Qt-使用绝对位置将项目添加到QGraphicsScene

Question

This is probably a dump question but I'm really stuck with it right now. I'm trying to draw some squares in a QGraphicsScene and I want them to be aligned from position x = 0 towards the positive position of x coordinates. However they are aligned according to the alignment configuration of the QGraphicsView and the setting of position is only effective for the second item and upwards relative to the first item! This means that if I have a single item, then setting of it's position has no effect. Mainly this line seems not to be working:

graphicsView->setAlignment(Qt::AlignAbsolute);

This is my code:

QGraphicsScene *scene = new QGraphicsScene();
QGraphicsView *graphicsView;
graphicsView->setScene(scene);
graphicsView->setAlignment(Qt::AlignAbsolute);

for(int i = 0; i < 500; i+= 50)
{
    QGraphicsPolygonItem *item = new QGraphicsPolygonItem();

    item->setPen(QPen(QColor(qrand()%255,qrand()%255,qrand()%255)));
    item->setBrush(QBrush(QColor(255,251,253)));
    item->setPolygon(*myPolygon);
    graphicsView->scene()->addItem(item);
    item->setPos(i , 40);
    item->setFlag(QGraphicsItem::ItemIsMovable, true);
    item->setFlag(QGraphicsItem::ItemIsSelectable, true);
    graphicsView->show();
}

I do not know what the problem might be, so I tried the following code

const QRectF rect = QRectF(0, 0, ui->graphicsView->width(), ui->graphicsView->height());
ui->graphicsView->setScene(&_scene);
ui->graphicsView->fitInView(rect, Qt::KeepAspectRatio);
ui->graphicsView->setSceneRect(rect);

With respect to the previous four lines, the following output does not produce sizes even close to each other:

qDebug() << "scene =>" << _scene.width() << _scene.height();
qDebug() << "graphicview =>" << ui->graphicsView->width() << ui->graphicsView->height();

I highly appreciate your help.

Answer 1

It seems that Qt::AlignAbsolute does not do what you assume it does. What you actually need is Qt::AlignLeft | Qt::AlignTop Qt::AlignLeft | Qt::AlignTop .

It is explained here: http://qt-project.org/doc/qt-4.8/qgraphicsview.html#alignment-prop http://qt-project.org/doc/qt-4.8/qt.html#AlignmentFlag-enum

In my code, I only use graphicsView->setSceneRect(rect); and not fitInView() . The latter introduces a scaling and may hinder your understanding on what is going wrong.

Basically, I overwrite the QGraphicsview to re-implement resizeEvent() :

void AutohideView::resizeEvent(QResizeEvent *event)
{
    /* always resize the scene accordingly */
    if (scene())
        scene()->setSceneRect(QRect(QPoint(0, 0), event->size()));

    QGraphicsView::resizeEvent(event);
}

I do not alter alignment or any other options and use absolute coordinates in the range [ 0 , 0 .. scene()->width() , scene()->height() ] when positioning my items.