add Room libraries

In this task, you'll add the required Room component libraries to your Gradle files.

  1. Open module level gradle file, build.gradle (Module: InventoryApp.app) In the dependencies block, add the following dependencies for the Room library.
    // Room
    implementation "androidx.room:room-runtime:$room_version"
    kapt "androidx.room:room-compiler:$room_version"
    implementation "androidx.room:room-ktx:$room_version"

Entity class defines a table, and each instance of this class represents a row in the database table. The entity class has mappings to tell Room how it intends to present and interact with the information in the database. In your app, the entity is going to hold information about inventory items such as item name, item price and stock available.

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@Entity annotation marks a class as a database Entity class. For each Entity class a database table is created to hold the items. Each field of the Entity is represented as a column in the database, unless it is denoted otherwise (see Entity docs for details). Every entity instance that is stored in the database must have a primary key. The primary key is used to uniquely identify every record/entry in your database tables. Once assigned, the primary key cannot be modified, it represents the entity object as long as it exists in the database.

In this task, you will create an Entity class. Define fields to store the following inventory information for each item.

  • An Int to store the primary key.
  • A String to store the item name.
  • A double to store the item price.
  • An Int to store the quantity in stock.
  1. Open starter code in the Android Studio.
  2. Create a package called data under com.example.inventory base package.

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  1. Inside the data package, create a Kotlin class called Item. This class will represent a database entity in your app. In the next step you will add corresponding fields to store inventory information.
  2. Update the Item class definition with the following code. Declare id of type Int, itemName of type String, itemPrice of type Double, and quantityInStock of type Int as parameters for the primary constructor. Assign a default value of 0 to id. This will be the primary key, an ID to uniquely identify every record/entry in your Item table.
class Item(
   val id: Int = 0,
   val itemName: String,
   val itemPrice: Double,
   val quantityInStock: Int
)

Data classes

Data classes are primarily used to hold data in Kotlin. They are marked with the keyword data. Kotlin data class objects have some extra benefits, the compiler automatically generates utilities for comparing, printing and copying such as toString(), copy(), and equals().

Example:

// Example data class with 2 properties.
data class User(val first_name: String, val last_name: String){
}

To ensure consistency and meaningful behavior of the generated code, data classes have to fulfill the following requirements:

  • The primary constructor needs to have at least one parameter.
  • All primary constructor parameters need to be marked as val or var.
  • Data classes cannot be abstract, open, sealed or inner.

To learn more about Data classes, check out the documentation.

  1. Convert the Item class to a data class by prefixing its class definition with data keyword.
data class Item(
   val id: Int = 0,
   val itemName: String,
   val itemPrice: Double,
   val quantityInStock: Int
)
  1. Above the Item class declaration, annotate the data class with @Entity. Use tableName argument to give the item as the SQLite table name.
@Entity(tableName = "item")
data class Item(
   ...
)
  1. To identify the id as the primary key, annotate the id property with @PrimaryKey. Set the parameter autoGenerate to true so that Room generates the ID for each entity. This guarantees that the ID for each item is unique.
@Entity(tableName = "item")
data class Item(
   @PrimaryKey(autoGenerate = true)
   val id: Int = 0,
   ...
)
  1. Annotate the remaining properties with @ColumnInfo. The ColumnInfo annotation is used to customise the column associated with the particular field. For example, when using the name argument, you can specify a different column name for the field rather than the variable name. Customize the property names using parameters as shown below. This approach is similar to using tableName to specify a different name to the database.
import androidx.room.ColumnInfo
import androidx.room.Entity
import androidx.room.PrimaryKey

@Entity
data class Item(
   @PrimaryKey(autoGenerate = true)
   val id: Int = 0,
   @ColumnInfo(name = "name")
   val itemName: String,
   @ColumnInfo(name = "price")
   val itemPrice: Double,
   @ColumnInfo(name = "quantity")
   val quantityInStock: Int
)

Data Access Object (DAO)

The Data Access Object (DAO) is a pattern used to separate the persistence layer with the rest of the application by providing an abstract interface. This isolation follows the single responsibility principle, which you have seen in the previous codelabs.

The functionality of the DAO is to hide all the complexities involved in performing the database operations in the underlying persistence layer from the rest of the application. This allows the data access layer to be changed independently of the code that uses the data.

