What is a StreamController, and how does it let you manually create and control your own custom Stream, pushing values into it yourself whenever you choose to?

A StreamController gives you direct manual control over creating and populating your own custom Stream, exposing a sink property that lets you add new values into the underlying stream at any point your own code decides to, and a corresponding stream property that other parts of your code can then separately listen to in order to receive those values, which is especially useful for situations like building your own custom event bus or bridging data coming from a source that does not already naturally expose itself as a proper Dart Stream.

final controller = StreamController<String>();

controller.sink.add('Hello');
controller.stream.listen((value) => print(value));