Move QGraphicsView top left to a certain point in QGraphicsScene manually - qt

Question

I have a QGraphicsView subclass, holding a QGraphicsScene subclass, inside a QWidget.
The QGraphicsScene has an actual drawing rectangle with top let corner at (0,0) (so the scene really has a QRectF(-x,-y,w,h) of values irrelevant to the problem).
On startup, actually in the showEvent of the widget, I set the size of the scene to fit the custom sized rectangle. That also centers it.
What I would like, is to match the (0,0) point of the scene to the top left of the view, on start.
Unlike other questions with the same title, I actually need to move the view, not the items inside the scene.
Image 1: actual start, image 2: desired
I am trying:
void MyWidget::showEvent(QShowEvent *event)
{
static bool startUp = true;
if(startUp)
{
QSizeF sz = m_scene->getActualSize();
ui->graphicsView->fitInView(QRectF(0, 0, sz.width(), sz.height()), Qt::KeepAspectRatio);
QPointF zero = ui->graphicsView->mapFromScene(QPointF(0,0));
ui->graphicsView->translate(-zero.x(), -zero.y()); // or positive
startUp = false;
}
QWidget::showEvent(event);
}
note: I use the fitInView to get the desired zoom - and it also brings the desired rectangle in focus - but centered in the view.
I also tried
void GraphicsView::alignToZero()
{
QPointF zero = this->mapFromScene(QPointF(0,0));
this->scrollContentsBy(-zero.x(), -zero.y());
}
void MyWidget::showEvent(QShowEvent *event)
{
static bool startUp = true;
if(startUp)
{
QSizeF sz = m_scene->getActualSize();
ui->graphicsView->fitInView(QRectF(0, 0, sz.width(), sz.height()), Qt::KeepAspectRatio);
ui->graphicsView->alignToZero();
startUp = false;
}
QWidget::showEvent(event);
}
Still no result.
Also tried
void MyWidget::showEvent(QShowEvent *event)
{
static bool startUp = true;
if(startUp)
{
QSizeF sz = m_scene->getActualSize();
ui->graphicsView->fitInView(QRectF(0, 0, sz.width(), sz.height()), Qt::KeepAspectRatio);
QPointF zero = ui->graphicsView->mapFromScene(QPointF(0,0));
//ui->graphicsView->setAlignment(Qt::AlignLeft | Qt::AlignTop);
ui->graphicsView->horizontalScrollBar()->setValue(zero.x());
ui->graphicsView->verticalScrollBar()->setValue(zero.y());
startUp = false;
}
QWidget::showEvent(event);
}
This, with or without the alignment setting (which I think may have unwanted effects elsewhere), moves the view... too far.

Answer 1

If I understand your question correctly, to achieve desired effect you can use method
QGraphicsView::setAlignment(Qt::Alignment alignment):
ui->graphicsView->setAlignment( Qt::AlignTop | Qt::AlignLeft );
Which is commented in your code.
This method with those arguments aligns view top-left, you don't have to move or fit anything.

Answer 2

While there must be a better way... I figured out a way to get my 0's aligned by doing fitInView twice:
void MyWidget::showEvent(QShowEvent *event)
{
static bool startUp = true;
if(startUp)
{
QSizeF sz = m_scene->getActualSize();
// First I do a normal `fitInView` that centers the rectangle
ui->graphicsView->fitInView(QRectF(0, 0, sz.width(), sz.height()), Qt::KeepAspectRatio);
// Then, knowing the shift, I fit it again adding twice the shift to the size.
QPointF zero = ui->graphicsView->mapToScene(QPoint(0,0));
ui->graphicsView->fitInView(QRectF(0, 0, sz.width() - 2 * zero.x(), sz.height() - 2 * zero.y()), Qt::KeepAspectRatio);
startUp = false;
}
QWidget::showEvent(event);
}