What is the equivalent of Nuget? How do I add dependencies?

In the .NET ecosystem, native Xamarin projects and Xamarin.Forms projects
had access to Nuget and the built-in package management system.
Flutter apps contain a native Android app, native iOS app and Flutter app.

In Android, you add dependencies by adding to your Gradle build script.
In iOS, you add dependencies by adding to your Podfile.

Flutter uses Dart’s own build system, and the Pub package manager.
The tools delegate the building of the native Android and iOS wrapper apps
to the respective build systems.

In general, use pubspec.yaml to declare
external dependencies to use in Flutter.
A good place to find Flutter packages is on pub.dev.

Application lifecycle

How do I listen to application lifecycle events?

The observable lifecycle events are:

The application is in an inactive state and is not receiving user input.
This event is iOS only.