Add "Memento" pattern, conceptual example from book.

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• Wait for the approval of the pull request Add "Observer" pattern. Official example from java. #19

Memento pattern

Memento is a behavioral design pattern that lets you save and restore the previous state of an
object without revealing the details of its implementation.

Tutorial: here.

Conceptual Editor example

This example uses the Memento pattern alongside the Command pattern for storing snapshots of the
complex text editor’s state and restoring an earlier state from these snapshots when needed.

More detailed explanation on RefactoringGuru.

Diagram:

Client code:

void main() {
  final editor = Editor('New Document');
  final firstState = Command.makeBackup(editor);
  editor.text += ' add text';
  final secondState = Command.makeBackup(editor);

  print('Current state: "${editor.text}"');

  firstState.undo();
  print('First state: "${editor.text}"');

  secondState.undo();
  print('Second state: "${editor.text}"');
}

Output:

Current state: "New Document add text"
First state: "New Document"
Second state: "New Document add text"