Graphics_view_zoom.h

#include <QObject>
#include <QGraphicsView>

/*!
 * This class adds ability to zoom QGraphicsView using mouse wheel. The point under cursor
 * remains motionless while it's possible.
 *
 * Note that it becomes not possible when the scene's
 * size is not large enough comparing to the viewport size. QGraphicsView centers the picture
 * when it's smaller than the view. And QGraphicsView's scrolls boundaries don't allow to
 * put any picture point at any viewport position.
 *
 * When the user starts scrolling, this class remembers original scene position and
 * keeps it until scrolling is completed. It's better than getting original scene position at
 * each scrolling step because that approach leads to position errors due to before-mentioned
 * positioning restrictions.
 *
 * When zommed using scroll, this class emits zoomed() signal.
 *
 * Usage:
 *
 *   new Graphics_view_zoom(view);
 *
 * The object will be deleted automatically when the view is deleted.
 *
 * You can set keyboard modifiers used for zooming using set_modified(). Zooming will be
 * performed only on exact match of modifiers combination. The default modifier is Ctrl.
 *
 * You can change zoom velocity by calling set_zoom_factor_base().
 * Zoom coefficient is calculated as zoom_factor_base^angle_delta
 * (see QWheelEvent::angleDelta).
 * The default zoom factor base is 1.0015.
 */
#ifndef GRAPHICS_VIEW_ZOOM_H_
#define GRAPHICS_VIEW_ZOOM_H_

class Graphics_view_zoom : public QObject {
Q_OBJECT
public:
    Graphics_view_zoom(QGraphicsView* view);
    void gentle_zoom(double factor);
    void set_modifiers(Qt::KeyboardModifiers modifiers);
    void set_zoom_factor_base(double value);

private:
    QGraphicsView* _view;
    Qt::KeyboardModifiers _modifiers;
    double _zoom_factor_base;
    QPointF target_scene_pos, target_viewport_pos;
    bool eventFilter(QObject* object, QEvent* event);

signals:
    void zoomed();
};

#endif