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我想要一些关于如何在单击“面板”后更新“状态字段”的建议。

下面的程序演示了这个问题。该程序绘制了两个框架。您可以将左框架想象成某种绘图区域，而右框架包含“红色”和“绿色”按钮。单击标记为“红色”的按钮后，状态字段的文本将更新为“当前颜色：红色”。标记为“绿色”的按钮将文本更新为“当前颜色：绿色”。

用户单击左侧面板后如何更改状态字段的文本？例如，将其更改为“您成功单击了绘图面板”。

为什么我不能像在按钮的“on command”中那样在“on click”中执行此操作？（请参阅下面源代码中的注释。）

非常感谢。

module Main where

import Graphics.UI.WX

-- | NOP (= No Operation)
data Command = Nop
             | Red
             | Green
               deriving (Eq)

main :: IO ()
main
  = start hello


hello :: IO ()
hello 
    = do  currentCommand <- varCreate $ Nop               -- current command performed on next click on "pDrawingarea"

          status <- statusField    [text := "Welcome."]

          -- Frames and Panels
          f            <- frame   [ text := "Demo"
                                  , bgcolor := lightgrey ]

          pButtons     <- panel f [ bgcolor := lightgrey]
          pDrawingarea <- panel f [ on paint := draw
                                  , bgcolor := lightgrey
                                  ]

          set pDrawingarea [on click :=  do drawingAreaOnClick status currentCommand pDrawingarea
                                            -- set status [text := "User clicked on the panel."]
                                            -- Problem: uncommenting the line above shows the problem
                           ]

          bRed <- button pButtons [text := "Red",  on command := do varSet currentCommand Red
                                                                    set status [text := "Current color: Red"]
                                 ]

          bGreen <- button pButtons [text := "Green",  on command := do varSet currentCommand Green
                                                                        set status [text := "Current color: Green"]
                                    ]

          set pButtons [ layout := column 1 [ hstretch.expand $ widget bRed
                                            , hstretch.expand $ widget bGreen
                                            ]
                       ]

          set f [ statusBar := [status]
                , layout := row 3 [
                                    minsize (sz 600 500) $ stretch.expand $  widget pDrawingarea
                                  , vstretch.expand $ rule 3 500
                                  , minsize (sz 200 500) $ vstretch.expand $ widget pButtons
                                  ]    
                ]

          return ()

draw ::  DC a -> Rect -> IO ()
draw  dc viewArea
    = do putStrLn "Imagine some code to repaint the screen."


drawingAreaOnClick :: statusField -> Var Command -> Panel () -> Point -> IO ()
drawingAreaOnClick sf command panel pt
    = do c <- varGet command
         case c of 
            Red   -> do putStrLn "Imagine some code to do red painting"
            Green -> do putStrLn "Imagine some code to do green painting"
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1 回答 1

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在这个问题上花了很多时间后，我找到了解决方案。

解决方法是改变定义

drawingAreaOnClick :: statusField -> Var Command -> Panel () -> Point -> IO ()

至

drawingAreaOnClick :: Textual x =>  x -> Var Command -> Panel () -> Point -> IO ()

因为“statusField”本身是“Textual”类的成员，所以我不明白这个问题。

为了完整起见，我会提到我也切换了 GHC 版本。最初的问题发生在 GHC 7.8.4 上，我找到的解决方案适用于 GHC 7.10.3。我不能说 GHC 版本是否会影响问题。

供参考完整的工作代码：

module Main where

import Graphics.UI.WX

-- | NOP (= No Operation)
data Command = Nop
             | Red
             | Green
               deriving (Eq)

main :: IO ()
main
  = start hello


hello :: IO ()
hello 
    = do  currentCommand <- varCreate Nop               -- current command performed on next click on "pDrawingarea"


          status <- statusField    [text := "Welcome."]

          -- not needed:     currentStatus <- varCreate status


          -- Frames and Panels
          f            <- frame   [ text := "Demo"
                                  , bgcolor := lightgrey ]

          pButtons     <- panel f [ bgcolor := lightgrey]
          pDrawingarea <- panel f [ on paint := draw
                                  , bgcolor := lightgrey
                                  ]

          set pDrawingarea [on click :=  do drawingAreaOnClick status currentCommand pDrawingarea
                                            -- set status [text := "User clicked on the panel."]
                                            -- Problem: uncommenting the line above shows the problem
                           ]

          bRed <- button pButtons [text := "Red",  on command := do varSet currentCommand Red
                                                                    set status [text := "Current color: Red"]
                                 ]

          bGreen <- button pButtons [text := "Green",  on command := do varSet currentCommand Green
                                                                        set status [text := "Current color: Green"]
                                                                        --sf <- varGet currentStatus
                                                                        -- set sf [text := "yyy"]

                                    ]

          set pButtons [ layout := column 1 [ hstretch.expand $ widget bRed
                                            , hstretch.expand $ widget bGreen
                                            ]
                       ]

          set f [ statusBar := [status]
                , layout := row 3 [
                                    minsize (sz 600 500) $ stretch.expand $  widget pDrawingarea
                                  , vstretch.expand $ rule 3 500
                                  , minsize (sz 200 500) $ vstretch.expand $ widget pButtons
                                  ]    
                ]

          return ()

draw ::  DC a -> Rect -> IO ()
draw  dc viewArea
    = do putStrLn "Imagine some code to repaint the screen."


drawingAreaOnClick ::  Textual x =>  x -> Var Command -> Panel () -> Point -> IO ()
drawingAreaOnClick sf command panel pt
    = do c <- varGet command
         set sf [text := "Drawing on the screen."]
         case c of 
            Red   -> do putStrLn "Imagine some code to do red painting"
            Green -> do putStrLn "Imagine some code to do green painting"
于 2016-10-11T09:37:12.633 回答