屏幕阅读器的活动 AccessibilityHierarchy。若未激活任何层次结构，可能会显示 null。
您需要一个可访问性分层结构，才能通过屏幕阅读器向用户显示任何内容。
如果屏幕阅读器已关闭，则没有活动的分层结构。如果在设置活动分层结构时设备上的屏幕阅读器已关闭，则活动分层结构将自动设置为 null。
对于所有受支持的平台，请参阅辅助支持。
如果平台不支持屏幕阅读器，并且代码未在 Unity 编辑器中运行，则会抛出 PlatformNotSupportedException。
分配活动分层结构时，会向操作系统发出通知，表明屏幕已发生重大更改。该通知通过调用 IAccessibilityNotificationDispatcher.SendScreenChanged（使用 null 参数）发送。
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