Observer-观察者模式
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.. note:: is a publish/subscribe pattern, which allows a number of observer objects to see an event. Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

有各种不同的叫法::

    Subject-Observer
    Publisher-Subscriber(一对多关系，可同步也可异步)
    Producer-Consumer(多对多关系，一般是异步)
    EventEmitter-EventListener
    Dispatcher-Listener



参考
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* The Observer Pattern In Go: https://levelup.gitconnected.com/the-observer-design-pattern-in-go-d564048fe9f6