Adding a Flutter screen to an Android app

This guide describes how to add a single Flutter screen to an existing Android app. A Flutter screen can be added as a normal, opaque screen, or as a see-through, translucent screen. Both options are described in this guide.

Add a normal Flutter screen

Step 1: Add FlutterActivity to AndroidManifest.xml

Flutter provides FlutterActivity to display a Flutter experience within an Android app. Like any other Activity, FlutterActivity must be registered in your AndroidManifest.xml. Add the following XML to your AndroidManifest.xml file under your application tag:

<activity
  android:name="io.flutter.embedding.android.FlutterActivity"
  android:theme="@style/LaunchTheme"
  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
  android:hardwareAccelerated="true"
  android:windowSoftInputMode="adjustResize"
  />

The reference to @style/LaunchTheme can be replaced by any Android theme that want to apply to your FlutterActivity. The choice of theme dictates the colors applied to Android’s system chrome, like Android’s navigation bar, and to the background color of the FlutterActivity just before the Flutter UI renders itself for the first time.

Step 2: Launch FlutterActivity

With FlutterActivity registered in your manifest file, add code to launch FlutterActivity from whatever point in your app that you’d like. The following example shows FlutterActivity being launched from an OnClickListener.

• Java
• Kotlin

myButton.setOnClickListener(new OnClickListener() {
  @Override
  public void onClick(View v) {
    startActivity(
      FlutterActivity.createDefaultIntent(currentActivity)
    );
  }
});

myButton.setOnClickListener {
  startActivity(
    FlutterActivity.createDefaultIntent(this)
  )
}

The previous example assumes that your Dart entrypoint is called main(), and your initial Flutter route is ‘/’. The Dart entrypoint can’t be changed using Intent, but the initial route can be changed using Intent. The following example demonstrates how to launch a FlutterActivity that initially renders a custom route in Flutter.

• Java
• Kotlin

myButton.addOnClickListener(new OnClickListener() {
  @Override
  public void onClick(View v) {
    startActivity(
      FlutterActivity
        .withNewEngine()
        .initialRoute("/my_route")
        .build(currentActivity)
      );
  }
});

myButton.setOnClickListener {
  startActivity(
    FlutterActivity
      .withNewEngine()
      .initialRoute("/my_route")
      .build(this)
  )
}

Replace "/my_route" with your desired initial route.

The use of the withNewEngine() factory method configures a FlutterActivity that internally create its own FlutterEngine instance. This comes with a non-trivial initialization time. The alternative approach is to instruct FlutterActivity to use a pre-warmed, cached FlutterEngine, which minimizes Flutter’s initialization time. That approach is discussed next.

Step 3: (Optional) Use a cached FlutterEngine

Every FlutterActivity creates its own FlutterEngine by default. Each FlutterEngine has a non-trivial warm-up time. This means that launching a standard FlutterActivity comes with a brief delay before your Flutter experience becomes visible. To minimize this delay, you can warm up a FlutterEngine before arriving at your FlutterActivity, and then you can use your pre-warmed FlutterEngine instead.

To pre-warm a FlutterEngine, find a reasonable location in your app to instantiate a FlutterEngine. The following example arbitrarily pre-warms a FlutterEngine in the Application class:

• Java
• Kotlin

public class MyApplication extends Application {
  public FlutterEngine flutterEngine;
  
  @Override
  public void onCreate() {
    super.onCreate();
    // Instantiate a FlutterEngine.
    flutterEngine = new FlutterEngine(this);

    // Start executing Dart code to pre-warm the FlutterEngine.
    flutterEngine.getDartExecutor().executeDartEntrypoint(
      DartEntrypoint.createDefault()
    );

    // Cache the FlutterEngine to be used by FlutterActivity.
    FlutterEngineCache
      .getInstance()
      .put("my_engine_id", flutterEngine);
  }
}

class MyApplication : Application() {
  lateinit var flutterEngine : FlutterEngine

  override fun onCreate() {
    super.onCreate()

    // Instantiate a FlutterEngine.
    flutterEngine = FlutterEngine(this)

    // Start executing Dart code to pre-warm the FlutterEngine.
    flutterEngine.dartExecutor.executeDartEntrypoint(
      DartExecutor.DartEntrypoint.createDefault()
    )

    // Cache the FlutterEngine to be used by FlutterActivity.
    FlutterEngineCache
      .getInstance()
      .put("my_engine_id", flutterEngine)
  }
}

The ID passed to the FlutterEngineCache can be whatever you want. Make sure that you pass the same ID to any FlutterActivity or FlutterFragment that should use the cached FlutterEngine. Using FlutterActivity with a cached FlutterEngine is discussed next.