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In this task, you define a Data Access Object (DAO) for the Room. Data access objects are the main components of Room that are responsible for defining the interface that accesses the database.

The DAO you will create will be a custom interface providing convenience methods for querying/retrieving, inserting, deleting, and updating the database. Room will generate an implementation of this class at compile time.

For common database operations, the Room library provides convenience annotations, such as @Insert, @Delete, and @Update. For everything else, there is the @Query annotation. You can write any query that's supported by SQLite.

As an added bonus, as you write your queries in Android Studio, the compiler checks your SQL queries for syntax errors.

For the inventory app, you need to be able to do the following:

  • Insert or add a new item.
  • Update an existing item to update name, price, and quantity.
  • Get a specific item based on its primary key, id.
  • Get all items, so you can display them.
  • Delete an entry in the database.

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Now, implement the item DAO in your app:

  1. In the data package, create Kotlin class ItemDao.kt.
  2. Change the class definition to interface and annotate with @Dao.
@Dao
interface ItemDao {
}
  1. Inside the body of the interface, add an @Insert annotation. Below the @Insert, add an insert() function that takes an instance of the Entity class item as its argument. The database operations can take a long time to execute, so they should run on a separate thread. Make the function a suspend function, so that this function can be called from a coroutine.
@Insert
suspend fun insert(item: Item)
  1. Add an argument OnConflict and assign it a value of OnConflictStrategy.IGNORE. The argument OnConflict tells the Room what to do in case of a conflict. The OnConflictStrategy.IGNORE strategy ignores a new item if it's primary key is already in the database. To know more about the available conflict strategies, check out the documentation.
@Insert(onConflict = OnConflictStrategy.IGNORE)
suspend fun insert(item: Item)

Now the Room will generate all the necessary code to insert the item into the database. When you call insert() from your Kotlin code, Room executes a SQL query to insert the entity into the database. (Note: The function can be named anything you want; it doesn't have to be called insert().)

  1. Add an @Update annotation with an update() function for one item. The entity that's updated has the same key as the entity that's passed in. You can update some or all of the entity's other properties. Similar to the insert() method, make the following update() method suspend.
@Update
suspend fun update(item: Item)
  1. Add @Delete annotation with a delete() function to delete item(s). Make it a suspend method. The @Delete annotation deletes one item, or a list of items. (Note: You need to pass the entity(s) to be deleted, if you don't have the entity you may have to fetch it before calling the delete() function.)
@Delete
suspend fun delete(item: Item)

There is no convenience annotation for the remaining functionality, so you have to use the @Query annotation and supply SQLite queries.

  1. Write a SQLite query to retrieve a particular item from the item table based on the given id. You will then add Room annotation and use a modified version of the following query in the later steps. In next steps, you will also change this into a DAO method using Room.
  2. Select all columns from the item
  3. WHERE the id matches a specific value.

Example:

SELECT * from item WHERE id = 1
  1. Change the above SQL query to use with the Room annotation and an argument. Add a @Query annotation, supply the query as a string parameter to the @Query annotation. Add a String parameter to @Query that is a SQLite query to retrieve an item from the item table.
  2. Select all columns from the item
  3. WHERE the id matches the :id argument. Notice the :id. You use the colon notation in the query to reference arguments in the function.
@Query("SELECT * from item WHERE id = :id")
  1. Below the @Query annotation add getItem() function that takes an Int argument and returns a Flow<Item>.
@Query("SELECT * from item WHERE id = :id")
fun getItem(id: Int): Flow<Item>

Using Flow or LiveData as return type will ensure you get notified whenever the data in the database changes. It is recommended to use Flow in the persistence layer. The Room keeps this Flow updated for you, which means you only need to explicitly get the data once. This is helpful to update the inventory list, which you will implement in the next codelab. Because of the Flow return type, Room also runs the query on the background thread. You don't need to explicitly make it a suspend function and call inside a coroutine scope.

You may need to import Flow from kotlinx.coroutines.flow.Flow.

  1. Add a @Query with a getItems() function:
  2. Have the SQLite query return all columns from the item table, ordered in ascending order.
  3. Have getItems() return a list of Item entities as Flow. Room keeps this Flow updated for you, which means you only need to explicitly get the data once.
@Query("SELECT * from item ORDER BY name ASC")
fun getItems(): Flow<List<Item>>
  1. Though you won't see any visible changes, run your app to make sure it has no errors.

