EwmhDesktops: Cache property values to avoid needless property changes

XMonad/Hooks/EwmhDesktops.hs

@@ -26,6 +26,7 @@ module XMonad.Hooks.EwmhDesktops (
 
 import Codec.Binary.UTF8.String (encode)
 import Control.Applicative((<$>))
+import Data.IORef
 import Data.List
 import Data.Maybe
 import Data.Monoid
@@ -70,6 +71,17 @@ ewmhDesktopsStartup = setSupported
 -- of the current state of workspaces and windows.
 ewmhDesktopsLogHook :: X ()
 ewmhDesktopsLogHook = ewmhDesktopsLogHookCustom id
+
+
+data CacheState = CacheState { cachedWs :: [WindowSpace] }
+                deriving (Eq)
+
+-- |
+-- Cache last property state to avoid needless property changes.
+cachedState :: IORef CacheState
+cachedState = newIORef []
+{-# NOINLINE cachedState #-}
+
 -- |
 -- Generalized version of ewmhDesktopsLogHook that allows an arbitrary
 -- user-specified function to transform the workspace list (post-sorting)
@@ -78,6 +90,10 @@ ewmhDesktopsLogHookCustom f = withWindowSet $ \s -> do
     sort' <- getSortByIndex
     let ws = f $ sort' $ W.workspaces s
 
+    cached <- readIORef cachedState
+    unless (cached == ws) $ do
+        writeIORef cachedState ws
+
         -- Number of Workspaces
         setNumberOfDesktops (length ws)
 
@@ -98,8 +114,6 @@ ewmhDesktopsLogHookCustom f = withWindowSet $ \s -> do
 
         setActiveWindow
 
-    return ()
-
 
 -- |
 -- Intercepts messages from pagers and similar applications and reacts on them.