How does airline detect if mode has been changed?

[begbaj]

I'm trying to write a plugin for Neovim where I need an autocommand event that indicates me that the mode has been changed. I'm pretty new to vim, so I'm still learning it, and writing this plugin helped me a lot to understand Vim, but I'm struggling to find a way to launch an event every time mode changes. I tried even to look at the source code of airline but I couldn't find the part where in fact it detects if mode has been changed.
I've been into this for 4 days now and I swear I'm becoming crazy xD
I searched a lot on internet, but I couldn't find what I'm looking for, but airline does exactly what I need.
One more thing to note is that I don't want to change the status line, I don't need that.
Thanks in advance.

[chrisbra]

newer Vims have the ModeChanged autocommand

[begbaj]

I completely missed that, didn't know this autocommand was added in Neovim pre-release. Thank you so much!