Undo Redo in WPF/C# in an already functional application

Question

I have done some research already as to how I can achieve the title of this question. The app I am working on has been under development for a couple of years or so (slow progress though, you all know how it is in the real world). It is now a requirement for me to put in Undo/Redo multiple level functionality. It's a bit late to say "you should have thought about this before you started" ... well, we did think about it - and we did nothing about it and now here it is. From searching around SO (and external links) I can see that the two most common methods appear to be ...

Command Pattern

Memento Pattern

The command pattern looks like it would be a hell of a lot of work, I can only imagine it throwing up thousands of bugs in the process too so I don't really fancy that one.

The Memento pattern is actually a lot like what I had in my head for this. I was thinking if there was some way to quickly take a snapshot of the object model currently in memory, then I would be able to store it somewhere (maybe also in memory, maybe in a file). It seems like a great idea, the only problem I can see for this, is how it will integrate with what we have already written. You see the app as we have it draws images in a big panel (potentially hundreds) and then allows the user to manipulate them either via the UI or via a custom built properties grid. The entire app is linked up with a big observer pattern. The second anything changes, events are fired and everything that needs to update does. This is nice but I cant help thinking that if a user is entering text into a texfield on the properties grid there will be a bit of delay before the UI catches up (seems as everytime the user presses a key, a new snapshot will be added to the undo list). So my question to you is ....

• Do you know of any good alternatives to the Memento pattern that might work.
• Do you think the Memento pattern will fit in here or will it slow the app down too much.
• If the Memento pattern is the way to go, what is the most efficient way to make a snapshot of the object model (i was thinking serialising it or something)
• Should the snapshots be stored in memory or is it possible to put them into files?

If you have got this far, thankyou kindly for reading. Any input you have will be valuable and very much appreciated.

Answer 1

The critical things for undo/redo are

• knowing what state you need to save and restore
• knowing when you need to save the state

Adding undo/redo after the fact is always a painful thing to do - (I know this comment is of no use to you now, but it's always best to design support into the application framework before you start, as it helps people use undo-friendly patterns throughout development).

Possibly the simplest approach will be a memento-based one:

• Locate all the data that makes up your "document". Can you unify this data in some way so that it forms a coherent whole? Usually if you can serialise your document structure to a file, the logic you need is in the serialisation system, so that gives you a way in. The down side to using this directly is usually that you will usually have to serialise everything so your undo will be huge and slow. If possible, refactor code so that (a) there is a common serialisation interface used throughout the application (so any and every part of your data can be saved/restored using a generic call), and (b) every sub-system is encapsulated so that modifications to the data have to go through a common interface (rather than lots of people modifying member variables directly, they should all call an API provided by the object to request that it makes changes to itself) and (c) every sub-portion of the data keeps a "version number". Every time an alteration is made (through the interface in (b)) it should increment that version number. This approach means you can now scan your entire document and use the version numbers to find just the parts of it that have changed since you last looked, and then serialise the minimal amount to save and restore the changed state.

  • Provide a mechanism whereby a single undo step can be recorded. This means allowing multple systems to make changes to the data structure, and then when everything has been updated, triggering an undo recording. Working out when to do this may be tricky, but it can usually be accomplished by scanning your document for changes (see above) in your message loop, when your UI has finished processing each input event.

Beyond that, I'd advise going for a command based approach, because there are many benefits to it besides undo/redo.

Answer 2

Well , Here is my thought on this problem.

1- You need multi level undo/redo functionality. so you need to store user actions performed which can be stored in a stack.

2- Your second problem how to identify what has been changed by a operation i think through Memento pattern , it is quite a challenge. Memento is all about toring initial object state in your memory.

either , you need to store what is changed by a operation so that you can use this information to undo the opertions.

Command pattern is designed for the Undo/Redo functionality and i would say that its late but its worth while to implement the design which is being used for several years and works for most of the applications.

Answer 3

If performance allows it you could serialize your domain before each action. A few hundred objects is not much if the objects aren't big themselves.

Since your object graph is probably non trivial (i.e. uses inheritance, cycles,...) the integrated XmlSerializer and JsonSerializers are out of question. Json.net supports these, but does some lossy conversions on some types (local DateTimes, numbers,...) so it's bad too.

I think the protobuf serializers need either some form of DTD(.proto file) or decoration of all properties with attributes mapping their name to a number, so it might not be optimal.

BinaryFormatter can serialize most stuff, you just need to decorate all classes with the [Serializable] attribute. But I haven't used it myself, so there might be pitfalls I'm not aware of. Perhaps related to Singletons or events.

Answer 4

You may find the Monitored Undo Framework to be useful. http://muf.codeplex.com/

It uses something similar to the memento pattern, by monitoring for changes as they happen and allows you to put delegates on the undo stack that will reverse / redo the change.

I considered an approach that would serialize / deserialize the document but was concerned about the overhead. Instead, I monitor for changes in the model (or view model) on a property by property bases. Then, as needed, I use the MUF library to "batch" related changes so that they undo / redo as a unit of change.

The fact that you have your UI setup to react to changes in the underlying model is good. It sounds like you could inject the undo / redo logic there and the changes would bubble up to the UI.

I don't think that you'd see much lag or performance degradation. I have a similar application, with a diagram that we render based on the data in the model. We've had good results with this so far.

You can find more info and documentation on the codeplex site at http://muf.codeplex.com/. The library is also available via NuGet, with support for .NET 3.5, 4.0, SL4 and WP7.