最近一次 FixedUpdate 已启动的独立于 TimeScale 的时间（只读）。此为自游戏启动以来的时间（以秒为单位）。
Returns the same value if called multiple times in a single frame. Unlike fixedTimeAsDouble this value is not affected by timeScale. Double precision version of fixedUnscaledTime. This offers more precision than a float or single value, particularly over extended periods of real-world time. In almost all cases, choose the fixedUnscaledTimeAsDouble equivalent over fixedUnscaledTime.