Best way to create a long line (or cross line) cursor in Qt GraphicsView

The easy way to create the long cross line cursor (as long as viewport) is create a cross line graphicsItem, when mouse moved, set the item's pos property. But this way will be very slow when the scene is complex, because it should update the whole viewport to update the cursor's pos.

The another easy way is setCursor(QCursor(..)),use a QPixmap to define the long cross line, this way will very fast , but the cursor will exce开发者_C百科ed the viewport rect.

Is there another way to show a long cross line cursor fastly?

Thanks very much!

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If I understand correctly, you want to draw an horizontal line and a vertical line, crossing at the cursor position, and being as large as the viewport is.

A poosible solution would be to reimplement QGraphicsScene::drawForeground() to draw the two lines with the painter.

The problem is that the scene doesn't know about the mouse position. This means the view will have to track it and inform the scene when the mouse position has changed.

To do that, you'll have to create your own GraphicsScene (inheriting QGraphicsScene) and your own GraphicsView (inheriting QGraphicsView).

On your GraphicsView constructor, you'll have to start mouse tracking. This will make your you receive a mouseMoveEvent each time the mouse moves inside the view :

GraphicsViewTrack::GraphicsViewTrack(QWidget* parent) : QGraphicsView(parent) {
    setMouseTracking(true);
}

void GraphicsViewTrack::mouseMoveEvent(QMouseEvent* pEvent) {
    QPointF MousePos = this->mapToScene(pEvent->pos());
    emit mousePosChanged(MousePos.toPoint());
}

As you can see in the code snippet above, the view is emitting a signal (mousePosChanged) to which the scene will be connected. This signal contains the mouse position, converted to the scene's coordinates.

Now, on the scene side, you have to add a slot which will be called when the mouse position changed, store the new mouse position in a member variable and reimplement QGraphicsScene::drawForeground() :

void GraphicsSceneCross::drawForeground(QPainter* painter, const QRectF& rect) {
    QRectF SceneRect = this->sceneRect();

    painter->setPen(QPen(Qt::black, 1));
    painter->drawLine(SceneRect.left(), m_MousePos.y(), SceneRect.right(), m_MousePos.y());
    painter->drawLine(m_MousePos.x(), SceneRect.top(), m_MousePos.x(), SceneRect.bottom());
}

void GraphicsSceneCross::onMouseChanged(QPoint NewMousePos) {
    m_MousePos = NewMousePos; // Store the mouse position in a member variable
    invalidate(); // Tells the scene it should be redrawn
}

The last thing to do is connect the GraphicsView's signal to the GraphicsScene slot.

I'll let you check if this solution is acceptable performance wise.

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Based on Jerome answer and using python I created this code in my QGraphicsScene subclass:

def drawForeground(self, painter, rect):
    if self.guidesEnabled:
        painter.setClipRect(rect)
        painter.setPen(self.guidePen)
        painter.drawLine(self.coords.x(), rect.top(), self.coords.x(), rect.bottom())
        painter.drawLine(rect.left(), self.coords.y(), rect.right(), self.coords.y())

def mouseMoveEvent(self, event):
    self.coords = event.scenePos()
    self.invalidate()

It should be straighforward for you to write its appropiate C++ code. Note that I take advantage of the rect argument passed by Qt Api framework and I clip the painter to that area, since it's the visible area to be drawn.

I also cache the pen object, since I realized in other experiments that creating objects while painting will penalty performace and doing it so you also give the user the chance to set a custom pen in your program options.

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I have found a way to do this! I develops under Windows system, so can use lower GDI api jumping out of Qt's painting system. The detail is get the HDC of the QGraphicsView's viewPort. Then in the QMouseEvent of QGraphicsView use "MoveToEx" and "LineTo" drawing two lines on the viewport,then I should do is erase the "old" cursor, It's easy to do this using "setROP2(HDC dc,R2_NOT)",then draw the old Cursor stored again. This method doesn't enter the QPainter system, so the GraphicsItems under the cursor will not be repaint.

To solve the filker problem when the mouse moves fast, I don't use "double buffer". I used QTimer to update the cursor at CPU's idle. The detail is in QMouseEvent ,don't update the cursor at time ,but store the position to a list, When CPU idle, draw the cursor at the list of positions

I wish this will help others who meets the same problem with me. Thanks Jérôme, who gave me useful tip of QGraphicsScene.