In this task, you create a RoomDatabase that uses the Entity and DAO that you created in the previous task. The database class defines the list of entities and data access objects. It is also the main access point for the underlying connection.

The Database class provides your app with instances of the DAOs you've defined. In turn, the app can use the DAOs to retrieve data from the database as instances of the associated data entity objects. The app can also use the defined data entities to update rows from the corresponding tables, or to create new rows for insertion.

You need to create an abstract RoomDatabase class, annotated with @Database. This class has one method that either creates an instance of the RoomDatabase if it doesn't exist, or returns the existing instance of the RoomDatabase.

Here's the general process for getting the RoomDatabase instance:

  • Create a public abstract class that extends RoomDatabase. The new abstract class you defined acts as a database holder. The class you defined is abstract, because Room creates the implementation for you.
  • Annotate the class with @Database. In the arguments, list the entities for the database and set the version number.
  • Define an abstract method or property that returns an ItemDao Instance and the Room will generate the implementation for you.
  • You only need one instance of the RoomDatabase for the whole app, so make the RoomDatabase a singleton.
  • Use Room's Room.databaseBuilder to create your (item_database) database only if it doesn't exist. Otherwise, return the existing database.

Create the Database

  1. In the data package, create a Kotlin class ItemRoomDatabase.kt.
  2. In the ItemRoomDatabase.kt file, make ItemRoomDatabase class as an abstract class that extends RoomDatabase. Annotate the class with @Database. You will fix the missing parameters error in the next step.
@Database
abstract class ItemRoomDatabase : RoomDatabase() {}
  1. The @Database annotation requires several arguments, so that Room can build the database.
  • Specify the Item as the only class with the list of entities.
  • Set the version as 1. Whenever you change the schema of the database table, you'll have to increase the version number.
  • Set exportSchema to false, so as not to keep schema version history backups.
@Database(entities = [Item::class], version = 1, exportSchema = false)
  1. The database needs to know about the DAO. Inside the body of the class, declare an abstract function that returns the ItemDao. You can have multiple DAOs.
abstract fun itemDao(): ItemDao
  1. Below the abstract function, define a companion object. The companion object allows access to the methods for creating or getting the database using the class name as the qualifier.
 companion object {}
  1. Inside the companion object, declare a private nullable variable INSTANCE for the database and initialize it to null. The INSTANCE variable will keep a reference to the database, when one has been created. This helps in maintaining a single instance of the database opened at a given time, which is an expensive resource to create and maintain.

Annotate INSTANCE with @Volatile. The value of a volatile variable will never be cached, and all writes and reads will be done to and from the main memory. This helps make sure the value of INSTANCE is always up-to-date and the same for all execution threads. It means that changes made by one thread to INSTANCE are visible to all other threads immediately.

@Volatile
private var INSTANCE: ItemRoomDatabase? = null
  1. Below INSTANCE, while still inside the companion object, define a getDatabase()method with a Context parameter that the database builder will need. Return a type ItemRoomDatabase. You'll see an error because getDatabase() isn't returning anything yet.
fun getDatabase(context: Context): ItemRoomDatabase {}
  1. Multiple threads can potentially run into a race condition and ask for a database instance at the same time, resulting in two databases instead of one. Wrapping the code to get the database inside a synchronized block means that only one thread of execution at a time can enter this block of code, which makes sure the database only gets initialized once.

Inside getDatabase(), return INSTANCE variable or if INSTANCE is null, initialize it inside a synchronized{} block. Use the elvis operator(?:) to do this. Pass in this the companion object, that you want to be locked inside the function block. You will fix the error in the later steps.

return INSTANCE ?: synchronized(this) { }
  1. Inside the synchronized block, create a val instance variable, and use the database builder to get the database. You will still have errors which you will fix in the next steps.
val instance = Room.databaseBuilder()
  1. At the end of the synchronized block, return instance.
return instance
  1. Inside the synchronized block, initialize the instance variable, and use the database builder to get a database. Pass in the application context, the database class, and a name for the database, item_database to the Room.databaseBuilder().
val instance = Room.databaseBuilder(
   context.applicationContext,
   ItemRoomDatabase::class.java,
   "item_database"
)

Android Studio will generate a Type Mismatch error. To remove this error, you'll have to add a migration strategy and build() in the following steps.

