Model Subclassing Reference

Lazy Population of Model Data

Lazy population of model data effectively allows requests for information about the model to be deferred until it is actually needed by views.

Some models need to obtain data from remote sources, or must perform time-consuming operations to obtain information about the way the data is organized. Since views generally request as much information as possible in order to accurately display model data, it can be useful to restrict the amount of information returned to them to reduce unnecessary follow-up requests for data.

In hierarchical models where finding the number of children of a given item is an expensive operation, it is useful to ensure that the model's rowCount() implementation is only called when necessary. In such cases, the hasChildren() function can be reimplemented to provide an inexpensive way for views to check for the presence of children and, in the case of QTreeView, draw the appropriate decoration for their parent item.

Whether the reimplementation of hasChildren() returns true or false, it may not be necessary for the view to call rowCount() to find out how many children are present. For example, QTreeView does not need to know how many children there are if the parent item has not been expanded to show them.

If it is known that many items will have children, reimplementing hasChildren() to unconditionally return true is sometimes a useful approach to take. This ensures that each item can be later examined for children while making initial population of model data as fast as possible. The only disadvantage is that items without children may be displayed incorrectly in some views until the user attempts to view the non-existent child items.