Sets the function to be called when a site is requesting
authentication. The URL requested and the security realm from the
server are passed in the arguments url and realm.
The function returns a Future which should complete when the
authentication has been resolved. If credentials cannot be
provided the Future should complete with false. If
credentials are available the function should add these using
addCredentials before completing the Future with the value
true.
If the Future completes with true the request will be retried using the updated credentials. Otherwise response processing will continue normally.
Source
set authenticate(Future<bool> f(Uri url, String scheme, String realm));