QGraphicsItem仅通过X轴移动对象

Question

我有一个问题只能通过x轴移动我的对象.我知道你需要有功能的东西QVariant itemChange ( GraphicsItemChange change, const QVariant & value ).我找到了这样的东西:

QVariant CircleItem::itemChange(GraphicsItemChange change, const QVariant &value)
{
    if (change == ItemPositionChange)
            return QPointF(pos().x(), value.toPointF().y());
    return QGraphicsItem::itemChange( change, value );
}

但它不起作用.我是Qt的新手,所以我不知道,如何改变这个东西.这是我的GraphicsItem的代码:

#include "circleitem.h"

CircleItem::CircleItem()
{
    RectItem = new RoundRectItem();
    MousePressed = false;
    setFlag( ItemIsMovable );
}

void CircleItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    if( MousePressed )
    {
        painter->setBrush( QBrush( QColor( 0, 0, 255 ) ) );
        painter->setPen( QPen( QColor( 0, 0, 255 ) ) );
    }
    else
    {
        painter->setBrush( QBrush( QColor( 255, 255, 255 ) ) );
        painter->setPen( QPen( QColor( 255, 255, 255 ) ) );
    }
    painter->drawEllipse( boundingRect().center(), boundingRect().height() / 4 - 7, boundingRect().height() / 4 - 7 );
}

QRectF CircleItem::boundingRect() const
{
    return RectItem->boundingRect();
}

void CircleItem::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
    MousePressed = true;

    update( QRectF().x(), boundingRect().y(), boundingRect().width(), boundingRect().height() );
    QGraphicsItem::mousePressEvent( event );

}

void CircleItem::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    MousePressed = false;

    update( QRectF().x(), boundingRect().y(), boundingRect().width(), boundingRect().height() );
    QGraphicsItem::mouseReleaseEvent( event );

}

QVariant CircleItem::itemChange(GraphicsItemChange change, const QVariant &value)
{
    if (change == ItemPositionChange)
            return QPointF(pos().x(), value.toPointF().y());
    return QGraphicsItem::itemChange( change, value );
}

谢谢回答.

Answer 1

阅读QGraphicsItem :: ItemPositionChange的文档.它说:

项目的位置发生变化.如果启用了ItemSendsGeometryChanges标志,并且该项的本地位置相对于其父项发生更改(即,作为调用setPos()或moveBy()的结果),则会发送此通知.value参数是新位置(即QPointF).你可以调用pos()来获得原始位置.交付此通知时,请勿在itemChange()中调用setPos()或moveBy(); 相反,您可以从itemChange()返回新的,调整后的位置.在此通知之后,如果位置发生更改,QGraphicsItem会立即发送ItemPositionHasChanged通知.

在我们的代码中,我没有看到你设置了这个ItemSendsGeometryChanges标志,所以正确的构造函数如下:

CircleItem::CircleItem() // where is parent parameter?
{
    RectItem = new RoundRectItem();
    MousePressed = false;
    setFlag(ItemIsMovable | ItemSendsGeometryChanges);
}