Qt Unclear on the Concept

Qt makes a big deal of using the Model-View architecture for its tables. One creates a table model by customizing QAbstractTableModel. Then one makes the table visible by creating a QTableView and linking it to the customized model.

All well and good, but unfortunately their design leaks view considerations into the model, as I only realized while finishing the character panel. It occurred to me that I had implemented a little character database in chardata.py (as described in the preceding post); so why had I based that class on QObject instead of QAbstractTableModel? I actually started to change this, and then stopped.

You customize QAbstractTableModel by adding overriding definitions of these methods:

• rowCount() to return the number of unique rows.
• columnCount() to return the number of columns.
• data(index, role) to return both data and metadata for one cell.
• headerData(index, role) to return both data and metadata for one header cell.

There's no debate about rowCount(); it is certainly the job of the data model to know how many primary keys there are to show. The trouble starts with columnCount(). The number of columns is a matter of how the data are to be presented to the user. As it says in Wikipedia, "the model captures the application's behavior in terms of its problem domain, independent of the user interface." The data model can know how many items are in the tuple related to one key, but how many of those are to be shown in this table, and in what sequence? That's the view's domain.

Things get worse with the data(index, role) method. There's no issue when the "role" passed is Qt.DisplayRole; then the return is one datum from the row. (Although one might quibble that the same datum could be displayed different ways; and this design forces the model to decide how to format each datum.) The issue is that the "role" code passed to data() can also be Qt.ToolTipRole, Qt.StatusTipRole, Qt.TextAlignmentRole, Qt.ForegroundRole and several other "roles" all related strictly to the display of the data.

It is (in my humble opinion) no business of the data model to know whether a given datum should be shown in red or black, left- or right-aligned, or what its tooltip should say.

The real breakdown of MVC is in headerData(index, role). The name at the top of a table column has nothing to do with the data model. I am especially sensitive to this because I am trying to make sure that all user-visible strings pass through QCoreApplication.translate(), so there is some hope of a properly-localized UI. Column header titles like "Symbol", "Count", and "Value" need to be translated. Same for tooltip and statustip strings! But (again, in my so-humble opinion), nothing the data model knows about should ever need translation. Translation should only ever be needed by the user-facing View component.

Tl;Dr: The Qt Model-View architecture forces the table model to perform many view-related things: deciding how to display each datum, providing column header and tooltip texts, and knowing presentation attributes such as color and alignment. That's just wrong.

Not that anything can or should be done about it at this point. I implemented the table model in the charview module.