I want to run a function when I start typing in the terminal.
Specifically I want to run the fzf function, for which I currently need to press ctrl-r to trigger it. I would like any keystroke to trigger it so history always appears when I type.
Only the first keystroke should run the function, because running it multiple time toggles between path and filename selection.
I have bound the Up and Down arrow keys to history-search-backward and history-search-forward respectively. So when I type something and then press Up or Down, it does a history search starting with typed letters. This works for me as I don't want to do a history search for every command entered.
I know this isn't exactly what you're looking for, but it's close.
# Bind up and down arrows to do backward and forward history search if [[ $- == *i* ]] then bind '"\e[A": history-search-backward' bind '"\e[B": history-search-forward' fi 
Ctrl-r needs to be fired before typing anything. But yeah, different strokes for different folks.
up-line-or-search and down-line-or-search for up and down arrows. This has the additional advantage of "search backward/forward in the history for a line beginning with the first word in the buffer" over the default (very similar to history-search-backward) And I use history-incremental-search-backward and history-incremental-search-forward for C-r and C-s.
bindprobably won't work, as I don't think you can meet the "first keystroke only" requirement. A faux command line with a high speed read loop might work. Using the terminal in raw mode or modifying the bash source code are probably the most likely avenues. Also, consider selector instead of fzf.