Question

I have a main window with some widgets on it, each needs its own graphic. I would like to use QPainter to draw shapes, lines, etc. on them, but only after a specific event, like the press of a button.

The problem is, if I just create a QPainter in any function, it won’t work:

QPainter::setPen: Painter not active

The QPainter methods can only be called inside a paintEvent(QPaintEvent *) function! This raises the following problems:

1. I have to derive my custom classes for all the widgets I would like to paint on, so I can’t use the Designer to place my widgets. This can get frustrating with a large number of widgets.

2. The widgets redraw themselves after each paint event of the window, like moving it around, or moving other windows in front of it. I do a lot of drawing in those widgets, so they will visibly blink in these cases.

Is there a better and simpler way to solve this? I started to think about just displaying images, and re-manufacturing those images only when the specific buttons are pressed. I doubt that it’s the most elegant solution…

Answer 1

You can use custom widgets in the designer: Creating Custom Widgets for Qt Designer.

Qt Designer’s plugin-based architecture allows user-defined and third party custom widgets to be edited just like you do with standard Qt widgets.

For your second question, one of the approaches is to create a QPixmap for each of your widgets. When your widget’s appearance needs to be changed, you draw in that pixmap (using QPainter‘s constructor that takes a QPaintDevice – QPixmap is a QPaintDevice).

In your widget’s paintEvent function, you simply fill your widget with that “cache” pixmap. This way, you only do the (potentially expensive) painting when it’s actually necessary.