feat: implement support for monitoring D-Bus signals

Implements support for using `dbus-monitor`, which allows direct monitoring of
signals, which are send when the settings changes.

This reduces the system load, as continuous polling is no longer required, as
well as improving the responsiveness, as the theme is now updated immediately,
instead of waiting for the next polling-cycle to complete.

If `dbus-monitor` is not available, the polling method will be used as a
fallback.

Author: FineFindus

lua/auto-dark-mode/init.lua

@@ -54,6 +54,8 @@ M.state = {
 	system = nil,
 	---@type table
 	query_command = {},
+	---@type string?
+	monitor_command = nil,
 }
 
 ---@return nil
@@ -92,6 +94,10 @@ M.init = function()
 			"string:org.freedesktop.appearance",
 			"string:color-scheme",
 		}
+
+        if vim.fn.executable("dbus-monitor") ~= 0 then
+            M.state.monitor_command = "dbus-monitor --session type=signal,interface=org.freedesktop.portal.Settings,member=SettingChanged,path=/org/freedesktop/portal/desktop,arg0='org.freedesktop.appearance',arg1='color-scheme'"
+        end
 	elseif M.state.system == "Windows_NT" or M.state.system == "WSL" then
 		local reg = "reg.exe"
 

lua/auto-dark-mode/interval.lua

@@ -16,9 +16,9 @@ local uv = vim.uv or vim.loop
 ---@return Appearance?
 local function parse_query_response(stdout, stderr)
 	if M.state.system == "Linux" then
-        if stderr ~= "" then
-            return nil;
-        end
+            if stderr ~= "" then
+                return nil;
+            end
 
 		-- https://github.com/flatpak/xdg-desktop-portal/blob/c0f0eb103effdcf3701a1bf53f12fe953fbf0b75/data/org.freedesktop.impl.portal.Settings.xml#L32-L46
 		-- 0: no preference
@@ -67,6 +67,33 @@ local function sync_theme(appearance)
 	end
 end
 
+-- Uses a subprocess to monitor the system for the current dark mode setting.
+-- The callback is called with the plaintext stdout response of the query.
+---@param callback? fun(stdout: string, stderr: string): nil
+---@return nil
+M.monitor_dark_mode = function (callback)
+    -- if no callback is provided, use a no-op
+    callback = callback or function() end
+
+    vim.fn.jobstart(M.state.monitor_command,
+        {
+            on_stdout = function (_, data, _)
+                data = table.concat(data, "")
+
+                if string.match(data, "uint32") then
+                    -- check if this was a signal update with a new value,
+                    -- as otherwise the fallback option will be incorrectly triggered
+                    callback(data, "")
+                end
+            end,
+            on_stderr = function (_, data, _)
+                data = table.concat(data, "")
+                callback("", data)
+            end
+        }
+    );
+end
+
 -- Uses a subprocess to query the system for the current dark mode setting.
 -- The callback is called with the plaintext stdout response of the query.
 ---@param callback? fun(stdout: string, stderr: string): nil
@@ -151,7 +178,13 @@ M.start = function(options, state)
 	-- act as if the timer has finished once to instantly sync on startup
 	timer_callback()
 
+    -- if the system supports monitoring, prefer that over polling updates
+    if M.state.monitor_command then
+        M.monitor_dark_mode(M.parse_callback)
+    else
 	M.start_timer()
+    end
+
 end
 
 return M