enum Key 表示 调号类型，定义如下

enum class Key {
      C_B = -7,  // 7个降号
      G_B,        // 6个降号
      D_B,        // 5个降号
      A_B,        // 4个降号
      E_B,        // 3个降号
      B_B,        // 2个降号
      F,           // 1个降号
      C,           //  无升降号
      G,           // 1个升号
      D,           // 1个升号
      A,           // 1个升号
      E,           // 1个升号
      B,           // 1个升号
      F_S,        // 1个升号
      C_S,        // 1个升号
      MIN     = Key::C_B,
      MAX     = Key::C_S,
      INVALID = Key::MIN - 1,
      NUM_OF  = Key::MAX - Key::MIN + 1,
      DELTA_ENHARMONIC = 12
      };

KeySigEvent 表示一个调号信息

class KeySigEvent {
      Key _key            { Key::INVALID     };          // -7 -> +7
      KeyMode _mode       { KeyMode::UNKNOWN };
      bool _custom        { false            };
      bool _forInstrumentChange{ false          };
      QList<KeySym> _keySymbols;

      void enforceLimits();

   public:
      KeySigEvent() {}
      KeySigEvent(const KeySigEvent&);

      bool operator==(const KeySigEvent& e) const;
      bool operator!=(const KeySigEvent& e) const { return !(*this == e); }

      void setKey(Key v);
      void print() const;

      Key key() const            { return _key;                    }
      KeyMode mode() const       { return _mode;                   }
      void setMode(KeyMode m)    { _mode = m;                      }
      bool custom() const        { return _custom;                 }
      void setCustom(bool val)   { _custom = val; _key = Key::C;   }
      bool isValid() const       { return _key != Key::INVALID;    }
      bool isAtonal() const      { return _mode == KeyMode::NONE;  }
      void setForInstrumentChange(bool forInstrumentChange) { _forInstrumentChange = forInstrumentChange; }
      bool forInstrumentChange() const{ return _forInstrumentChange; }
      void initFromSubtype(int);    // for backward compatibility
      QList<KeySym>& keySymbols()             { return _keySymbols; }
      const QList<KeySym>& keySymbols() const { return _keySymbols; }
      };

KeyList 用来保存 tick 的 调号信息，可以用查询某tick的调号

class KeyList : public std::map<const int, KeySigEvent> {

   public:
      KeyList() {}
      KeySigEvent key(int tick) const;
      KeySigEvent prevKey(int tick) const;
      void setKey(int tick, KeySigEvent);
      int nextKeyTick(int tick) const;
      int currentKeyTick(int tick) const;
      void read(XmlReader&, Score*);
      };

}

Staff 里包含 KeyList

class Staff final : public ScoreElement {
   public:
      enum class HideMode { AUTO, ALWAYS, NEVER, INSTRUMENT };

   private:
      Part* _part       { 0 };

      ClefList clefs;
      ClefTypeList _defaultClefType;

      KeyList _keys;

调用 Staff::keySigEvent 获得调号

KeySigEvent Staff::keySigEvent(const Fraction& tick) const
      {
      return _keys.key(tick.ticks());
      }