Switching themes automatically in lightline.vim

Category: vim

Tags: vim

I have recently switched my Neovim setup to use lightline.vim and at the same time I was experimenting with colour schemes. Getting lightline to switch colours along with the rest of the editor is non-trivial, but I think I have figured it out, so I am going to share my setup.

A word about colour schemes

If you are already an expert on Vim colour schemes feel free to skip this section.

When you execute the command :colorscheme derp Vim will search the runtimepaths (see :h 'runtimepath') for the file colors/derp.vim and source it. If there are multiple such files, the first one found will be loaded. This means that you cannot have two definitions for derp where one overrides or complements the other, only one of them will be sourced. See :h :colorschme for full details.

Vim does not really have a notion of a colour scheme, the :colorscheme command only locates the file and sources it, which in turn executes all the ex-commands contained in the file. Usually these commands set various colours used by Vim, but in theory the commands could do anything. It also means that setting colours is not limited to a colour scheme file, we can set our colours anywhere.

As a side effect of executing :colorscheme the variable g:colors_name will be set to the name of the colour scheme. This variable allows us to act as if Vim had self-contained colour schemes. Actually, there is a small catch: a colour scheme has to explicitly set this variable, it will not be set automatically. So if the author of the derp colour scheme forgot to set it when the scheme is loaded, Vim will think that no colour scheme was set. In that case it would be best to send a patch to the author or fix it locally with an autocommand:

augroup colorscheme-overrides
    autocmd ColorScheme derp let g:colors_name = 'derp'
augroup END

How to do it

Here is our strategy: when the user changes the colour scheme we call a function which finds the correct lightline colour palette file, sources it, and then re-initialises lightline. The following code can be placed anywhere in your Vim configuration; personally I prefer to create the file plugin/lightline.vim in my Vim configuration directory.

First we set up an autocommand to call whenever the colour scheme changes.

augroup lightline-events
    autocmd ColorScheme * call s:onColorSchemeChange(expand("<amatch>"))
augroup END

Next we must actually write that function. How do we find out which file to source? The lightline manual describes for plugin authors how to create their own schemes (see :h lightline-colorscheme). The relevant portion is this:

In each file, one global variable is defined. For example, in the
landscape.vim file, you see

    let g:lightline#colorscheme#landscape#palette = s:p

If we recall how autoloading works, this means that we are interested in the file lightline/colorscheme/landscape.vim (see :h autoload). But what if the names do not match? The NeoSolarized colour scheme is a variant of Solarized and does not provide its own lightline scheme, so I would like to load the Solarized scheme which comes bundled with lightline instead. For this purpose a dictionary is good enough:

let s:colour_scheme_map = {'NeoSolarized': 'solarized'}

With all this out of the way we can finally write the callback function.

function! s:onColorSchemeChange(scheme)
    " Try a scheme provided already
    execute 'runtime autoload/lightline/colorscheme/'.a:scheme.'.vim'
    if exists('g:lightline#colorscheme#{a:scheme}#palette')
        let g:lightline.colorscheme = a:scheme
    else  " Try falling back to a known colour scheme
        let l:colors_name = get(s:colour_scheme_map, a:scheme, '')
        if empty(l:colors_name)
            let g:lightline.colorscheme = l:colors_name
    call lightline#init()
    call lightline#colorscheme()
    call lightline#update()

The reason we execute runtime with the file name as argument is to make sure it is sourced, otherwise the following if block would not find the variable.

We are almost done, there are just a few things to take care of. First we need to call our function if it is defined after the colour scheme has been set. In my case I set my colour scheme in my init.vim (or vimrc for Vim), which is going to be sourced before plugin/lightline.vim, so I have to add the following line after the function definition:

call s:onColorSchemeChange(g:colors_name)