font_manager.h

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#ifndef FONT_MANAGER_H
#define FONT_MANAGER_H
 
#include <QFont>
#include <QMutex>
#include <QObject>

class FontManager : public QObject
{
    Q_OBJECT
public:
    enum class ZoomScope {
        AppWide,
        ContentOnly
    };

    static FontManager *instance();
 
    // --- Value accessors: static only (single instance, no instance
    //     duplicates, so there is no static/instance name clash) ---

    static QFont font();
    static QFont monospaceFont();
    static QFont zoomedFont();
    static QFont zoomedMonospaceFont();

    static qreal zoomFactor();
 
    // --- Mutators & queries ---

    void setRegularFont(const QString &fontName);

    void setMonospaceFont(const QString &fontName);
 
    void zoomIn();
    void zoomOut();
    void resetZoom();
    void setZoomLevel(int level);
    int  zoomLevel() const { return zoom_level_; }
 
signals:
    void applicationFontChanged(QFont font);
    void monospaceFontChanged(QFont font);
    void zoomChanged();
 
protected:
    explicit FontManager(QObject *parent = nullptr);
    ~FontManager();
 
    // Catches QEvent::ApplicationFontChange on qApp.  Distinguishes our own
    // push from an external/OS change by comparing the now-current
    // qApp->font() against applicationFont() (the value we would push): when
    // they match the change is ours and is ignored; when they differ the OS
    // changed the font and we adopt it (unless a theme/pref overrides regular).
    bool eventFilter(QObject *watched, QEvent *event) override;
 
private:
    static FontManager *instance_;
    static QMutex       mutex_;
 
    QFont     base_regular_;
    QFont     base_monospace_;
    int       zoom_level_ = 0;
    ZoomScope zoom_scope_;
 
    // True when a theme or pref explicitly set the regular font; while set,
    // an external OS font change is NOT adopted (our override wins).
    bool regular_overridden_ = false;
 
    QFont applicationFont() const;          
    QFont zoomed(const QFont &base) const;  
    void  applyApplicationFont();           
    void  applyZoom();                      
    void  syncMonospacePref();              
 
    static QFont fontFromName(const QString &name);  
    static QFont guaranteeMonospace(const QFont &font);
};
 
#endif /* FONT_MANAGER_H */