QWidget update events but no visual update

Question

Using Qt4.8 on a Mint Linux 12, I implemented a simple window containing a QTableView to show contents of a model. The model data is continually updated (log messages) and the dataChanged() signal is emitted on a regular basis (i.e. every 100ms).

The problem I see is stuttering visual updates on the table.

I installed an event filter on the window that counts updateRequest-type events, which should trigger a widget repaint (also on child widgets, i.e. the tableView). These come in with an average time of ~170ms between them and a standard deviation of ~90ms (which is rather large, I guess). However, the perceived visual update rate is only two or three times a second and I wonder why. It seems that not all updateRequest events trigger a widget repaint or that the window system swallows visual updates.

As a second test, I forced the window to update itself by calling repaint or update every 100ms. Using repaint, I saw a corresponding increase in updateRequest-type events and a decrease of the standard deviation of the gaps; with update, the number did not increase. However, there was only a moderate increase of perceived update rate in both cases.

Also: Is there a good method to measure how often a widget is actually really repainted without having to overload its paintEvent handler? Maybe something from QTest?

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Update: I extended my event filter to also catch paintEvent-type events. There are only a one-digit number of those versus > 1000 updateRequest-type events.

Answer 1

You should instrument the event dispatcher's aboutToBlock() and awake() signals, and measure time between them using a QElapsedTimer. The instance of the event dispatcher for the current thread is returned by the static QAbstractEventDispatcher::instance().

If the event loop sleeps for a very small fraction of your time measurement window, it means there's too much stuff going on in the GUI thread. You can keep a count of how long the event loop slept, say, during the last second. If it's below 10%, you can expect slow updates and whatnot. Remember that update events are queued with Qt::LowEventPriority. They will be preempted by standard queued signals and almost all other events.

Answer 2

QApplication::compressEvent discards QEvent::UpdateRequest, if there is already one unprocessed. Thus even repaint(s) can return immediately without painting, if the event is discarded. You can check, if your updateRequest events are discarded by overwriting compressEvent.