What is BLoC pattern in Flutter?

The pattern is deliberately one-directional. The interface sends events in; the component holds all the logic and emits states out; the interface renders whatever state it is handed and makes no decisions of its own. Because the logic sits in a plain class receiving events and emitting states, you can test it without building a single widget.

The cost is boilerplate. Every interaction needs an event class, a state class and a handler, which is a poor trade on a three-screen app where holding state locally in a widget would do. It starts paying for itself when several screens depend on the same data, when there are separate user roles seeing different views of it, or when someone other than the original author has to change the logic safely.