如何在Gtk2hs Cairo中检查渲染状态是否保存，以防止在Haskell中调整窗口大小时缩放图像

Question

我正试图在gtk2hs-cairo中绘制一些简单的图像。我知道您可以使用save命令保存当前状态，并使用restore命令恢复。是否有一种方法可以检查当前是否保存了状态。我不希望我的图像在调整窗口大小时缩放，或者有更好的方法来防止调整大小。我不想每次调整窗口大小时都重新计算图像。

Answer 1

save和restore操作实际上与图像是否重新计算无关。然而，gtk2hs中包含了一个演示，展示了如何缓存执行Cairo操作的结果，请参阅cairo/demo/Clock.hs，尤其是main:的第320-404行

  let redrawStaticLayers = do
        (width, height) <- widgetGetSize window
        drawWin <- widgetGetDrawWindow window
        background <- createImageSurface FormatARGB32 width height
        foreground <- createImageSurface FormatARGB32 width height
        let clear = do
              save
              setOperator OperatorClear
              paint
              restore
        renderWith background $ do
          clear
          drawClockBackground True width height
        renderWith foreground $ do
          clear
          drawClockForeground True width height
        writeIORef backgroundRef (Just background)
        writeIORef foregroundRef (Just foreground)
  onRealize window redrawStaticLayers
  sizeRef <- newIORef (initialSize, initialSize)
  timeoutHandlerRef <- newIORef Nothing
  window `on` configureEvent $ do
    (w,h) <- eventSize
    liftIO $ do
    size <- readIORef sizeRef
    writeIORef sizeRef (w,h)
    when (size /= (w,h)) $ do
      background <- readIORef backgroundRef
      foreground <- readIORef foregroundRef
      maybe (return ()) surfaceFinish background
      maybe (return ()) surfaceFinish foreground
      writeIORef backgroundRef Nothing
      writeIORef foregroundRef Nothing
      timeoutHandler <- readIORef timeoutHandlerRef
      maybe (return ()) timeoutRemove timeoutHandler
      handler <- timeoutAddFull (do
        writeIORef timeoutHandlerRef Nothing
        redrawStaticLayers
        widgetQueueDraw window
        return False
        ) priorityDefaultIdle 300
      writeIORef timeoutHandlerRef (Just handler)
    return False
  window `on` exposeEvent $ do
    drawWin <- eventWindow
    exposeRegion <- eventRegion
    liftIO $ do
    (width, height) <- drawableGetSize drawWin
    background <- readIORef backgroundRef
    foreground <- readIORef foregroundRef
    renderWithDrawable drawWin $ do
      region exposeRegion
      clip
      save
      setOperator OperatorSource
      setSourceRGBA 0 0 0 0
      paint
      restore
      case background of
        Nothing -> drawClockBackground False width height
        Just background -> do
          setSourceSurface background 0 0
          paint
      drawClockHands (isJust background) width height
      case foreground of
        Nothing -> drawClockForeground False width height
        Just foreground -> do
          setSourceSurface foreground 0 0
          paint
    return True