Understanding QGraphicsSceneMouseEvent::buttonDownScenePos() in Qt Widgets

Purpose

• This position is expressed in the scene's coordinate system, which is independent of the widget's on-screen location.
• In Qt's graphics scene framework, QGraphicsSceneMouseEvent::buttonDownScenePos() is a method used to retrieve the position of the mouse cursor at the moment a mouse button is pressed within a QGraphicsScene.

Context

• QGraphicsSceneMouseEvent is a class that encapsulates information about mouse events that happen within a QGraphicsScene.
• QGraphicsScene is a class that manages items in a two-dimensional scene. It provides a coordinate system for these items and handles mouse and keyboard events that occur within the scene.

How it Works

1. When a mouse button is pressed within a QGraphicsScene, a corresponding QGraphicsSceneMouseEvent object is generated.
2. This object stores various details about the event, including the button that was pressed, the position of the cursor at the time of the press, and other modifiers like Ctrl or Shift.
3. QGraphicsSceneMouseEvent::buttonDownScenePos() specifically accesses the position of the cursor in the scene's coordinate system at the moment the button was pressed. This position is represented by a QPointF object, which holds floating-point coordinates for both the X and Y axes.

Example Usage

#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsView>

class MyScene : public QGraphicsScene {
public:
    void mousePressEvent(QGraphicsSceneMouseEvent *event) override {
        QPointF scenePos = event->buttonDownScenePos();
        qDebug() << "Mouse button pressed at scene position:" << scenePos;

        // Perform some action based on the scene position where the button was pressed
        // ...
    }
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    MyScene scene;
    QGraphicsView view(&scene);
    view.show();

    return app.exec();
}

In this example:

• This position can then be used for various purposes, such as displaying it for debugging or performing actions specific to that location in the scene.
• Inside the method, event->buttonDownScenePos() is used to get the scene position where the button was pressed.
• When a mouse button is pressed within the scene, this method is triggered.
• A custom MyScene class inherits from QGraphicsScene and overrides the mousePressEvent method.

• Understanding the scene coordinate system is crucial for working with mouse events in Qt's graphics framework. The scene coordinates are independent of the widget's on-screen location, allowing for more flexible calculations and transformations.
• QGraphicsSceneMouseEvent provides other methods to access information about the mouse event, such as button(), modifiers(), and screenPos().

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Dragging an Item

This example shows how to track the mouse position when a button is pressed on an item in the scene, and then update the item's position as the mouse is dragged:

#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QGraphicsItem>

class DraggableItem : public QGraphicsItem {
public:
    void mousePressEvent(QGraphicsSceneMouseEvent *event) override {
        dragOffset = event->buttonDownScenePos() - pos();
        setCursor(Qt::ClosedHandCursor);
        event->setAccepted(true);
    }

    void mouseMoveEvent(QGraphicsSceneMouseEvent *event) override {
        if (event->buttons() & Qt::LeftButton) {
            setPos(event->scenePos() - dragOffset);
        }
    }

    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event) override {
        setCursor(Qt::ArrowCursor);
    }

private:
    QPointF pos() const { return mapToScene(boundingRect().center()); }
    QPointF dragOffset;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QGraphicsScene scene;
    DraggableItem *item = new DraggableItem;
    item->setRect(0, 0, 50, 50);
    scene.addItem(item);

    QGraphicsView view(&scene);
    view.show();

    return app.exec();
}

• When the mouse button is released (mouseReleaseEvent), the cursor is reset to the default arrow.
• During dragging (mouseMoveEvent), the item's position is updated based on the current scene position and the drag offset.
• When the mouse button is pressed on the item (mousePressEvent), buttonDownScenePos() is used to calculate the offset between the click position and the item's center.
• The DraggableItem class inherits from QGraphicsItem and implements event handlers for mouse press, move, and release events.

Selecting Items

This example demonstrates how to use buttonDownScenePos() to create a rubber band selection for items in the scene:

#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QGraphicsItem>
#include <QRubberBand>

class MyScene : public QGraphicsScene {
public:
    void mousePressEvent(QGraphicsSceneMouseEvent *event) override {
        if (event->button() == Qt::LeftButton) {
            selectionStart = event->buttonDownScenePos();
            selectionRect = QRect(selectionStart, QPoint());
            selectionRubberBand = new QRubberBand(QRubberBand::Rectangle, this);
            selectionRubberBand->show();
        }
        event->setAccepted(true);
    }

    void mouseMoveEvent(QGraphicsSceneMouseEvent *event) override {
        if (selectionRubberBand) {
            selectionRect.setTopLeft(selectionStart);
            selectionRect.setBottomRight(event->scenePos());
            selectionRubberBand->setGeometry(selectionRect);
        }
    }

    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event) override {
        if (selectionRubberBand) {
            // Handle selection based on items within selectionRect (code omitted for brevity)
            delete selectionRubberBand;
            selectionRubberBand = nullptr;
        }
    }

private:
    QPointF selectionStart;
    QRect selectionRect;
    QRubberBand *selectionRubberBand;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    MyScene scene;
    // Add items to the scene

    QGraphicsView view(&scene);
    view.show();

    return app.exec();
}

• When the mouse button is released (mouseReleaseEvent), the selection logic can
• A QRubberBand object is created to visualize the selection rectangle as the mouse is dragged (mouseMoveEvent). The rectangle's geometry is updated based on the current scene position and the starting point.
• When the left mouse button is pressed (mousePressEvent), buttonDownScenePos() is used to store the starting point of the selection.
• The MyScene class overrides the mouse event handlers to manage the selection process.

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QGraphicsSceneMouseEvent::pos()

• If you only need the position where the event occurred, pos() can be a simpler option. However, if you specifically need the position at the moment a button was pressed, buttonDownScenePos() is more suitable.
• This method returns the current position of the mouse cursor at the time the event was triggered, regardless of whether a button was pressed or not.

Custom Event Handling

• For example, you could define a signal/slot mechanism that emits a signal whenever a button is pressed, along with the scene position.
• This approach gives you more control over how you capture and store mouse button press positions.
• You can create your own custom event handling mechanism within your QGraphicsScene class.

Combining with modifiers()

• By checking the modifiers along with the position obtained from pos(), you can create more sophisticated event handling logic based on specific button presses with modifier combinations.
• The modifiers() method of QGraphicsSceneMouseEvent returns information about any modifier keys (like Ctrl, Shift, Alt) that were held down during the event.
• In some cases, you might want to consider a combination of pos() and modifiers().

Choosing the Right Alternative

The best alternative depends on your specific needs:

• For more granular control or handling specific button press combinations, consider custom event handling or combining methods with modifiers().
• If you just need the current mouse position during any event, pos() can suffice.
• If you strictly require the position where a button was pressed within the scene, buttonDownScenePos() remains the most appropriate choice.