feat: allow configuring presets Feline on the fly

Add utility functions `add_preset` and `use_preset` to allow customizing
Feline presets on the fly.

USAGE.md

@@ -522,6 +522,39 @@ Here is a list of all possible vi_mode names used with the default color associa
 - `highlight_reset_triggers` - Feline automatically resets its cached highlights on certain autocommands to prevent the statusline colors from getting messed up. The value of `highlight_reset_triggers` can be set to a table containing a list of autocommands that'll trigger a highlight reset.<br><br>
   Default: `{'SessionLoadPost', 'ColorScheme'}`
 
+## Utility functions
+
+Feline provides a few utility functions that allow you to customize or modify Feline on the fly. These are discussed below.
+
+### Reset highlight
+
+If, for some reason, you want to clear all highlights that Feline sets (useful if you want to reload your entire Neovim config which may mess up highlights), you can do:
+
+```lua
+require('feline').reset_highlights()
+```
+
+And then Feline will automatically regenerate those highlights when it needs them, so you don't have to worry about setting the highlights yourself.
+
+### Adding and using presets
+
+If you want to add your own presets, you can do this through the `require('feline').add_preset` function, like this:
+
+```lua
+-- Components table for the preset
+local my_preset = {
+    -- Insert components here
+}
+
+require('feline').add_preset('my_preset_name', my_preset)
+```
+
+You can also use a preset using `require('feline').use_preset`, like this:
+
+```lua
+require('feline').use_preset('my_preset_name')
+```
+
 ## Example configuration
 
 You can check out the code in the [default preset](lua/feline/presets/default.lua) to see how the components in it are set up so you can get a good practical idea of how to use the tools that Feline gives you to create all kinds of different statusline components.
@@ -711,16 +744,6 @@ end
 
 Note that the values of both `vim.g.actual_curwin` and `vim.g.actual_curbuf` are strings, not numbers. So if you want to use them as a number, use `tonumber()` to convert the string to a number first, as shown in the second example.
 
-### Reset highlight
-
-If, for some reason, you want to clear all highlights that Feline sets (useful if you want to reload your entire Neovim config which may mess up highlights), you can do:
-
-```lua
-require('feline').reset_highlights()
-```
-
-And then Feline will automatically regenerate those highlights when it needs them, so you don't have to worry about setting the highlights yourself.
-
 ### Highlight section gaps
 
 By default, gaps between two sections inherit the highlight of the last element of the section. If you wish to customize the highlight of the gap between two sections, you can just add a component with only an `hl` value to the end of the first section. 

lua/feline/generator.lua

@@ -3,11 +3,8 @@ local api = vim.api
 
 local feline = require('feline')
 local providers = feline.providers
-local components_table = feline.components
 local colors = feline.colors
 local separators = feline.separators
-local disable = feline.disable
-local force_inactive = feline.force_inactive
 
 local M = {
     -- Cached highlights
@@ -21,6 +18,7 @@ end
 
 -- Check if current buffer is forced to have inactive statusline
 local function is_forced_inactive()
+    local force_inactive = feline.force_inactive
     local buftype = bo.buftype
     local filetype = bo.filetype
     local bufname = api.nvim_buf_get_name(0)
@@ -32,6 +30,7 @@ end
 
 -- Check if buffer is configured to have statusline disabled
 local function is_disabled()
+    local disable = feline.disable
     local buftype = bo.buftype
     local filetype = bo.filetype
     local bufname = api.nvim_buf_get_name(0)
@@ -365,6 +364,8 @@ end
 
 -- Generate statusline by parsing all components and return a string
 function M.generate_statusline(is_active)
+    local components_table = feline.components
+
     if not components_table or is_disabled() then
         return ''
     end

lua/feline/init.lua

@@ -6,6 +6,7 @@ local cmd = api.nvim_command
 
 local utils = require('feline.utils')
 local gen = utils.lazy_require('feline.generator')
+local presets = utils.lazy_require('feline.presets')
 
 local M = {}
 
@@ -60,6 +61,23 @@ function M.reset_highlights()
     gen.highlights = {}
 end
 
+-- Add a new preset for Feline (useful for plugins which intend to extend Feline)
+function M.add_preset(name, value)
+    presets[name] = value
+end
+
+-- Use a preset
+function M.use_preset(name)
+    if presets[name] then
+        M.components = presets[name]
+    else
+        api.nvim_err_writeln(string.format(
+            "Preset '%s' not found!",
+            name
+        ))
+    end
+end
+
 -- Setup Feline using the provided configuration options
 function M.setup(config)
     -- Check if Neovim version is 0.5 or greater
@@ -71,8 +89,8 @@ function M.setup(config)
     -- Check if termguicolors is enabled
     if not opt.termguicolors:get() then
         api.nvim_err_writeln(
-            'Feline needs \'termguicolors\' to be enabled to work properly\n' ..
-            'Please do `:help \'termguicolors\'` in Neovim for more information'
+            "Feline needs 'termguicolors' to be enabled to work properly\n" ..
+            "Please do `:help 'termguicolors'` in Neovim for more information"
         )
         return
     end
@@ -97,7 +115,6 @@ function M.setup(config)
         M.components = config.components
     else
         local preset = config.preset
-        local presets = require('feline.presets')
 
         -- If a valid preset isn't provided, then use the default preset if nvim-web-devicons
         -- exists, else use the noicons preset

lua/feline/presets/init.lua

@@ -1,7 +1,33 @@
 -- Use lazy_require to only load the preset that's are being used
+local api = vim.api
 local lazy_require = require('feline.utils').lazy_require
 
-return {
+local default_presets = {
     default = lazy_require('feline.presets.default'),
     noicon = lazy_require('feline.presets.noicon')
 }
+
+local custom_presets = {}
+
+local presets_mt = {
+    __index = function(_, key)
+        if default_presets[key] then
+            return default_presets[key]
+        elseif custom_presets[key] then
+            return custom_presets[key]
+        end
+    end,
+    __newindex = function(_, key, val)
+        if default_presets[key] or custom_presets[key] then
+            api.nvim_err_writeln(string.format(
+                "Preset '%s' already exists!",
+                key
+            ))
+        else
+            custom_presets[key] = val
+        end
+    end,
+    __metatable = false
+}
+
+return setmetatable({}, presets_mt)