Qt5 QTreeView with custom model and large data very slow scrolling

Question

I have custom data that I need to display in a QTreeView. I have derived my model from QAbstractTableModel, and made my own implementations of rowCount(), columnCount(), data(), and headerData(). The model has a local QList> to support it, and the data() function is defined to read the values out of that list of lists directly corresponding to the row and column received in the QModelIndex parameter. There are two issues I'm running into.

The first is that the load of a very large file is quite slow, which is understandable. The second is that the scroll action is painfully slow, which I am not really understanding. Turns out that if I pull the scroll handle down, the GUI hangs for about 20 seconds, and then pops back. If I pull the handle a greater distance down, the hang time increases accordingly. If I pull the handle all the way to the bottom of the scroll bar, after waiting for the application to become responsive again, I can pull the handle up and down and get much better response.

It seems to me that QTreeView is only asking for a small chunk of the available data, but when I have pulled the scroll handle all the way to the bottom of the scroll bar, once the application becomes responsive again, it has by that point read all the data.

Is there a way to program for a much more responsive experience with scrolling for large data? I don't mind a longer wait up front, so just something like forcing the view to read all data from the model up front would work.

I have also thought that I could go back to just deriving from QAbstractItemView and controlling how it requests and stores data, only allowing for storing the viewed data, plus a buffer of entries before and after the viewed data. That of course would mean I'd have to control the scroll bar, since the handle sizing would indicate a small amount of data, and I would like it to look to the user as it should for the size of data they are dealing with. Not really wanting to go there if I don't have to.

Answer 1

Two things:

• Re-implement fetchMore() and canFetchMore() in your model. See this implementation example. Basically, the two functions allow lazy initialization of your data and should stop ui freezes.
• Replace your usage of reset() and dataChanged() to use the insert and remove functionality. Right now, you are forcing the view to recalc which of 100,000 items to show.

Answer 2

use

treeview view;
view.setUniformRowHeights(true);

Then view do n't hangs.