  1. Add the required migration strategy to the builder. Use .fallbackToDestructiveMigration().

Normally, you would have to provide a migration object with a migration strategy for when the schema changes. A migration object is an object that defines how you take all rows with the old schema and convert them to rows in the new schema, so that no data is lost. Migration is beyond the scope of this codelab. A simple solution is to destroy and rebuild the database, which means that the data is lost.

.fallbackToDestructiveMigration()
  1. To create the database instance, call .build(). This should remove the Android Studio errors.
.build()
  1. Inside the synchronized block, assign INSTANCE = instance.
INSTANCE = instance
  1. At the end of the synchronized block, return instance. Your final code should look like this:
import android.content.Context
import androidx.room.Database
import androidx.room.Room
import androidx.room.RoomDatabase

@Database(entities = [Item::class], version = 1, exportSchema = false)
abstract class ItemRoomDatabase : RoomDatabase() {

   abstract fun itemDao(): ItemDao

   companion object {
       @Volatile
       private var INSTANCE: ItemRoomDatabase? = null
       fun getDatabase(context: Context): ItemRoomDatabase {
           return INSTANCE ?: synchronized(this) {
               val instance = Room.databaseBuilder(
                   context.applicationContext,
                   ItemRoomDatabase::class.java,
                   "item_database"
               )
                   .fallbackToDestructiveMigration()
                   .build()
               INSTANCE = instance
               return instance
           }
       }
   }
}
  1. Build your code to make sure there are no errors.

Implement Application class

In this task you will instantiate the database instance in the Application class.

  1. Open InventoryApplication.kt, create a val called database of the type ItemRoomDatabase. Instantiate the database instance by calling getDatabase() on ItemRoomDatabase passing in the context. Use lazy delegate so the instance database is lazily created when you first need/access the reference (rather than when the app starts). This will create the database (the physical database on the disk) on the first access.
import android.app.Application
import com.example.inventory.data.ItemRoomDatabase

class InventoryApplication : Application(){
   val database: ItemRoomDatabase by lazy { ItemRoomDatabase.getDatabase(this) }
}

You will use this database instance later in the codelab when creating a ViewModel instance.

You now have all the building blocks for working with your Room. This code compiles and runs, but you have no way of telling if it actually works. So, this is a good time to add a new item to your Inventory database to test your database. To accomplish this, you need a ViewModel to talk to the database.

You have thus far created a database and the UI classes were part of the starter code. To save the app's transient data and to also access the database, you need a ViewModel. Your Inventory ViewModel will interact with the database via the DAO, and provide data to the UI. All database operations will have to be run away from the main UI thread, you'll do that using coroutines and viewModelScope.

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Create Inventory ViewModel

  1. In the com.example.inventory package, create a Kotlin class file InventoryViewModel.kt.
  2. Extend the InventoryViewModel class from the ViewModel class. Pass in the ItemDao object as a parameter to the default constructor.
class InventoryViewModel(private val itemDao: ItemDao) : ViewModel() {}
  1. At the end of the InventoryViewModel.kt file outside the class, add InventoryViewModelFactory class to instantiate the InventoryViewModel instance. Pass in the same constructor parameter as the InventoryViewModel that is the ItemDao instance. Extend the class from the ViewModelProvider.Factory class. You will fix the error regarding the unimplemented methods in the next step.
class InventoryViewModelFactory(private val itemDao: ItemDao) : ViewModelProvider.Factory {
}
  1. Click on the red bulb and select Implement Members, or you can override the create() method inside the ViewModelProvider.Factory class as follows, which takes any class type as an argument and returns a ViewModel object.
override fun <T : ViewModel?> create(modelClass: Class<T>): T {
   TODO("Not yet implemented")
}
  1. Implement the create() method. Check if the modelClass is the same as the InventoryViewModel class and return an instance of it. Otherwise, throw an exception.
if (modelClass.isAssignableFrom(InventoryViewModel::class.java)) {
   @Suppress("UNCHECKED_CAST")
   return InventoryViewModel(itemDao) as T
}
throw IllegalArgumentException("Unknown ViewModel class")

Populate the ViewModel

In this task, you will populate the InventoryViewModel class to add inventory data to the database. Observe the Item entity and Add Item screen in the Inventory app.

