It’s sometimes not practical to rewrite your entire application in Flutter all at once. For those situations, Flutter can be integrated into your existing application piecemeal, as a library or module. That module can then be imported into your Android app to render a part of your app’s UI in Flutter.
Get Started
Add a single Flutter screen

First, we need to add FlutterActivity to AndroidManifest.xml.
Next, we use a cached FlutterEngine to launch FlutterActivity.
Add a Flutter Fragment

We just add a FlutterFragment to an Activity with a cached FlutterEngine.
Add a Flutter View
FlutterView can be added to any Android native view.
First, we need to create a cached FlutterEngine.
Next, when the FlutterView is added to a resumed Activity’s view hierarchy and is visible, call attachToFlutterEngine.

Thanks for reading this and I hope this helped you add Flutter to your existing Android App project. You can find the code for this tutorial on my GitHub.