每个观察者决定观察什么，所以整个工作流是建立在单个决定基础之上，而不是一个过载流程控制器。
Each observer decides what to observe, so the overall workflow is based on individual decisions rather than an overriding flow controller.
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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