@Entity
data class Item(
   @PrimaryKey(autoGenerate = true)
   val id: Int = 0,
   @ColumnInfo(name = "name")
   val itemName: String,
   @ColumnInfo(name = "price")
   val itemPrice: Double,
   @ColumnInfo(name = "quantity")
   val quantityInStock: Int
)

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You need the name, price, and stock in hand for that particular item in order to add an entity to the database. Later in the codelab, you will use the Add Item screen to get these details from the user. In the current task, you use three strings as input to the ViewModel, convert them to an Item entity instance, and save it to the database using the ItemDao instance. It's time to implement.

  1. In the InventoryViewModel class, add a private function called insertItem() that takes in an Item object and adds the data to the database in a non-blocking way.
private fun insertItem(item: Item) {
}
  1. To interact with the database off the main thread, start a coroutine and call the DAO method within it. Inside the insertItem() method, use the viewModelScope.launch to start a coroutine in the ViewModelScope. Inside the launch function, call the suspend function insert() on itemDao passing in the item. The ViewModelScope is an extension property to the ViewModel class that automatically cancels its child coroutines when the ViewModel is destroyed.
private fun insertItem(item: Item) {
   viewModelScope.launch {
       itemDao.insert(item)
   }
}

Import kotlinx.coroutines.launch, androidx.lifecycle.viewModelScope

com.example.inventory.data.Item, if not automatically imported.

  1. In the InventoryViewModel class, add another private function that takes in three strings and returns an Item instance.
private fun getNewItemEntry(itemName: String, itemPrice: String, itemCount: String): Item {
   return Item(
       itemName = itemName,
       itemPrice = itemPrice.toDouble(),
       quantityInStock = itemCount.toInt()
   )
}
  1. Still inside the InventoryViewModel class, add a public function called addNewItem() that takes in three strings for item details. Pass in item detail strings to getNewItemEntry() function and assign the returned value to a val named newItem. Make a call to insertItem() passing in the insertItem to add the new entity to the database. This will be called from the UI fragment to add Item details to the database.
fun addNewItem(itemName: String, itemPrice: String, itemCount: String) {
   val newItem = getNewItemEntry(itemName, itemPrice, itemCount)
   insertItem(newItem)
}

Notice that you did not use viewModelScope.launch for addNewItem(), but it is needed above in insertItem() when you call a DAO method. The reason is that the suspend functions are only allowed to be called from a coroutine or another suspend function. The function itemDao.insert(item)is a suspend function.

You have added all the required functions to add entities to the database. In the next task you will update the Add Item fragment to use the above functions.

  1. In AddItemFragment.kt, at the beginning of the AddItemFragment class create a private val called viewModel of the type InventoryViewModel. Use the by activityViewModels() Kotlin property delegate to share the ViewModel across fragments. You will fix the error in the next step.
private val viewModel: InventoryViewModel by activityViewModels {
}
  1. Inside the lambda, call the InventoryViewModelFactory() constructor and pass in the ItemDao instance. Use the database instance you created in one of the previous tasks to call the itemDao constructor.
private val viewModel: InventoryViewModel by activityViewModels {
   InventoryViewModelFactory(
       (activity?.application as InventoryApplication).database
           .itemDao()
   )
}
  1. Below the viewModel definition, create a lateinit var called item of the type Item.
 lateinit var item: Item
  1. The Add Item screen contains three text fields to get the item details from the user. In this step, you will add a function to verify if the text in the TextFields are not empty. You will use this function to verify user input before adding or updating the entity in the database. This validation needs to be done in the ViewModel and not in the Fragment. In the InventoryViewModel class, add the following public function called isEntryValid().
fun isEntryValid(itemName: String, itemPrice: String, itemCount: String): Boolean {
   if (itemName.isBlank() || itemPrice.isBlank() || itemCount.isBlank()) {
       return false
   }
   return true
}
  1. In AddItemFragment.kt, below the onCreateView() function create a private function called isEntryValid() that returns a Boolean. You will fix the missing return value error in the next step.
private fun isEntryValid(): Boolean {
}
  1. In the AddItemFragment class, implement the isEntryValid() function. Call the isEntryValid() function on the viewModel instance, passing in the text from the text views. Return the value of the viewModel.isEntryValid() function.
private fun isEntryValid(): Boolean {
   return viewModel.isEntryValid(
       binding.itemName.text.toString(),
       binding.itemPrice.text.toString(),
       binding.itemCount.text.toString()
   )
}
  1. In the AddItemFragment class below the isEntryValid() function, add another private function called addNewItem() with no parameters and return nothing. Inside the function, call isEntryValid() inside the if condition.
private fun addNewItem() {
   if (isEntryValid()) {
   }
}
  1. Inside the if block, call the addNewItem()method on the viewModel instance. Pass in the item details entered by the user, use the binding instance to read them.
if (isEntryValid()) {
   viewModel.addNewItem(
   binding.itemName.text.toString(),
   binding.itemPrice.text.toString(),
   binding.itemCount.text.toString(),
   )
}
  1. Below the if block, create a val action to navigate back to the ItemListFragment. Call findNavController().navigate(), passing in the action.
val action = AddItemFragmentDirections.actionAddItemFragmentToItemListFragment()
findNavController().navigate(action)

