I'm trying to write a bitmap editor for a mobile device (i.e. a limited version of Photoshop). The user's document consists of ~4 bitmaps around 1000x500 in size each.
I want a robust and efficient undo/redo system that's as simple as possible. I'm aiming for about ~0.2s to undo or redo an edit. I'm looking for some feedback on my current intended approach or for some new ideas I can use. I think what I have is too complex so I'm cautious about proceeding so just knowing it's about the best I could do would be good.
I've experimented with combinations of using the Command pattern and the Memento pattern for my undo/redo system. Some conclusions I've come to so far are:
I don't have enough memory and I cannot write memory to disk fast enough to use a memento to support an "unexecute" operation on the previous command in many situations e.g. if the user does several individual paint strokes very quickly, I won't be able to store bitmaps that represent what the user painted over without making the user wait for them to be saved.
If I restore the document to its initial state and replay all the commands except for the last one to implement undo, this is far too slow after even a modest number of commands e.g. replaying 10 paint strokes or 5 smudge strokes takes ~1s which is too sluggish.
I can get around the previous point by saving the whole document in the background periodically to disk and restoring to this checkpoint before playing back commands. To undo further back than the last checkpoint, we reload the checkpoint before this and replay the commands.
Approach 2 with 3 works OK except saving the entire document gets slower and slower as more layers are added and it's already slow with 4 bitmaps (~5 - 10 seconds wait). I therefore need to modify 3 so that I only save what has changed since last time.
Since many commands operate on only one layer, it makes sense to only s