观察者必须注册或者绑定到目标对象，在目标对象中维护了一个观察者列表，当目标对象的状态更改之后，它将通知列表中的观察者。

The observer registers with or attaches to the subject, which maintains a list of observers that it will notify when a particular state change occurs.

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可识别依赖关系的注册中心能向使用者通知提供者方面的更改情况，如某个服务变成了已弃用状态。

A registry aware of dependencies can then notify consumers of changes in providers, such as when a service becoming deprecated.

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通过提供过滤器，可以告知osgi服务注册中心只需通知您指定服务中的状态更改。

By providing a filter, you're telling the OSGi service registry to only notify you of state changes in the specified service.

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