controlling size of QItemDelegate dynamically

Question

I have a couple of questions regarding the sizes of QItemDelegates in a QListView:

I have a QListView using a QItemDelegate which renders a widget in the delegate's custom paint() method like so:

self.thumbnail = MyCustomWidget()
self.thumbnail.render(painter, QtCore.QPoint(option.rect.x(), option.rect.y()))

This, however, shows the item with a 250x260 image in a QListView, even though the MyCustomWidget().sizeHint() is 250x250 and it's maximumSize() returns 250x250 as well.

I found that the culprit is the QListView's spacing, which I had set to 10. If I set the spacing to 100, I still get the QItemDelegates size of 250x260, but if I just don't use setSpacing() at all it renders as expected at 250x250. The spacing seems to alter the option.rect that is passed into the paint method, causing the incorrect size. I do need that spacing, so I'm a bit confused why the QListView's spacing alters the QItemDelegates's size? Is this a bug?

I can work around this by rendering a QPixmap first, then have the painter draw the QPixmap instead of rendering to the painter directly:

self.thumbnail = MyCustomWidget()
pixmap = QtGui.QPixmap(self.thumbnail.size())
self.thumbnail.render(pixmap)
painter.drawPixmap(option.rect.topLeft(), pixmap)

This yields 250x250 images which is what I need, but I don't understand why the first method doesn't render the correct size when I use setSpacing?!

Now, the bigger challenge is how to dynamically scale the size of the QItemDelegate's via a QSlider: I have a QSlider in the QListView that is supposed to scale the items so the user can chose to see smaller but more items in the current view. I tested the resizing of a standalone instance of MyCustomWidget() and it works just fine.

However, the delegates won't scale as expected. This is my delegate code: class Delegate(QtGui.QItemDelegate):

def __init__(self, parent = None):

    super(Delegate, self).__init__(parent)
    self.scaleValue = 100 # size in percent (as returned by QSlider)

def paint(self, painter, option, index):
    proxyModel = index.model()
    item = proxyModel.sourceModel().itemFromIndex(proxyModel.mapToSource(index))
    self.thumbnail = ElementThumbnail(item)
    self.thumbnail.scale(self.scaleValue)

    pixmap = QtGui.QPixmap(self.thumbnail.size())
    self.thumbnail.render(pixmap)
    painter.drawPixmap(option.rect.topLeft() * self.scaleValue / 100.0, pixmap)

    super(Delegate, self).paint(painter, option, index)

def setScaleValue(self, value):
    self.scaleValue = value

def sizeHint(self, option, index):
    return ElementThumbnail.thumbSize  * self.scaleValue / 100.0

and in the QListView I am using this slot connected to the slider's valueChanges signal:

def scaleThumbnails(self, value):
    self.itemDelegate().setScaleValue(value)
    self.update()

The result is that the QSlider will crop the QItemDelegates but not scale them, because the QItemDelegate's sizeHint() is only called when the QListView is first shown. Additionally, I need to make sure that when the widgets are (eventually) scaled down, the layout of QListView is recalculated and more items are fit inside the visible area.

So in a nutshell my questions are:

How can I scale QItemDelegates dynamically inside a QListView?
How can I force the QListView to recalculate it's layout after the delegate size has been changed?

edit: as for issue 2: QAbstractItemView.doItemsLayout seems to do the trick. Still wondering about issue 1 though

Thanks, frank

Frank Rueter Frank Rueter · Accepted Answer · 2016-08-15T04:45:12

Turns out it was the context of my code that was the issue, not the delegate. I was scaling the widget before rendering it to a pixmap, which of course made the editor scale properly, but not the item when it wasn't in edit state. So the solution is simply to scale the pixmap after rendering it from the widget, e.g.:

scaledPixmap = pixmap.scaled(pixmap.size() * self.scaleValue / 100.0)
painter.drawPixmap(option.rect.topLeft(), scaledPixmap)

controlling size of QItemDelegate dynamically

1 Answers