[google-calendar-api] Get refresh token google api

OAuth has two scenarios in real mode. The normal and default style of access is called online. In some cases, your application may need to access a Google API when the user is not present,It's offline scenarios . a refresh token is obtained in offline scenarios during the first authorization code exchange.

So you can get refersh_token is some scenarios ,not all.

you can have the content in https://developers.google.com/identity/protocols/OAuth2WebServer#offline .