What is the hook used by company-mode to perform autocompletion?

Question

According to describe-key (C-h k), every character I type runs self-insert-command and I suspect that self-insert-command calls some undocumented hook to perform text completition.

I am trying to debug company-mode, which occasionally fails to complete my input. So, describe-function says that self-insert-command calls post-self-insert-hook. I set it to nil and tried to type something that would be autocompleted — and it was completed anyway! In addition to this, self-insert-command calls expand-abbrev and auto-fill-function, but I tried to call both, and neither shows a popup text. So, it seems like every keypress runs some other hook used by company-mode.

I found that company-complete-common is the function that does autocompletition, so I set a breakpoint there to find this mysterious hook by using a backtrace. And… it didn't work! It works if I call company-complete-common interactively, but if I trigger completion by typing a character, and the same window pops up, the breakpoint is ignored!

I'm out of ideas, I hope somebody knows how this works.

Answer 1

Unless you invoke one of the completion commands directly (such as M-x company-complete-common), the completion maybe-happens and the popup is rendered in company-post-command.

It looks at the current command, sees if it satisfies the value of company-begin-commands, and calls company--perform.

The latter looks at the buffer contents, and checks whether the prefix value returned by the currently used backend satisfies company-minimum-prefix-length, if so then asks for completions from the backend.

If completions were found, company-post-command calls all currently configured frontends with an argument post-command. That's where the popup rendering happens.

When completion doesn't work (and you don't see any errors in *Messages*), check whether it works with M-x company-complete-common, at the same buffer position. If it does, you might need to figure out the exact command your keypress was calling (try C-h k), and then add it to company-begin-commands.