A fish shell function that emits an event when the current directory changes. As of version 2.1.1, function that capture events do not seem to work as autoloading functions, so the easiest thing is to place this in config.fish. As this uses the fish_prompt event, the change of directory will only be notes when a prompt is displayed. It would be …

fs_change_directory.fish

set -xg FS_CURRENT_DIR ""
function fs_change_directory -e fish_prompt
	set -l this_dir (PWD)
	if [ $this_dir != $FS_CURRENT_DIR ] 
		set -xg FS_CURRENT_DIR (pwd)
		emit directory_changed
	end
end 

fs_change_directory_v2.fish

function fs_change_directory --on-variable PWD
	emit directory_changed
end