It has been brought to my attention that I have not blogged in a while. That usually means it’s time for a new proxy model.
The latest one is KIdentityProxyModel. It maintains an identity relationship to its source model such that the structure in the proxy mirrors exactly the structure in the source model, without sorting, filtering, flattening or otherwise changing the structure.
What the KIdentityProxyModel does provide though is a base class suitable for data proxying. There are many uses for such a class. Typically, an implementation of QAbstractItemModel will have an implementation of the data() method with a large switch statement for returning the decoration, the tooltip, the what’s-this text etc. It becomes cumbersome and difficult to maintain. Better of course is to use separate proxy models instead of a switch statement. If your tooltip text is defined in a ToolTipProxyModel, you can compile it in on the desktop, and compile it out on mobile/touch platforms where they make no sense. Similarly, you can define a DecorationProxyModel to return images of different sizes for desktop and mobile use. In KDE PIM we already use it to color the folders in kmail if you are reaching an IMAP folder size limit. Another use I have for it is to make any model have checkable items with the KCheckableProxyModel. (Ok, so I made two new proxy models 🙂 ).
The primary reason for a class like this is separation of concerns, a separation of platform code and re-usable components. The overhead of using a KIdentityProxyModel is very small, so even using multiple proxies does not incur a great cost. I haven’t made any benchmarks, so I offer only proof by inspection.
After writing this class I was informed that something similar used to exist in Qt4 as QProxyModel, but was deprecated some time ago. I didn’t even know it existed. Its implementation is very similar to KIdentityProxyModel, but an important difference is that KIdentityProxyModel implements QAbstractProxyModel, while QProxyModel implements QAbstractItemModel. That means that chaining is possible with KIdentityProxyModel, but not with QProxyModel.