A Subsystem is initialized from a SubsystemDescriptorWithProvider for a given Subsystem (Example, Input, Display, etc.) and provides an interface to interact with that given Subsystem until it is Destroyed. After a Subsystem is created it can be Started or Stopped to turn on and off functionality (and improve performance). The base type for subsystems only exposes this functionality; this class is designed to be a base class for derived classes that expose more functionality specific to a given Subsystem.
注意：从同一 SubsystemDescriptor 中初始化第二个子系统将返回对现有子系统的引用，因为目前只允许单个子系统提供商使用一个子系统。
This subsystem base-class is deprecated. If you are creating a new subsystem type, derive from SubsystemWithProvider instead.