From the last code you have shown it looks like you don't want the regular undo/redo feature. Here is the regular idea of a QUndoCommand: When the user does an action you create a new command and push it onto the QUndoStack. Normally, the actions are undone through the undo stack. This might be through a shortcut (Ctrl+Z) or the menu. The QUndoCommand has both a redo() and an undo() function. When you push the command on the stack it will execute the redo function. When you undo the stack, you don't create a new command, but the undo stack will just execute the undo function of the same command that is already stored on the stack. And the stack does not drop your command immediately, so you could actually undo the undo operation (i.e. the redo). It allows you to redo the exact same command which is stored on the undo stack.
I am not sure if your double click action would fit right in to the undo/redo functionality. It is probably not what a user expects. The only way this could work out is if you have both an expand undo command and a collapse undo command. So, if you double click to expand you push the expand undo command to the undo stack. The use could now undo to collapse again and after redo to expand again. However, if the user double clicks again you would push a collapse undo command on the undo stack. What that means is that your implementation for redo() in your expand undo command is the same as the undo() in your collapse undo command and vice versa.
Before you write any undo commands you should figure out what to do when double clicking to expand or collapse. If this code works without first creating an undo command you can then put the code for expansion into the redo() function and the code for collapse into the undo() function of your expand undo command (and the other way around for your collapse command -- you might want to have ExpandCollapseUndoCommand which receives an additional boolean (or better an enum) to choose between expansion or collapse). If you can't figure out what to do without the undo command you will not be able to write the undo command.
