    {-# OPTIONS_GHC -fglasgow-exts #-} -- For deriving Data/Typeable
    {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, PatternGuards, TypeSynonymInstances #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  XMonad.Layout.DragPane
    -- Copyright   :  (c) Spencer Janssen <spencerjanssen@gmail.com>
    --                    David Roundy <droundy@darcs.net>,
    --                    Andrea Rossato <andrea.rossato@unibz.it>
    -- License     :  BSD3-style (see LICENSE)
    --
    -- Maintainer  :  Andrea Rossato <andrea.rossato@unibz.it>
    -- Stability   :  unstable
    -- Portability :  unportable
    --
    -- Layouts that splits the screen either horizontally or vertically and
    -- shows two windows.  The first window is always the master window, and
    -- the other is either the currently focused window or the second window in
    -- layout order.

    -----------------------------------------------------------------------------

    module XMonad.Layout.DragPane (
                                   -- * Usage
                                   -- $usage
                                    dragPane
                                  , DragPane, DragType (..)
                                  ) where

    import XMonad
    import Data.Unique

    import qualified XMonad.StackSet as W
    import XMonad.Util.Invisible
    import XMonad.Util.XUtils

    -- $usage
    -- You can use this module with the following in your @~\/.xmonad\/xmonad.hs@:
    --
    -- > import XMonad.Layout.DragPane
    --
    -- Then edit your @layoutHook@ by adding the DragPane layout:
    --
    -- > myLayouts = dragPane Horizontal 0.1 0.5 ||| Full ||| etc..
    -- > main = xmonad defaultConfig { layoutHook = myLayouts }
    --
    -- For more detailed instructions on editing the layoutHook see:
    --
    -- "XMonad.Doc.Extending#Editing_the_layout_hook"

    halfHandleWidth :: Integral a => a
    halfHandleWidth = 1

    handleColor :: String
    handleColor = "#000000"

    dragPane :: DragType -> Double -> Double -> DragPane a
    dragPane t x y = DragPane (I Nothing) t x y

    data DragPane a =
        DragPane (Invisible Maybe (Window,Rectangle,Int)) DragType Double Double
                 deriving ( Show, Read )

    data DragType = Horizontal | Vertical deriving ( Show, Read )

    instance LayoutClass DragPane a where
        doLayout d@(DragPane _ Vertical   _ _) = doLay id d
        doLayout d@(DragPane _ Horizontal _ _) = doLay mirrorRect d
        handleMessage = handleMess

    data SetFrac = SetFrac Int Double deriving ( Show, Read, Eq, Typeable )
    instance Message SetFrac

    handleMess :: DragPane a -> SomeMessage -> X (Maybe (DragPane a))
    handleMess d@(DragPane mb@(I (Just (win,_,ident))) ty delta split) x
        | Just e <- fromMessage x :: Maybe Event = do handleEvent d e
                                                      return Nothing
        | Just Hide             <- fromMessage x = do hideWindow win
                                                      return $ Just (DragPane mb ty delta split)
        | Just ReleaseResources <- fromMessage x = do deleteWindow win
                                                      return $ Just (DragPane (I Nothing) ty delta split)
        -- layout specific messages
        | Just Shrink <- fromMessage x = return $ Just (DragPane mb ty delta (split - delta))
        | Just Expand <- fromMessage x = return $ Just (DragPane mb ty delta (split + delta))
        | Just (SetFrac ident' frac) <- fromMessage x, ident' == ident = do
                                         return $ Just (DragPane mb ty delta frac)
    handleMess _ _ = return Nothing

    handleEvent :: DragPane a -> Event -> X ()
    handleEvent (DragPane (I (Just (win,r,ident))) ty _ _)
                (ButtonEvent {ev_window = thisw, ev_subwindow = thisbw, ev_event_type = t })
        | t == buttonPress && thisw == win || thisbw == win  = do
      mouseDrag (\ex ey -> do
                 let frac = case ty of
                            Vertical   -> (fromIntegral ex - (fromIntegral $ rect_x r))/(fromIntegral $ rect_width  r)
                            Horizontal -> (fromIntegral ey - (fromIntegral $ rect_x r))/(fromIntegral $ rect_width r)
                 sendMessage (SetFrac ident frac))
                (return ())
    handleEvent _ _  = return ()

    doLay :: (Rectangle -> Rectangle) -> DragPane a -> Rectangle -> W.Stack a -> X ([(a, Rectangle)], Maybe (DragPane a))
    doLay mirror (DragPane mw ty delta split) r s = do
      let r' = mirror r
          (left', right') = splitHorizontallyBy split r'
          left = case left' of Rectangle x y w h ->
                                   mirror $ Rectangle x y (w-halfHandleWidth) h
          right = case right' of
                    Rectangle x y w h ->
                        mirror $ Rectangle (x+halfHandleWidth) y (w-halfHandleWidth) h
          handr = case left' of
                    Rectangle x y w h ->
                        mirror $ Rectangle (x + fromIntegral w - halfHandleWidth) y (2*halfHandleWidth) h
          wrs = case reverse (W.up s) of
                  (master:_) -> [(master,left),(W.focus s,right)]
                  [] -> case W.down s of
                          (next:_) -> [(W.focus s,left),(next,right)]
                          [] -> [(W.focus s, r)]
      if length wrs > 1
         then case mw of
                I (Just (w,_,ident)) -> do
                        w' <- deleteWindow w >> newDragWin handr
                        return (wrs, Just $ DragPane (I $ Just (w',r',ident)) ty delta split)
                I Nothing -> do
                        w <- newDragWin handr
                        i <- io $ newUnique
                        return (wrs, Just $ DragPane (I $ Just (w,r',hashUnique i)) ty delta split)
         else return (wrs, Nothing)

    newDragWin :: Rectangle -> X Window
    newDragWin r = do
      let mask = Just $ exposureMask .|. buttonPressMask
      w <- createNewWindow r mask handleColor False
      showWindow  w
      d <- asks display
      liftIO $ lowerWindow d w
      return      w