最近一次 FixedUpdate 已启动的时间（只读）。此为自游戏启动以来的时间（以秒为单位）。
Fixed time is updated in regular intervals (equal to fixedDeltaTime) until timeAsDouble property is reached. Double precision version of fixedTime. This offers more precision than a float or single value, particularly over extended periods of real-world time. In almost all cases, choose the fixedTimeAsDouble equivalent over fixedTime.