Friday, April 24, 2009

Okay, take a deep breath...

I'll try and keep this short and sweet. Long story short, I'm still kindof obsessing over how to implement undo/redo in PiTiVi. The reason for this is pretty simple. Now that we have file support again, it's the one thing that keeps pitivi from feeling like a real application.

I decided to do a tiny bit of research on the subject (in other words, type things into google), and the main thing I learned is that there's no single answer to the problem, and every application ends up solving this in their own way. There's basically three main approaches to doing undo:
  • Brute Force (push a copy fo the entire document every time user makes an action)
  • State Capture (keep track of just the changes to the model at a low-to-intermediate level)
  • Command Pattern (basically what I presented in my last post, keep track of high-level user actions)
Then there's some largely-irrelevant, "magical" approaches, such as using low-level operating system features to directly and transparently keep track of when memory is read from / written to which won't really work for us because we're using Python and not C++, and because we actually want our code to work on more than one operating system.

In almost all cases, Undo support is very tightly coupled to the model side of the application, which is interesting because I had originally thought of it as more of a User Interface type of feature.

Brute force is pretty self explanatory. After every user action, a deep copy of the entire document is pushed onto a stack. Undo is basically just replacing the current document with one of these copies.

State capture is conceptually similar to the brute force approach, but more memory efficient in that only changes to a document are recorded. Depending on the architecture, it may be more or less work to implement a state-capture scheme. Inkscape, for example, has a dual-layer model. The top layer is domain-specific stuff, what they term the "svg" layer. The lower layer is a type-agnostic tree, what they term the XML layer. At the lower level, everything is just elements and attributes. Whenever this lower layer changes, the upper layer, and in turn the UI react (though not always the other way around). They use this lower layer to support undo/redo, and file input/output. Neat, but unfortunately PiTiVi isn't organized in quite this way. The nice thing about their approach is that it's clean: they can add, remove or change SVG-layer classes without affecting either file or undo functionality.

Most applications seem to orgnize themselves around the command pattern. This means that you essentially create an input language of "commands" which can be applied to "documents". Every user action is a command of some sort, including direct manipulation. The nice thing about this approach is that you can easily wrap it around an existing code base. Undo and Redo can then implemented by managing the history of commands applied to a document. With a little extra work, you can even allow for things like selective Undo/Redo, and use the same interface to support scripting. The down-side is that you have to manually specify every action and its inverse, and make sure that they work properly. This can, over time, become quite an onerous task. It's also not yet entirely clear how the command pattern fits with direct-manipulation (drag-and-drop type actions).

So, basically, I still don't know how I'm going to proceed, but at least I have some idea of what approaches have been tried in the past, and their associated trade-offs. I would favor some kind of state-capture-based approach, but this might require some labor-intensive refactoring the core classes. I think over the long run, the maintenance concerns associated with the command pattern would make this near-term refactoring more than worth it, and their may even be some cool python tricks (like using a meta-class) that would even minimize this maintenance effort. I guess the moral of the story is that this isn't strictly a UI problem, and so it's not something I'll be working on entirely on my own.

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