Import androidx.navigation.fragment.findNavController.

  1. The complete method should look like the following.
private fun addNewItem() {
       if (isEntryValid()) {
           viewModel.addNewItem(
               binding.itemName.text.toString(),
               binding.itemPrice.text.toString(),
               binding.itemCount.text.toString(),
           )
           val action = AddItemFragmentDirections.actionAddItemFragmentToItemListFragment()
           findNavController().navigate(action)
       }
   }
}
  1. To tie everything together, add a click handler to the Save button. In the AddItemFragment class, above the onDestroyView() function, override the onViewCreated() function.
  2. Inside the onViewCreated() function, add a click handler to the save button, and call addNewItem()from it.
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
   super.onViewCreated(view, savedInstanceState)
   binding.saveAction.setOnClickListener {
       addNewItem()
   }
}
  1. Build and run your app. Tap the + Fab. In the Add Item screen, add the item details and tap Save. This action saves the data, but you cannot see anything yet in the app. In the next task, you will use the Database Inspector to view the data you saved.

193c7fa9c41e0819.png

View the database using Database Inspector

  1. Run your app on an emulator or connected device running API level 26 or higher, if you have not done so already. Database Inspector works best on emulator/devices running API level 26.
  2. In Android studio, select View > Tool Windows > Database Inspector from the menu bar.
  3. In the Database Inspector pane, select the com.example.inventory from the dropdown menu.
  4. The item_database in the Inventory app appears in the Databases pane. Expand the node for the item_database and select Item to inspect. If your Databases pane is empty, use your emulator to add some items to the database using the Add Item screen.
  5. Check the Live updates checkbox in the Database Inspector to automatically update the data it presents as you interact with your running app in the emulator or device.

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Congratulations! You have created an app that can persist the data using Room. In the next codelab, you will add a RecyclerView to your app to display the items on the database and add new features to the app like deleting and updating the entities. See you there!

The solution code for this codelab is in the GitHub repo and branch shown below.

To get the code for this codelab and open it in Android Studio, do the following.

Get the code

  1. Click on the provided URL. This opens the GitHub page for the project in a browser.
  2. On the GitHub page for the project, click the Code button, which brings up a dialog.

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  1. In the dialog, click the Download ZIP button to save the project to your computer. Wait for the download to complete.
  2. Locate the file on your computer (likely in the Downloads folder).
  3. Double-click the ZIP file to unpack it. This creates a new folder that contains the project files.

Open the project in Android Studio

  1. Start Android Studio.
  2. In the Welcome to Android Studio window, click Open an existing Android Studio project.

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Note: If Android Studio is already open, instead, select the File > New > Import Project menu option.

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  1. In the Import Project dialog, navigate to where the unzipped project folder is located (likely in your Downloads folder).
  2. Double-click on that project folder.
  3. Wait for Android Studio to open the project.
  4. Click the Run button 11c34fc5e516fb1c.png to build and run the app. Make sure it works as expected.
  5. Browse the project files in the Project tool window to see how the app was implemented.
  • Define your tables as data classes annotated with @Entity. Define properties annotated with @ColumnInfo as columns in the tables.
  • Define a data access object (DAO) as an interface annotated with @Dao. The DAO maps Kotlin functions to database queries.
  • Use annotations to define @Insert, @Delete, and @Update functions.
  • Use the @Query annotation with an SQLite query string as a parameter for any other queries.
  • Use Database Inspector to view the data saved in the Android SQLite database.

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