服务调用方法必须足够灵活以处理一些事件，比如经常在类似服务的外部提供者之间切换，而不需要更改应用程序。
The service invocation approach must be flexible enough to cope with events such as switching frequently between external providers of a similar service without requiring changes to the application.
如果网页中包含着需要经常查看、而且迅速更新的内容，比如查看社交网站上朋友们的状态，或者接收新闻源的最新消息，那么你就必须在不同页面间来回切换。
You have to constantly click back and forth among tabs if they contain fast-changing material you check often, like the status of your friends in social-networking services, or updates to news feeds.
VB管理网络链接，经常切换者需要。
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