_initial-route-cached-engine.md

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:

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)
  }
}

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);
  }
}

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 set up a method channel and explicitly instruct their Dart code to change Navigator routes.