Aligning QGraphicsItems to a grid when dragging and dropping - qt

Question

For example, if I wanted to display the player's inventory in a game using a QGraphicsView, how could I enforce a grid-based view where dragging and dropping items always results in them being aligned to a grid?

Answer 1

You can handle the appropriate mouse events in a QGraphicsScene subclass:
#include <QApplication>
#include <QGraphicsItem>
#include <QGraphicsScene>
#include <QGraphicsSceneMouseEvent>
#include <QGraphicsView>
#include <QMainWindow>
#include <math.h>
class GridScene : public QGraphicsScene
{
public:
GridScene() :
mCellSize(25, 25)
{
}
protected:
// Efficiently draws a grid in the background.
// For more information: http://www.qtcentre.org/threads/5609-Drawing-grids-efficiently-in-QGraphicsScene?p=28905#post28905
void drawBackground(QPainter *painter, const QRectF &rect)
{
qreal left = int(rect.left()) - (int(rect.left()) % mCellSize.width());
qreal top = int(rect.top()) - (int(rect.top()) % mCellSize.height());
QVarLengthArray<QLineF, 100> lines;
for (qreal x = left; x < rect.right(); x += mCellSize.width())
lines.append(QLineF(x, rect.top(), x, rect.bottom()));
for (qreal y = top; y < rect.bottom(); y += mCellSize.height())
lines.append(QLineF(rect.left(), y, rect.right(), y));
painter->drawLines(lines.data(), lines.size());
}
void mousePressEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
mDragged = qgraphicsitem_cast<QGraphicsItem*>(itemAt(mouseEvent->scenePos(), QTransform()));
if (mDragged) {
mDragOffset = mouseEvent->scenePos() - mDragged->pos();
} else
QGraphicsScene::mousePressEvent(mouseEvent);
}
void mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
if (mDragged) {
// Ensure that the item's offset from the mouse cursor stays the same.
mDragged->setPos(mouseEvent->scenePos() - mDragOffset);
} else
QGraphicsScene::mouseMoveEvent(mouseEvent);
}
void mouseReleaseEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
if (mDragged) {
int x = floor(mouseEvent->scenePos().x() / mCellSize.width()) * mCellSize.width();
int y = floor(mouseEvent->scenePos().y() / mCellSize.height()) * mCellSize.height();
mDragged->setPos(x, y);
mDragged = 0;
} else
QGraphicsScene::mouseReleaseEvent(mouseEvent);
}
private:
// The size of the cells in the grid.
const QSize mCellSize;
// The item being dragged.
QGraphicsItem *mDragged;
// The distance from the top left of the item to the mouse position.
QPointF mDragOffset;
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QMainWindow w;
w.resize(400, 400);
w.show();
QGraphicsView view(&w);
view.setScene(new GridScene());
view.resize(w.size());
view.setSceneRect(0, 0, view.size().width(), view.size().height());
view.show();
view.scene()->addRect(0, 0, 25, 25)->setBrush(QBrush(Qt::blue));
return a.exec();
}
#include "main.moc"