Use /org/freedesktop/UPower/devices/DisplayDevice as self.device

DisplayDevice is a "special" device object that represent what a widget should
display, however it does *not* give information about Line Power. E.g. when a
laptop has no battery but is plugged on the mains, DisplayDevice says that no
device should be displayed and the icon is set to "battery-missing" (which is
in fact true).

However, if we are working, it means that we must be getting power from
somewhere, so let's display something in the tooltip anyway.

Command line example

$ upower -i /org/freedesktop/UPower/devices/DisplayDevice
  power supply:         no
  updated:              Sun 05 Nov 2017 04:19:48 PM GMT (415 seconds ago)
  has history:          no
  has statistics:       no
  unknown
    warning-level:       none
    icon-name:          'battery-missing-symbolic'

See also https://upower.freedesktop.org/docs/UPower.html#UPower.GetDisplayDevice

power_widget.lua

@@ -23,7 +23,6 @@ local naughty = require("naughty")
 
 local power = require("upower_dbus")
 local WarningLevel = power.enums.BatteryWarningLevel
-local DeviceType = power.enums.DeviceType
 
 local spawn_with_shell = awful.spawn.with_shell or awful.util.spawn_with_shell
 local icon_theme_dirs = { -- The trailing slash is mandatory!
@@ -44,9 +43,11 @@ end
 
 function widget:update()
   self.device:update_mappings()
+
   self:set_image(build_icon_path(self.device))
 
   if self.device.IsPresent then
+
     local percentage = math.floor(self.device.Percentage)
     local warning_level = self.device.warninglevel
 
@@ -60,6 +61,7 @@ function widget:update()
           text = percentage .. "% remaining"})
     end
   else
+    -- We don't know how we're powered, but we must be somehow!
     self.tooltip:set_text("Plugged In")
   end
 end
@@ -68,12 +70,8 @@ function widget:init()
   local manager = power.Manager
   self.manager = manager
 
-  local devices = {}
-  for _, d in ipairs(self.manager.devices) do
-    devices[d.type] = d
-  end
-
-  self.device = devices[DeviceType.Battery] or devices[DeviceType["Line Power"]]
+  -- https://upower.freedesktop.org/docs/UPower.html#UPower.GetDisplayDevice
+  self.device = power.create_device("/org/freedesktop/UPower/devices/DisplayDevice")
 
   self.device:on_properties_changed(
     function ()

rockspec/power_widget-0.3.3-1.rockspec

@@ -0,0 +1,27 @@
+package = "power_widget"
+version = "0.3.3-1"
+source = {
+   url = "git://github.com/stefano-m/awesome-power_widget",
+   tag = "v0.3.3"
+}
+description = {
+   summary = "A Power widget for the Awesome Window Manager",
+   detailed = [[
+    Monitor your power devices in Awesome with UPower and DBus.
+    ]],
+   homepage = "https://github.com/stefano-m/awesome-power_widget",
+   license = "GPL v3"
+}
+supported_platforms = {
+   "linux"
+}
+dependencies = {
+   "lua >= 5.1",
+   "upower_dbus >= 0.3.0, < 0.4"
+}
+build = {
+   type = "builtin",
+   modules = {
+      power_widget = "power_widget.lua"
+   }
+}