QGraphicsScene：如何将项目移动映射到QUndoCommand？

Question

I'm trying to create a subclass of QUndoCommand that represents a movement of one or more QGraphicsItem . If several items are moved at once (by selecting them first), this should be represented as a single command within the QUndoStack .

Since the whole movement logic (including selections) is already implemented by QGraphicsScene , I'm not sure what would be the best place to create the undo commands.

The undoframework example provided by Qt subclasses QGraphicsScene and overrides mousePressEvent() / mouseReleaseEvent() to perform some manual hit testing. Is this really the way to go?

I'm afraid that this approach could miss some special cases such that the generated undo commands do not reflect exactly the same movement that has been performed by the internal Qt implementation. Some items could have the ItemIsMovable flag unset for example. Also, when moving multiple items at once, it seems better to store just the delta movement instead of old and new position.

It would be great if you could provide some short sample code/sketch for clarification.

Answer 1

I personally would wrap and encapsulate QGraphicsScene and implement the undo/redo stuff there. This allows to define your own and cleaner interface, free of the clutter of all the functionality you won't need. It will also make it easier to replace QGraphicsScene with another API if such a need arises in the future and makes your core stuff more portable.

Many of the high level classes in Qt, especially those for graphics, are not exactly what I'd call "cutting edge", the same applies even more so to the rudimentary examples Qt provides for them. There are several professional graphics applications that use Qt, but none of them uses stuff like QGraphicsScene . I suppose the reason for this is also the reason I extrapolated from my own experience using Qt - as big as a framework it may be, there is a lot of stuff that's missing, and a lot of stuff that just doesn't really work the way people have come to expect from professional software, or even common logic in such workflows. Qt was developed by very good programmers, but it seems like they didn't have someone experienced in the actual graphics applications workflow to advise them.

Focus on a wrapper that features the functionality you need, and make it work with QGraphicsScene instead of using it directly. This way there is less chance that you miss something, your design is more self contained and cleaner too. The same applies to the undo API as well, don't feel obligated to use Qt's, I myself have tried it and ended up implementing my own, as I found the one provided by Qt was rather "stiff".

In the case of moving several items at once - yes, that should create a single undo entry, which will undo the moving of a selection group, which will feature all the selected items. Whether you will use a delta or position - it is entirely up to you. The delta does make a little more sense when you move multiple selected items, since the selection group is not really an item with position.

Another consideration, if you plan on making history persistent across different sessions - you can't rely on pointers, since those will be arbitrary each time you run your application. My strategy is to implement a unique integer ID and an associative registry which stores the id and pointer for each item. Then you use the ID for the history, and that will internally map to the pointer of the actual item.