Add "Observer" pattern. Official example from java.

[ilopX]

Observer pattern

Observer is a behavioral design pattern that lets you define a subscription mechanism to notify
multiple objects about any events that happen to the object they’re observing.

Editor example

In this example, the Observer pattern lets the text editor object notify other service objects about
changes in its state.

More detailed explanation on RefactoringGuru.

Origin source code:

This example rewrite from java example.

Diagram:

Client code:

void main() {
  final editor = Editor();
  editor.events
    ..subscribe(
      'open',
      LogOpenListener('log.txt'),
    )
    ..subscribe(
      'save',
      EmailNotificationListener('admin@example.com'),
    );

  try {
    editor.openFile('test.txt');
    editor.saveFile();
  } catch (e) {
    print(e);
  }
}

Output:

Save to log "log.txt": Someone has performed "open" operation with the following file: "test.txt"
Email to "admin@example.com": Someone has performed "save" operation with the following file: "test.txt"