Special command class which marks an Undoable Period. It doesn't perform a command itself, but marks a start and an end to other commands to be threated as a single undoable/redoable unit. It has special features: redo will re-do all contained commands and will put the CommandManager in a special state which blocks any new commands being queued during redo. Undo will not clear the queues after a queue is completely undone. The advantage of an undoable period is that it doesn't call methods to do things (like a command would) and therefore can be used to mark a method which creates new objects as an undoable period while all actions performed on datastructures inside that method are tracked in the queue of this command. An Undoable Period is to be used with undo/redo areas where new objects are created and stored in datastructures. Re-doing these kind of areas with normal commands will re-create new instances which causes problems with follow up commands if these are re-done too: those commands work on the previous objects perhaps, causing problems along te way. With an undoable period you can prevent this from happening as any method which is called through a command should be replaced with a normal method call and inside the method you should mark the code as an undoable period.

public class UndoablePeriodCommand : Command<Object>

Public Class UndoablePeriodCommand
	Inherits Command(Of Object)