wibox.drawable: Add visibility tracking callback

This new function is called whenever the visibility of the drawable
changes. Later commits can use this for explicitly tracking the lifetime
of drawables instead of using magic weak tables.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/titlebar.lua

@@ -282,6 +282,7 @@ local function new(c, args)
             position = position
         }
         ret = drawable(d, context, "awful.titlebar")
+        ret:_inform_visible(true)
         local function update_colors()
             local args_ = bars[position].args
             ret:set_bg(get_color("bg", c, args_))
@@ -298,6 +299,9 @@ local function new(c, args)
         -- Update the colors when focus changes
         c:connect_signal("focus", update_colors)
         c:connect_signal("unfocus", update_colors)
+
+        -- Inform the drawable when it becomes invisible
+        c:connect_signal("unmanage", function() ret:_inform_visible(false) end)
     else
         bars[position].args = args
         ret = bars[position].drawable

lib/wibox/drawable.lua

@@ -266,6 +266,9 @@ function drawable:set_fg(c)
     self._do_complete_repaint()
 end
 
+function drawable:_inform_visible(visible)
+end
+
 local function emit_difference(name, list, skip)
     local function in_table(table, val)
         for _, v in pairs(table) do

lib/wibox/init.lua

@@ -169,6 +169,11 @@ local function new(args)
     ret._drawable = wibox.drawable(w.drawable, { wibox = ret },
         "wibox drawable (" .. object.modulename(3) .. ")")
 
+    ret._drawable:_inform_visible(w.visible)
+    w:connect_signal("property::visible", function()
+        ret._drawable:_inform_visible(w.visible)
+    end)
+
     for k, v in pairs(wibox) do
         if type(v) == "function" then
             ret[k] = v