Quick Start¶
Overview¶
This page contains information to quickly get Realm Database integrated into your app. Before you begin, ensure you have:
This page contains only the essential information that you need to set up a MongoDB Realm application. If you prefer to follow a guided tutorial that shows you step-by-step how to set up a working app, check out the Android Tutorial where you'll build a mobile app that connects to the Task Tracker backend.
Initialize Realm¶
Before you can use Realm in your app, you must initialize the Realm library. Your application should initialize Realm just once each time the application runs.
To initialize the Realm library, provide an Android
context
to the Realm.init()
static function. You can provide
an Activity, Fragment, or Application context
for initialization with no
difference in behavior. You can initialize the Realm library
in the onCreate()
method of an application subclass to
ensure that you only initialize Realm once each time the
application runs.
Realm.init(this) // context, usually an Activity or Application
If you create your own Application
subclass, you must add it to your
application's AndroidManifest.xml
to execute your custom
application code. Set the android.name
property of your manifest's
application definition to ensure that Android instantiates your Application
subclass before any other class when a user launches your application.
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.mongodb.example"> <application
android:name=".MyApplicationSubclass"
...
/> </manifest>
Initialize the App¶
To use MongoDB Realm features such as authentication and sync, you must access your Realm app using your Realm app ID. You can find your Realm app ID in the Realm UI.
val appID : String = YOUR_APP_ID; app = App(AppConfiguration.Builder(appID) .build())
If Android Studio does not recognize the Realm
, App
, or
AppConfiguration
types, there could be a problem with the
your Gradle build configuration. To fix the issue:
- Clean your project with
Build > Clean Project
- Rebuild your project based on your updated
build.gradle
file withBuild > Rebuild Project
- Revisit the Install the Android SDK guide to make sure that you installed the dependencies correctly.
Define Your Object Model¶
Your application's data model defines the structure of data stored within Realm Database and synchronized to and from MongoDB Realm. You can define your application's data model in two ways:
- Via JSON document schemas in MongoDB Realm with Realm Schema.
- Via Kotlin or Java classes in your application code with Realm Object Models.
This quick start uses the latter approach, which defines your schema using classes in your mobile application code. To define your Realm app's object model in this way, you need to enable Developer Mode.
Once you've enabled Developer Mode, add the following class definitions to your application code:
See Configure Your Data Model for more information on Realm Schema, object models, and Developer Mode.
enum class TaskStatus(val displayName: String) { Open("Open"), InProgress("In Progress"), Complete("Complete"), } open class Task(_name: String = "Task", project: String = "My Project") : RealmObject() { var _id: ObjectId = ObjectId() var name: String = _name var status: String = TaskStatus.Open.name var statusEnum: TaskStatus get() { // because status is actually a String and another client could assign an invalid value, // default the status to "Open" if the status is unreadable return try { TaskStatus.valueOf(status) } catch (e: IllegalArgumentException) { TaskStatus.Open } } set(value) { status = value.name } }
Authenticate a User¶
When you have enabled anonymous authentication in the Realm UI, users can immediately log into your app without providing any identifying information:
val credentials: Credentials = Credentials.anonymous() app.loginAsync(credentials) { if (it.isSuccess) { Log.v("QUICKSTART", "Successfully authenticated anonymously.") val user: User? = app.currentUser() // interact with realm using your user object here } else { Log.e("QUICKSTART", "Failed to log in. Error: ${it.error}") } }
Realm provides many additional ways to authenticate, register, and link users.
Open a Realm¶
Once you have enabled Realm Sync and authenticated a
user, you can open a synced realm. Use the
SyncConfiguration
to control the specifics of how your application
synchronizes data with MongoDB Realm, including the partition, how long to wait before a request times out, whether
to allow synchronous reads or writes to a realm on the UI thread,
and more.
val partitionValue: String = "My Project" val config = SyncConfiguration.Builder(user, partitionValue) .build() val backgroundThreadRealm : Realm = Realm.getInstance(config)
Create, Read, Update, and Delete Objects¶
Once you have opened a realm, you can modify the objects within that realm in a write transaction block.
By default, you can only read or write to a realm in your
application's UI thread using asynchronous transactions. That is,
you can only use Realm
methods whose name ends with the word
Async
in the main thread of your Android application unless you
explicitly allow the use of synchronous methods.
This restriction exists for the benefit of your application users:
performing read and write operations on the UI thread can lead to
unresponsive or slow UI interactions, so it's usually best to handle
these operations either asynchronously or in a background thread.
