Does a signal or event exist for when a QGraphicsView or QWidget is done being painted or rendered?

问题

I'm trying to time an application to see how long it takes to load up some information, and paint a graph. My function loads up the data first, then draws the graph.

The timing is fairly simple, it calls an external function that gets msecs since some date. The problem is even if I set t1 in the beginning and t2 right after I call the draw function, t2 will return before the QGraphicsView is actually updated. (I know, it makes sense why this should be asynchronous)

For instance when I load a large file, it will return with 700 msecs after I subtract the two values, but the actual rendering doesn't finish until a few seconds later.

I've looked all over the web and scoured the Qt documentation. I can find tons of information on updating widgets yourself, but nothing on any kind of signal or event that is fired off after rendering finishes.

Even the QGraphicsScene::changed signal appears to only be fired off when the scene changes underneath, not when rendering is done and the user can SEE the changes.

Any help on how to do this?

回答1:

Does a signal or event exist for when a QGraphicsView or QWidget is done being painted or rendered?

As far as I know, it does not exist. (looked for something similar)

user can SEE the changes

As far as I know, Qt uses double buffering, so if painting is finished, it doesn't mean that user can see the changes.

Any help on how to do this?

If you want to know when painting has finished, then...

You can subclass QGraphicsScene and implement your own drawItems, drawBackground or drawForeground. This is NOT simple (because item painting algorithm is complicated), but you'll be able to tell when every item has finished painted.
You can fire/emit signals from within paintEvent (QWidget-based classes) or paint() (QGraphicsItem/QGraphicsObject-based classes). You'll need to use your own classes, obviously, and you'll have to subclass either QGraphicsView, or items you're drawing within view, or QGraphicsScene.
You could also create proxy QPainter class, and this way you'll be able to know what exactly is being paitned and when.

Having said that I suspect you're approaching your problem incorrectly.

If you're only trying to draw a graph, then there's no reason for you to know when painting is finished.
If painting is finished, it doesn't mean user can see the result.
Paint events might be called more than once.

Recommended approach:

Receive/read the data (you're drawing in your graph) from external source using threads or timer events (you'll need to read it in small chunks if you're using timer events, obviously), then update the graph from time to time, and let Qt handle repainting.

How exactly does this allow me to detect the amount of time it takes from when I choose to open a file to when all the data is loaded and the graph is drawn and is visible?

You can detect when paintEvent has finished painting by subclassing whatever widget you're using to paint Graph, overriding paintEvent and firing signal from within paintEvent, calling a subroutine or doing whatever you want.
There is no warranty that paintEvent will be called only once.
To measure how slow individual routine is and locate bottlenecks, use profilers. VerySleepy, AQTime, and so on.
Instead of measuring how long it takes to load AND display data, it will make much more sense to measure separately loading time and display time. This is a GUI application, not a game engine, so you do not control precisely when something is being drawn.

回答2:

I have not testet it, but I think by subclassing QGraphicsScene and reimplementing the render method you can measure the render time.

#include <QGraphicsScene>
#include <QTime>

class MyGraphicsScene : public QGraphicsScene 
{
Q_OBJECT
public:
void render ( QPainter * painter, const QRectF & target = QRectF(), const QRectF & source = QRectF(), Qt::AspectRatioMode aspectRatioMode = Qt::KeepAspectRatio ) {
   QTime t;
   t.start();
   QGraphicsScene::render (painter, target, source, aspectRatioMode);
   qDebug("render time %d msec", t.elapsed());
}
};

来源：https://stackoverflow.com/questions/17448194/does-a-signal-or-event-exist-for-when-a-qgraphicsview-or-qwidget-is-done-being-p

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c++

events

timer