观察者控制器：当一个日程条目或者待办理列表添加、更新或者删除时，通告所有在日程模型中注册的观察者。
Observer controller observer controller Notifies all observers that have been registered with the agenda model when an entry or to-do list is added, updated or deleted.
观察者控制器：当一个日程条目或者待办理列表添加、更新或者删除时，通告所有在日程模型中注册的观察者。
Observer controller observer controller Notifies all observers that have been registered with the agenda model when an entry or to-do list is added, updated or deleted.
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