However, if your application requires the use of synchronous
realm reads or writes on the UI thread, you can explicitly allow
the use of synchronous methods with the following
SyncConfiguration
options:
val config = SyncConfiguration.Builder(app.currentUser(), PARTITION) .allowQueriesOnUiThread(true) .allowWritesOnUiThread(true) .build() Realm.getInstanceAsync(config, object : Realm.Callback() { override fun onSuccess(realm: Realm) { Log.v( "EXAMPLE", "Successfully opened a realm with reads and writes allowed on the UI thread." ) } })
To create a new Task
, instantiate an instance of the
Task
class and add it to the realm in a write block:
val task : Task = Task("New Task", partitionValue) backgroundThreadRealm.executeTransaction { transactionRealm -> transactionRealm.insert(task) }
You can retrieve a live collection of all items in the realm:
// all tasks in the realm val tasks : RealmResults<Task> = backgroundThreadRealm.where<Task>().findAll()
You can also filter that collection using a filter:
// you can also filter a collection val tasksThatBeginWithN : List<Task> = tasks.where().beginsWith("name", "N").findAll() val openTasks : List<Task> = tasks.where().equalTo("status", TaskStatus.Open.name).findAll()
To modify a task, update its properties in a write transaction block:
val otherTask: Task = tasks[0]!! // all modifications to a realm must happen inside of a write block backgroundThreadRealm.executeTransaction { transactionRealm -> val innerOtherTask : Task = transactionRealm.where<Task>().equalTo("_id", otherTask._id).findFirst()!! innerOtherTask.status = TaskStatus.Complete.name }
Finally, you can delete a task by calling the deleteFromRealm()
method in a write transaction block:
val yetAnotherTask: Task = tasks.get(0)!! val yetAnotherTaskId: ObjectId = yetAnotherTask._id // all modifications to a realm must happen inside of a write block backgroundThreadRealm.executeTransaction { transactionRealm -> val innerYetAnotherTask : Task = transactionRealm.where<Task>().equalTo("_id", yetAnotherTaskId).findFirst()!! innerYetAnotherTask.deleteFromRealm() }
Watch for Changes¶
You can watch a realm, collection, or object for changes by attaching a custom
OrderedRealmCollectionChangeListener
with the addChangeListener()
method:
// all tasks in the realm val tasks : RealmResults<Task> = realm.where<Task>().findAllAsync() tasks.addChangeListener(OrderedRealmCollectionChangeListener<RealmResults<Task>> { collection, changeSet -> // process deletions in reverse order if maintaining parallel data structures so indices don't change as you iterate val deletions = changeSet.deletionRanges for (i in deletions.indices.reversed()) { val range = deletions[i] Log.v("QUICKSTART", "Deleted range: ${range.startIndex} to ${range.startIndex + range.length - 1}") } val insertions = changeSet.insertionRanges for (range in insertions) { Log.v("QUICKSTART", "Inserted range: ${range.startIndex} to ${range.startIndex + range.length - 1}") } val modifications = changeSet.changeRanges for (range in modifications) { Log.v("QUICKSTART", "Updated range: ${range.startIndex} to ${range.startIndex + range.length - 1}") } })
Log Out¶
Once logged in, you can log out:
app.currentUser()?.logOutAsync() { if (it.isSuccess) { Log.v("QUICKSTART", "Successfully logged out.") } else { Log.e("QUICKSTART", "Failed to log out, error: ${it.error}") } }
Complete Example¶
Run the complete example by replacing the appId with your realm app ID.
