Communication between UMG and other blueprint

UE4 contacted for some time, from the beginning has been using BP (Blueprint), feeling BP function is indeed very powerful, but encountered some embarrassing questions during use, such as:
Although BP class can inherit C ++ classes, and C ++ classes can inherit BP class, but in the same way when using UMG bit helpless, and I think when an event such as the occurrence of at UMG (such as a button is clicked)
Inform other blueprints (here a checkpoint blueprint for example)?
Or a simple user login when the input is complete, you need to send the information about user name, password, etc. to the server, the same will happen very embarrassing thing. (Here I was thinking that when input is complete click Submit Get in C ++ to input information)

I hope they see the older generation can do a detailed tutorial busy, in this grateful!