Analysis of difficulties in diagramscene project

1. Engineering effect

This project is very similar miscrosoft visio Software flow chart components ：

 2. Project storage path

Examples\Qt-XX.XX.XX\widgets\graphicsview\diagramscene Under the table of contents ,XX.XX.XX by Qt Version number of , Such as ：5.14.1.

3. Analysis of engineering difficulties

There are two difficult points in this project , One of the codes is as follows ：

void DiagramScene::mouseReleaseEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
    if (line != nullptr && myMode == InsertLine) {
        QList<QGraphicsItem *> startItems = items(line->line().p1());
        if (startItems.count() && startItems.first() == line)
            startItems.removeFirst();
        QList<QGraphicsItem *> endItems = items(line->line().p2());
        if (endItems.count() && endItems.first() == line)
            endItems.removeFirst();

        removeItem(line);
        delete line;
//! [11] //! [12]

        if (startItems.count() > 0 && endItems.count() > 0 &&
            startItems.first()->type() == DiagramItem::Type &&
            endItems.first()->type() == DiagramItem::Type &&
            startItems.first() != endItems.first()) {
            DiagramItem *startItem = qgraphicsitem_cast<DiagramItem *>(startItems.first());
            DiagramItem *endItem = qgraphicsitem_cast<DiagramItem *>(endItems.first());
            Arrow *arrow = new Arrow(startItem, endItem);
            arrow->setColor(myLineColor);
            startItem->addArrow(arrow);
            endItem->addArrow(arrow);
            arrow->setZValue(-1000.0);
            addItem(arrow);
            arrow->updatePosition();
        }
    }
//! [12] //! [13]
    line = nullptr;
    QGraphicsScene::mouseReleaseEvent(mouseEvent);
}

The first 4~13 That's ok ： Detect dragging the mouse to scribe 、 When you release the mouse , The first item corresponding to the items below the starting and ending points of the line is not the line object itself , If yes, the line object is removed from startItems 、endItems Delete , Because the original intention of dragging the mouse to draw a line is to draw two non-linear item Connect . It's meaningless for a straight line to connect itself .

The first 16~28 That's ok ： If you remove startItems 、endItems after ,startItems 、endItems The container is not empty , prove startItems 、endItems Non linear objects are stored in （item）, Then take them out separately startItems 、endItems First of all , That is, take out two non-linear item, Create an arrow object （Arrow ）, Set up Arrow Object color 、z order , And set it to the non-linear line just taken out item Go to the object . Place the arrow object （Arrow ） Add to the scene and call  Arrow::updatePosition() Function to update the arrow object （Arrow ） Location . When calling  Arrow::updatePosition() when , Will be in these two again item Create a line object between .

Another difficulty is as follows ：

void Arrow::paint(QPainter *painter, const QStyleOptionGraphicsItem *,
				  QWidget *)
{
	if (myStartItem->collidesWithItem(myEndItem))
		return;

	QPen myPen = pen();
	myPen.setColor(myColor);
	qreal arrowSize = 20;
	painter->setPen(myPen);
	painter->setBrush(myColor);
//! [4] //! [5]

	QLineF centerLine(myStartItem->pos(), myEndItem->pos());
	QPolygonF endPolygon = myEndItem->polygon();
	QPointF p1 = endPolygon.first() + myEndItem->pos();
	QPointF intersectPoint;
	for (int i = 1; i < endPolygon.count(); ++i) {
		QPointF p2 = endPolygon.at(i) + myEndItem->pos();
		QLineF polyLine = QLineF(p1, p2);
		QLineF::IntersectionType intersectionType =
			polyLine.intersects(centerLine, &intersectPoint);
		if (intersectionType == QLineF::BoundedIntersection)
			break;
		p1 = p2;
	}

	setLine(QLineF(intersectPoint, myStartItem->pos()));
//! [5] //! [6]

	double angle = std::atan2(-line().dy(), line().dx());

	QPointF arrowP1 = line().p1() + QPointF(sin(angle + M_PI / 3) * arrowSize,
									cos(angle + M_PI / 3) * arrowSize);
	QPointF arrowP2 = line().p1() + QPointF(sin(angle + M_PI - M_PI / 3) * arrowSize,
									cos(angle + M_PI - M_PI / 3) * arrowSize);

	arrowHead.clear();
	arrowHead << line().p1() << arrowP1 << arrowP2;
//! [6] //! [7]
	painter->drawLine(line());
	painter->drawPolygon(arrowHead);
	if (isSelected()) {
		painter->setPen(QPen(myColor, 1, Qt::DashLine));
		QLineF myLine = line();
		myLine.translate(0, 4.0);
		painter->drawLine(myLine);
		myLine.translate(0,-8.0);
		painter->drawLine(myLine);
	}
}

The first 14~26 That's ok ： take myStartItem Item location point and myEndItem Create a line object at the item location point centerLine. take myEndItem Each vertex of the polygon where the item is located is taken out and converted into scene coordinates , Then judge whether each edge of the polygon is consistent with centerLine The intersection , If it intersects , Then take out the intersection , And use setLine function , Starting from the intersection ,myStartItem The position point of the item is the end point , change Arrow Line of class object .

The first 31~42： Draw a straight arrow .

The first 43~49： When the straight arrow is selected , On the top of the straight line drawn in the previous step 、 Draw below 1 A dotted line with a pen width of pixels , This gives the impression that the straight arrow is selected .

After this operation , will myStartItem、myEndItem Connected with straight arrows , And can realize the feeling of selection .

3 There are bug

This project DiagramScene Class setLineColor、setTextColor、setItemColor Functions exist bug, Now setLineColor Give an example of , Other similar .

void DiagramScene::setLineColor(const QColor &color)
{
    myLineColor = color;
    if (isItemChange(Arrow::Type)) {
        Arrow *item = qgraphicsitem_cast<Arrow *>(selectedItems().first());
        item->setColor(myLineColor);
        update();
    }
}

isItemChange Function as follows ：

bool DiagramScene::isItemChange(int type) const
{
    const QList<QGraphicsItem *> items = selectedItems();
    const auto cb = [type](const QGraphicsItem *item) { return item->type() == type; };
    return std::find_if(items.begin(), items.end(), cb) != items.end();
}

setLineColor Function means to check whether there is an arrow type item in the selected item , If there is , Then take out the first item in the selected item list , And set it to setLineColor Color of function parameters , The problem is ： The first item in the selected item list may not be an arrow type item , This is the following code ：

 Arrow *item = qgraphicsitem_cast<Arrow *>(selectedItems().first());

Back to item May be nullptr, namely  qgraphicsitem_cast<Arrow *> switch views , Lead to item by nullptr, This will cause the following code to crash . Try a box on the left , A box on the right , Then the two boxes are connected by straight arrows , Then check the box on the left , Then select the arrow , Then it collapsed .