If you're running this project in a fresh Android Studio project, you can
copy and paste this file into your application's MainActivity
-- just
remember to:
- change the package declaration so it matches your project
- replace the App ID placeholder with your Realm app's App ID
- update the
import
statements forTask
andTaskStatus
if you're using java
package com.mongodb.realm.examples.kotlin import org.bson.types.ObjectId import android.os.Bundle import androidx.appcompat.app.AppCompatActivity import android.util.Log import io.realm.OrderedRealmCollectionChangeListener import io.realm.Realm import io.realm.RealmObject import io.realm.RealmResults import io.realm.annotations.PrimaryKey import io.realm.annotations.Required import io.realm.kotlin.where import io.realm.mongodb.App import io.realm.mongodb.AppConfiguration import io.realm.mongodb.Credentials import io.realm.mongodb.User import io.realm.mongodb.sync.SyncConfiguration import java.util.concurrent.ExecutorService import java.util.concurrent.Executors import java.util.concurrent.FutureTask class MainActivity : AppCompatActivity() { lateinit var uiThreadRealm: Realm lateinit var app: App override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) Realm.init(this) // context, usually an Activity or Application val appID : String = YOUR_APP_ID; app = App(AppConfiguration.Builder(appID) .build()) val credentials: Credentials = Credentials.anonymous() app.loginAsync(credentials) { if (it.isSuccess) { Log.v("QUICKSTART", "Successfully authenticated anonymously.") val user: User? = app.currentUser() val partitionValue: String = "My Project" val config = SyncConfiguration.Builder(user, partitionValue) .build() uiThreadRealm = Realm.getInstance(config) addChangeListenerToRealm(uiThreadRealm) val task : FutureTask<String> = FutureTask(BackgroundQuickStart(app.currentUser()!!), "test") val executorService: ExecutorService = Executors.newFixedThreadPool(2) executorService.execute(task) } else { Log.e("QUICKSTART", "Failed to log in. Error: ${it.error}") } } } fun addChangeListenerToRealm(realm : Realm) { // all tasks in the realm val tasks : RealmResults<Task> = realm.where<Task>().findAllAsync() tasks.addChangeListener(OrderedRealmCollectionChangeListener<RealmResults<Task>> { collection, changeSet -> // process deletions in reverse order if maintaining parallel data structures so indices don't change as you iterate val deletions = changeSet.deletionRanges for (i in deletions.indices.reversed()) { val range = deletions[i] Log.v("QUICKSTART", "Deleted range: ${range.startIndex} to ${range.startIndex + range.length - 1}") } val insertions = changeSet.insertionRanges for (range in insertions) { Log.v("QUICKSTART", "Inserted range: ${range.startIndex} to ${range.startIndex + range.length - 1}") } val modifications = changeSet.changeRanges for (range in modifications) { Log.v("QUICKSTART", "Updated range: ${range.startIndex} to ${range.startIndex + range.length - 1}") } }) } override fun onDestroy() { super.onDestroy() // the ui thread realm uses asynchronous transactions, so we can only safely close the realm // when the activity ends and we can safely assume that those transactions have completed uiThreadRealm.close() app.currentUser()?.logOutAsync() { if (it.isSuccess) { Log.v("QUICKSTART", "Successfully logged out.") } else { Log.e("QUICKSTART", "Failed to log out, error: ${it.error}") } } } class BackgroundQuickStart(val user: User) : Runnable { override fun run() { val partitionValue: String = "My Project" val config = SyncConfiguration.Builder(user, partitionValue) .build() val backgroundThreadRealm : Realm = Realm.getInstance(config) val task : Task = Task("New Task", partitionValue) backgroundThreadRealm.executeTransaction { transactionRealm -> transactionRealm.insert(task) } // all tasks in the realm val tasks : RealmResults<Task> = backgroundThreadRealm.where<Task>().findAll() // you can also filter a collection val tasksThatBeginWithN : List<Task> = tasks.where().beginsWith("name", "N").findAll() val openTasks : List<Task> = tasks.where().equalTo("status", TaskStatus.Open.name).findAll() val otherTask: Task = tasks[0]!! // all modifications to a realm must happen inside of a write block backgroundThreadRealm.executeTransaction { transactionRealm -> val innerOtherTask : Task = transactionRealm.where<Task>().equalTo("_id", otherTask._id).findFirst()!! innerOtherTask.status = TaskStatus.Complete.name } val yetAnotherTask: Task = tasks.get(0)!! val yetAnotherTaskId: ObjectId = yetAnotherTask._id // all modifications to a realm must happen inside of a write block backgroundThreadRealm.executeTransaction { transactionRealm -> val innerYetAnotherTask : Task = transactionRealm.where<Task>().equalTo("_id", yetAnotherTaskId).findFirst()!! innerYetAnotherTask.deleteFromRealm() } // because this background thread uses synchronous realm transactions, at this point all // transactions have completed and we can safely close the realm backgroundThreadRealm.close() } } } enum class TaskStatus(val displayName: String) { Open("Open"), InProgress("In Progress"), Complete("Complete"), } open class Task(_name: String = "Task", project: String = "My Project") : RealmObject() { var _id: ObjectId = ObjectId() var name: String = _name var status: String = TaskStatus.Open.name var statusEnum: TaskStatus get() { // because status is actually a String and another client could assign an invalid value, // default the status to "Open" if the status is unreadable return try { TaskStatus.valueOf(status) } catch (e: IllegalArgumentException) { TaskStatus.Open } } set(value) { status = value.name } }
Output¶
Running the above code should produce output resembling the following:
Successfully authenticated anonymously. Updated range: 0 to 1 Deleted range: 0 to 1 Successfully logged out.