可用于确定 XR 设备是否处于活动状态的只读值。
When true, Unity is accepting input from the HMD and rendering to the HMD's display(s). This can become false if a device is disconnected, a device could not be intialized (see XRSettings.LoadDeviceByName), or XRSettings.enabled is set to false.
XR 输出将自动镜像到主显示器（如果适用）。这可以通过 XRSettings.showDeviceView 进行控制。
主窗口仍然由 Screen 和相关 API 控制。