Developing in makehuman - 4 - Undo-redo

When we started working on 1.0, one of the first features written was undo-redo. Having this from the start saves us a lot of time later as we add this functionality to each kind of model modification immediately. It is important that every modification is undo-able, since just one undo-able modification would leave the user without the possibility to undo anything. So it's crucial that if you write a plugin which modifies the model, you also make undo work.

The Application class has several methods to work with actions. An action is a class with two methods, do and undo. If the action itself does the modification you can use app.do to add it to the undo stack. If you did the modification yourself already during user interaction, you can add the action using app.did. The application won't call the do method of the action in that case. If you want to make your own undo-redo buttons, you can use app.undo and app.redo.

To illustrate, here is the action we use to change the hair color:

class Action:
  def __init__(self, human, before, after, postAction = None):
    self.name = "Change hair color"
    self.human = human
    self.before = before
    self.after = after
    self.postAction = postAction
    
  def do(self):
    self.human.hairColor = self.after
    if self.postAction:
      self.postAction()
    return True
    
  def undo(self):
    self.human.hairColor = self.before
    if self.postAction:
      self.postAction()
    return True

The postAction is a handy way to specify a method to keep your gui in sync with the changes. In this case we update the color control to show the correct color when the user choses to undo or redo the hair color change.