With a pre-warmed, cached FlutterEngine, you now need to instruct your FlutterActivity to use the cached FlutterEngine instead of creating a new one. To accomplish this, use FlutterActivity’s withCachedEngine() builder:

• Java
• Kotlin

myButton.addOnClickListener(new OnClickListener() {
  @Override
  public void onClick(View v) {
    startActivity(
      FlutterActivity
        .withCachedEngine("my_engine_id")
        .build(currentActivity)
      );
  }
});

myButton.setOnClickListener {
  startActivity(
    FlutterActivity
      .withCachedEngine("my_engine_id")
      .build(this)
  )
}

When using the withCachedEngine() factory method, pass the same ID that you used when caching the desired FlutterEngine.

Now, when you launch FlutterActivity, there is significantly less delay in the display of Flutter content.

Initial route with a cached engine

The concept of an initial route is available when configuring a FlutterActivity or a FlutterFragment with a new FlutterEngine. However, FlutterActivity and FlutterFragment don’t offer the concept of an initial route when using a cached engine. This is because a cached engine is expected to already be running Dart code, which means it’s too late to configure the initial route.

Developers that would like their cached engine to begin with a custom initial route can configure their cached FlutterEngine to use a custom initial route just before executing the Dart entrypoint. The following example demonstrates the use of an initial route with a cached engine:

• Java
• Kotlin

public class MyApplication extends Application {
  @Override
  public void onCreate() {
    super.onCreate();
    // Instantiate a FlutterEngine.
    flutterEngine = new FlutterEngine(this);
    // Configure an initial route.
    flutterEngine.getNavigationChannel().setInitialRoute("your/route/here");
    // Start executing Dart code to pre-warm the FlutterEngine.
    flutterEngine.getDartExecutor().executeDartEntrypoint(
      DartEntrypoint.createDefault()
    );
    // Cache the FlutterEngine to be used by FlutterActivity or FlutterFragment.
    FlutterEngineCache
      .getInstance()
      .put("my_engine_id", flutterEngine);
  }
}

class MyApplication : Application() {
  lateinit var flutterEngine : FlutterEngine
  override fun onCreate() {
    super.onCreate()
    // Instantiate a FlutterEngine.
    flutterEngine = FlutterEngine(this)
    // Configure an initial route.
    flutterEngine.navigationChannel.setInitialRoute("your/route/here");
    // Start executing Dart code to pre-warm the FlutterEngine.
    flutterEngine.dartExecutor.executeDartEntrypoint(
      DartExecutor.DartEntrypoint.createDefault()
    )
    // Cache the FlutterEngine to be used by FlutterActivity or FlutterFragment.
    FlutterEngineCache
      .getInstance()
      .put("my_engine_id", flutterEngine)
  }
}

By setting the initial route of the navigation channel, the associated FlutterEngine displays the desired route upon initial execution of the runApp() Dart function.

Changing the initial route property of the navigation channel after the initial execution of runApp() has no effect. Developers who would like to use the same FlutterEngine between different Activitys and Fragments and switch the route between those displays need to setup a method channel and explicitly instruct their Dart code to change Navigator routes.

Add a translucent Flutter screen

Most full-screen Flutter experiences are opaque. However, some apps would like to deploy a Flutter screen that looks like a modal, for example, a dialog or bottom sheet. Flutter supports translucent FlutterActivitys out of the box.

To make your FlutterActivity translucent, make the following changes to the regular process of creating and launching a FlutterActivity.

Step 1: Use a theme with translucency

Android requires a special theme property for Activitys that render with a translucent background. Create or update an Android theme with the following property:

<style name="MyTheme" parent="@style/MyParentTheme">
  <item name="android:windowIsTranslucent">true</item>
</style>

Then, apply the translucent theme to your FlutterActivity.

<activity
  android:name="io.flutter.embedding.android.FlutterActivity"
  android:theme="@style/MyTheme"
  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
  android:hardwareAccelerated="true"
  android:windowSoftInputMode="adjustResize"
  />

Your FlutterActivity now supports translucency. Next, you need to launch your FlutterActivity with explicit transparency support.

Step 2: Start FlutterActivity with transparency

To launch your FlutterActivity with a transparent background, pass the appropriate BackgroundMode to the IntentBuilder:

• Java
• Kotlin

// Using a new FlutterEngine.
startActivity(
  FlutterActivity
    .withNewEngine()
    .backgroundMode(FlutterActivityLaunchConfigs.BackgroundMode.transparent)
    .build(context)
);

// Using a cached FlutterEngine.
startActivity(
  FlutterActivity
    .withCachedEngine("my_engine_id")
    .backgroundMode(FlutterActivityLaunchConfigs.BackgroundMode.transparent)
    .build(context)
);

// Using a new FlutterEngine.
startActivity(
  FlutterActivity
    .withNewEngine()
    .backgroundMode(FlutterActivityLaunchConfigs.BackgroundMode.transparent)
    .build(this)
);

// Using a cached FlutterEngine.
startActivity(
  FlutterActivity
    .withCachedEngine("my_engine_id")
    .backgroundMode(FlutterActivityLaunchConfigs.BackgroundMode.transparent)
    .build(this)
);

You now have a FlutterActivity with a transparent background.