The widgets that we've seen so far are stateless widgets, meaning that once created they are immutable, and they do not keep any state information. When you interact with your users, you expect things to change. For example, if you want to convert a measure from one system to another, the result must change, based on some user input.
The most basic way to deal with changes in Flutter is using State.
State is information that can be used when a widget is built and can change during the lifetime of a widget.
An important part of this definition is that state is information that can change, and the most obvious takeaway of this concept is that when you want to add interactivity to your app, you can use State. But, if you read this definition thoroughly, it also means that it's not the widget itself that will change, it's the...