Question

I’m using PyQt 4.4.

It’s best shown using some pictures. All nodes should have leafs from 0 to 99. They are being incrementally loaded using canFetchMore() and fetchMore(). But for some reason unknown for me this works only for the root node. (Picture 1)

If I collapse and expand a node it loads additional 10 values. (Picture 2 & 3)

It’s also strange, that it loads 10 values, as the code loads only 5 per call to fetchMore(), meaning this gets called 2 times before the code stops to load more data.

I’ve written a small example to demonstrate the problem, just run it with python test.py.
http://snipt.org/lLh

Does anyone know what causes this error?

Answer 1

I took a look at the Qt source (v4.5, though I don’t expect much difference between v4.4 and v4.5) for QAbstractItemView and QTreeView, and I don’t think they support incremental lazy loading of child nodes.

QAbstractItemView has no notion of trees, so it only calls fetchMore() on the top most index. It calls fetchMore() when:

• Geometry is updated
• The scroll bars are moved
• Rows are inserted
• The current item is changed as a result of an autoscrolling drag & drop operation

QTreeView additionally calls fetchMore() when:

• An item is expanded (this is essentially the only time it calls fetchMore() with a non-root index)
• The view’s layout needs to be relaid, such as with expandAll() and collapseAll()

I think the best solution would be to subclass QTreeView to make it call fetchMore() in the appropriate places and with the appropriate indices.