Check if a selection exists in vimscript

Question

I want to write a function in vimscript that echoes the selected text or, if no text is selected, the entire buffer.

How can I distinguish between these two cases?

Answer 1

Define two mappings, an :nmap using the entire buffer, and a :vmap for the selected text. Both can invoke the same function, passing an isVisual boolean flag or a mode argument.

Anything else (custom commands, direct function :call) would require an explicit hint, because in order to invoke them, visual mode as already been left (for command-line mode). You also cannot use the '<,'> marks for the detection, because they will keep the last selection even after it has been removed.