I think I must be missing something fundamental, or have a
misunderstanding to begin with. Or maybe I'm just doing something
wrong to begin with...
I'm working on an app, implementing Undo features. Undo basics are
simple: you supply the Undo manager an object, a method, and some
arguments that effectively "mirror" the operation just [about to be]
performed: when changing a yellow object blue, you pass the Undo
manager the object, the -setColor: method, and the current color, so
that upon invoking Undo the color is changed back to what it was.
Nice and symmetrical.
However, I have a case where symmetry is not involved. The program
manages "cards"; it uses custom code to manage the backing store file,
which is in a cross-platform format (i.e., it uses open, seek, write,
close et al, instead of, say, writing out dictionaries).
One of the operations I need to support Undo for is Delete Card. When
a card is deleted, the data in the file is deleted, and the GUI
removes an entry from the array controller managing the display. In
order to support Undo, I cache all the pertinent information that the
card would normally hold onto, and upon Undo being invoked, I recreate
a new card from scratch. (I cannot hold onto the card object itself
because it loads its data lazily; data is not loaded from the file
until it actually needs to be displayed on screen. So if I held onto
the object itself, some fields would be nil, and once the data is
deleted from the file, I can no longer get it back in order to restore
it. Not that this is really pertinent to my real question...)
I have Undo working just great: I delete a card, I cache its
attributes with the Undo manager, it gets deleted from the file, it's
removed from the display, I Undo, a new card gets created and
populated with the cached attributes, saved in the file and displayed,
all is well in the world.
The problem is Redo. This is not a symmetric operation, like changing
the color of an object on screen. All I need to do to Redo is call
the method that deletes the card. It seems I need to tell the Undo
manager to invoke a different method, with different arguments, when
Redo is invoked, but I've not found how to do that.
Anyone care to set me straight on this?
Thanks!
randy
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