当我们添加某些依赖库（Okhttp、Retrofit）时， Android studio 会报如下错误：
Error:A problem occurred configuring project ':app'.
> Could not resolve all dependencies for configuration ':app:_iranDebugCompile'.
> Could not resolve com.facebook.network.connectionclass:connectionclass:1.0.1.
Required by:
doctor:app:unspecified
> No cached version of com.facebook.network.connectionclass:connectionclass:1.0.1 available for offline mode.
需要注意的在最后一行， > No cached version of com.facebook.network.connectionclass:connectionclass:1.0.1 available for offline mode.
我们需要做的是，取消红色边框标记的选项，然后 build Gradle 。