QGraphicsView/Scene - multithreading nightmare

Question

Am I correct in thinking that the QGraphics* classes are not thread-safe? I am porting an old app to Qt and in the process attempting to make it multi-threaded too. I looked at the update code and I see no locks whatsoever.

I started off right, all my processing is done in a set of worker threads so that the GUI does not have to block. But as soon as I come to display my visual representation the whole thing falls down like a pack of cards as the update code attempts to read from the buffer the other thread is writing to.

Here is my test case:

1. Create a bunch of ellipse objects
2. Create a thread and pass it the scene pointer
3. In a loop modify any setting on any object in the scene.

Test function:

bool CBasicDocument::Update( float fTimeStep )
{
const QList<QGraphicsItem*> tObjects = items();

for( QList<QGraphicsItem*>::const_iterator tIter = tObjects.constBegin();
tIter != tObjects.constEnd();
++tIter )
 {
    QGraphicsEllipseItem* pElipse = (QGraphicsEllipseItem*)(*tIter);
   if( pElipse )
   {   
      pElipse->setPen( QPen( QColor( (int)(255.0f * sinf( fTimeStep )), (int)(255.0f * cosf( fTimeStep )), (int)(255.0f * sinf( fTimeStep )) ) ) );
   }
 }
return true;
}

I have been thinking about ways I can fix this and none of them are particularly pretty.

Ideally what I want to happen is when I change a setting on an object it is buffered until the next render call, but for the time being I'll settle with it not crashing!

At the moment I have four options:

1. Double buffer the whole scene maintaining two scene graphs in lockstep (one rendering, one updating). This is how our multithreaded game engine works. This is horrible here though because it will require double the CPU time and double the memory. Not to mention the logistics of maintaining both scene graphs.

2. Modify QGraphics* to be thread safe as above. This is probably the most practical approach but it will be a lot of work to get it done.

3. Push modifications to the scene into a queue and process them from the main thread.

4. Throw multithreading to the wind for the time being and just let my app stall when the document updates. (Not pretty given the data size for some documents)

None of them are particularly appealing and all require a massive amount of work.

Does anybody have any ideas or attempted multithreading QGraphicsScene before?

Cheers

Answer 1

I've always read it's a better idea to have all the GUI work happen in the main thread, no matter you're using GTK or Qt or else.

In your case I would go for option 2.

Modifying Qt's code to put locks here and there is the worst thing to do ihmo as you won't be able to sync your modified Qt with upstream without a significant amount of work.

Answer 2

I notice in your test function that you are already casting (using c-style casts, at that) the QGraphicsItem to a QGraphicsEllipseItem. If you only use a certain number of item types, it might be easier to subclass those types, and provide the thread-safe functions in those subclasses. Then cast down to those subclasses, and call what you want to from the other thread. They can handle buffering the changes, or mutex locking, or whatever you want to do for those changes.

An alternative would be to have one mutex, and each thread locks that when processing. This would likely slow you down a lot as you wait for the mutex to become available. You could also maintain a map of items->mutexes somewhere that both threads can get to, and each thread locks the mutex for the item it is working on while it is working with it. This would be simpler to layer in if you think the subclassing option would be too complex.