Wow! I go away for one evening and its like an explosion in my mail box ;)
First, I think the undo approach is an excellent one. Looks much cleaner than the clone/copy one and will have much less scope for coming back and biting us in the future. Some comments:
- I'm not sure you can just separate it as simply as value/reference. As models are under control of the end developer I see no reason why they would not create a detachable model but at some point change the object that it was referencing. Thus when they press the back button they have a page that is changing a different referenced object. We have something similar in an app I'm currently working on in that one page is a combined list/create/view/edit page so the page is the same but the id of the referenced business object changes over time. Solution: I think you have to clone the model in all undo cases.
- There are also some other options regarding staleness which we could define against a page:
* In some cases if a page is detected as stale there is actually no problem with just rendering the very latest version of the page. In fact this is desirable in some cases, for example I might want my shopping basket page to always show the latest basket contents so if the user goes back to a stale shopping basket page I can just send in return the latest page rendering instead.
* Another option might be that if a page is detected as stale then you always return to an associated known starting page - e.g. a summary page from which they can navigate forward again to the latest area of data that they need to work with.
By having the four options per page: stale error, undo, render latest and goto page we cover pretty much anything that most people would want to do. The advantage of this is also that we only need capture undo events for pages where this is specified as the mode (which is quite important for efficient clustering). We may also need to do something like specifying behaviour in relation to whether undo is held in the session or not (i.e. clustered/session recovery undo) and if not how the page should react - I'll put a post up on where I have got with clustering later today.
regards, Chris
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