The future AlbumWidget view will display a grid of thumbnails with the pictures attached to the selected Album. In Chapter 3, Dividing Your Project and Ruling Your Code, we designed the gallery-core library to be agnostic of how a picture should be displayed: a Picture class contains only a mUrl field.

In other words, the generation of the thumbnails has to be done in gallery-desktop rather than gallery-core. We already have the PictureModel class that is responsible for retrieving the Picture information, so it would be great to be able to extend its behavior with the thumbnail data.

This is possible in Qt with the use of the QAbstractProxyModel class and its subclasses. The goal of this class is to process data from a base QAbstractItemModel (sorting, filtering, adding data, and so on) and present it to the view by proxying the original model. To take a database analogy, you can view it as a projection over a table.

The QAbstractProxyModel class has two subclasses...