How to run a widget's PaintEvent on another QThread

[binary001]

Thanks for your replay.

I want to use and reload so many pictures (just like wallpaper transaction) from disk. If all pictures are preloaded, app size is very big.
Now I try to use setGraphicsEffect from QThread.

[stampede]

If all pictures are preloaded, app size is very big.

yeye_olive means that you should load them to memory when your app is running, not include them into app.exe.

Now I try to use setGraphicsEffect from QThread.

Now I try to use setGraphicsEffect from QThread.

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That probably won't work - all GUI related operations have to be used from the main thread, as already mentioned.

[d_stranz]

If all pictures are preloaded

You only need to have two pictures loaded at any time - the one you are fading in, and the one you are fading out. In your timeout handler, when the opacity reaches 0 (or 1, whatever), just emit a signal or call a method to load the next image (and swap the pixmaps so that the one that was faded in is now the one to fade out):

pixmap1 = pixmap2;
pixmap2.load( nextFileName );

pixmap1 = pixmap2;
pixmap2.load( nextFileName );

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It is not necessary to have two QPainter instances in the paint event. Just change the opacity on a single QPainter instance between drawing the two pixmaps.

The only thing you might want to preload is the list of picture file names. And you don't really even need to do that - you can create a QDirIterator as a member of your class, and simply call QDirIterator::next():

pixmap1 = pixmap2;
if ( myDirIterator.hasNext() == false )
    myDirIterator = QDirIterator( path-to-picture-directory );  // restart from the beginning
pixmap2.load( myDirIterator.next() );

pixmap1 = pixmap2;
if ( myDirIterator.hasNext() == false )
    myDirIterator = QDirIterator( path-to-picture-directory );  // restart from the beginning
pixmap2.load( myDirIterator.next() );

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This code swaps the pixmaps, then checks to see if there are any files remaining in the list. If there are still files remaining, it loads the next one. If not, it resets the iterator back to the beginning and then loads the first one.