ServiceListener 提供了服务器端组件可用性的更新，来响应 CommunityService 对象上所作的请求。
The ServiceListener provides updates on the availability of server-side components in response to requests made on the CommunityService object.
决定每个组件的可用性，以从0到1的数字表示。
Determine the availability of each component, as a number from 0 to 1.
参见 计算单独组件的可用性部分。
See the Calculating the availability of individual components section.
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