Add conceptual command pattern

[ilopX]

Command Pattern

Command is a behavioral design pattern that turns a request into a stand-alone object that contains
all information about the request.

Tutorial: here.

Diagram:

Client dode:

void main() {
  final mutStr = MutStr();

  final input1 = AddTextCommand('One', mutStr);
  final input2 = AddTextCommand('Three', mutStr);
  final input3 = InsertTextCommand(' Two ', mutStr, pos: 2);

  input1.execute();
  print('text = $mutStr'); // mutStr = "One"

  input2.execute();
  print('text = $mutStr'); // mutStr = "OneThree"

  input3.execute();
  print('text = $mutStr'); // mutStr = "One Two Three"

  input3.undo();
  print('text = $mutStr'); // mutStr = "OneThree"

  input2.undo();
  print('text = $mutStr'); // mutStr = "One "

  input1.undo();
  print('text = $mutStr'); // mutStr = ""
}

Output:

text = One
text = OneThree
text = One Two Three
text = OneThree
text = One
text =