Signal emitted in paintEvent stops drawing

[ofmrew]

I added an emit to the beginning of paintEvent and drawing stops, program continues, but if I add it to the end it works fine. Why?

Works

void MyCanvas::paintEvent(QPaintEvent *e)
{
    QPainter p(this);
    p.drawRect(drawableAreaRectangle);
drawing code.
    QString st;
    st.append("AspectRatios Canvas = ");
    st.append(QString::number(drawableAreaAspectRatio));
    st.append("  Rectangle = ");
    st.append(QString::number(drawableAspectRatio));
    emit message(st);
}
Does not work
void MyCanvas::paintEvent(QPaintEvent *e)
{
    QString st;
    st.append("AspectRatios Canvas = ");
    st.append(QString::number(drawableAreaAspectRatio));
    st.append("  Rectangle = ");
    st.append(QString::number(drawableAspectRatio));
    emit message(st);
    QPainter p(this);
drawing code

}

    connect(ui->canvas, &MyCanvas::message,
         this,
        [this](const QString& st)
        {   this->ui->statusBar->clearMessage();
            this->ui->statusBar->showMessage(st, 0);});

[ofmrew]

@ofmrew More info. I also get:

QBackingStore::endPaint() called with active painter on backingstore paint device
QBackingStore::endPaint() called with active painter on backingstore paint device

I added p.end(); and that stopped the above messages.

[SGaist]

Hi,

What does the method connected to message do ? Note that it's pretty unusual to emit in the paintEvent.

[JonB]

@SGaist
It's at the bottom of his post, scroll down. A lambda, writing to the statusbar.

I presume that does not re-enter his canvas' paint event?

[ofmrew]

@SGaist I know it is unusual, but the values required were available it there..

As for JonB's comment, I have no idea. But the code that works gives no errors.

[SGaist]

@JonB Thanks, the scroll didn't go far enough...

As I don't know what MyCanvas is, it could be triggering repaints every time, so re-triggering the emit etc.

[JonB]

@SGaist

As I don't know what MyCanvas is

https://doc.qt.io/archives/3.3/qcanvas.html ? ;-)

[ofmrew]

@JonB MyCanvas is a UI Widget that I have promoted. Simply that. The full code (that works) is posted below; note that is only one update(). It fails when the emit is moved before the first declaration of QPainter.

class MyCanvas : public QWidget
{
    Q_OBJECT
public:
    explicit MyCanvas(QWidget *parent = nullptr);
    void paintEvent(QPaintEvent *e);
    void resizeEvent(QResizeEvent *e);

    qreal       drawableAreaAspectRatio;
    QPointF     drawableAreaCenter;
    qreal       drawableAreaHeight;
    qreal       drawableAreaWidth;
    qreal       drawableAspectRatio{1.0};
    QRectF      drawableAreaRectangle;
signals:
    void message(QString st);
public slots:
    void    newAspectRatio(qreal ar);

};
MyCanvas::MyCanvas(QWidget *parent) : QWidget(parent)
{

}

void MyCanvas::paintEvent(QPaintEvent *e)
{
    QPainter p(this);
    QString st;
    p.drawRect(drawableAreaRectangle);
    p.setPen(Qt::red);
    if (drawableAspectRatio < drawableAreaAspectRatio){
        st.append("Height Constrained ");
        qreal widthDelta = drawableAreaHeight * drawableAspectRatio;
        qreal wd = widthDelta * 0.5;
        qreal cx = drawableAreaCenter.rx();
        p.drawRect(int(cx - wd), 20, int(wd + wd), int(drawableAreaHeight));
    }else{
        st.append("Width Constrained ");
        qreal heightDelta = drawableAreaHeight / drawableAspectRatio;
        qreal hd = (heightDelta * 0.5);
        qreal w = drawableAreaWidth;
        qreal cy = drawableAreaCenter.ry();
        p.drawRect(20.0, int(cy - hd), int(w), int(hd + hd));
    }
    p.end();
    st.append("AspectRatios Canvas = ");
    st.append(QString::number(drawableAreaAspectRatio));
    st.append("  Rectangle = ");
    st.append(QString::number(drawableAspectRatio));
    emit message(st);
}

void MyCanvas::resizeEvent(QResizeEvent *e)
{
    drawableAreaRectangle = QRectF(this->rect());
    drawableAreaAspectRatio = drawableAreaRectangle.width()/drawableAreaRectangle.height();
    drawableAreaRectangle.adjust(20.0, 20.0, -20.0, -20.0);
    drawableAreaCenter = drawableAreaRectangle.center();
    drawableAreaHeight = drawableAreaRectangle.height();
    drawableAreaWidth = drawableAreaRectangle.width();
}

void MyCanvas::newAspectRatio(qreal ar)
{
    drawableAspectRatio = ar;
    this->update();
}