Dragging object around a QWidget in Qt

Question

I am trying to make a custom widget in Qt Creator that supports dragging objects around. At its simplest form, the widget has a QRect (or any other shape), on which I can click and then drag it around the widget. Once I release the mouse button, the QRect should stop being dragged.

In my QWidget class, I have this method

void ImageArea::mouseMoveEvent(QMouseEvent *event)
{
    QPoint mousePos = event->pos();

    qDebug() << mousePos.x();
    qDebug() << mousePos.y();
    qDebug() << "---------";
}

that can get the coordinates of the mouse as the pointer is moved around the screen. I have tried updating member variables for x and y, and then painting the QRect via the paintEvent method, but this doesn't work.

Does anyone have any suggestions?

score 2 · Answer 1 · edited Sep 10 '15 at 12:19

2

To get mouse move events, you must set the QWidget::mouseTracking property to true:

ImageArea::ImageArea( QWidget* p ) : QWidget( parent ) {
    ...
    setMouseTracking( true );
}

edited Sep 10 '15 at 12:19

Tarod

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answered Apr 04 '11 at 06:16

Frank Osterfeld

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score 0 · Answer 2 · answered Apr 04 '11 at 04:21

Implement paintEvent(QPaintEvent *) to draw the object(s) at the positions indicated by the current value(s) of their corresponding member variables.

After you've changed the values of one or more member variables (in mouseMoveEvent or wherever), call this->update(). That will tell Qt that it needs to call your paintEvent method again in the near future.

That should be all you need to do.

score 0 · Answer 3 · answered Apr 04 '11 at 05:25

Be sure to use the moveTo method to move the rectangle. Setting the x,y position directly may affect the size of the rectangle.

I don't see what you aren't doing, based on your question. Are you sure that the rectangles are in new positions when you paint them?

Maybe you are missing the update step Jeremy Friesner told to implement.

It seems that you are missing mouse button tracking. The easy way might be to get the mouse button states from QApplication::mouseButtons(). Although it might be slightly less efficient.

Dragging object around a QWidget in Qt

3 Answers3