Keep lines drawn by drawPolyLine() function as an object?

CJha

Hi, I am making poly-lines on my QWidget using drawPolyLine() function of QPainter. I have an algorithm that detects when my mouse is over one of these poly-lines. What I want to achieve is to highlight the given poly-line (such as by changing the colour) so that the user understands that the poly-line can be selected. The problem is to change the colour of the poly-line I have to call QWidget's paintEvent() again which is highly inefficient as it redraws everything else. Is there any way I can create a poly-line object that is displayed in a QWidget and whose colour can be changed easily without calling paintEvent()? I know I can use QGraphicsItem but for that I will have to move my entire thing from QWidget to QGraphicsView-QGraphicsScene and I would like to avoid that.

kshegunov

Is this what you're after?
https://doc.qt.io/qt-5/qpainterpath.html

CJha

@kshegunov Thanks, QPainterPath can work but I will still have to redraw it in paintEvent() I am looking for something that I can change the colour of without calling paintEvent() for the QWidget. Is it possible?

kshegunov

@CJha said in Keep lines drawn by drawPolyLine() function as an object?:

Is it possible?

Absolutely not. How is it supposed to be drawn if you intend to not draw it? It just can't happen.

JonB

@CJha
As you said originally, if you want graphical objects you can manipulate individually you need to move to QGraphicsItems. While you are painting on widgets, once you draw a line it's "gone", there is no object for you to access, and you need to redraw to recreate it.

CJha

@JonB & @kshegunov Thanks for clearing it up. I just wasn't sure if it is possible or not because I could not find any reference to any such thing anywhere.

kshegunov

@JonB said in Keep lines drawn by drawPolyLine() function as an object?:

need to move to QGraphicsItems

Or directly paint the prepared QPainterPaths, although the graphics item is going to do that for you, so there's some convenience.