Use the magic of modeling to prevent double-loading images

src/Model.purs

@@ -31,11 +31,13 @@ data ImageError
 
 data ImageData
     = Unloaded String -- file ID of encrypted data
+    | Loading
     | ILoaded String -- blob url of decrypted image
     | IError ImageError
 
 instance showImageData :: Show ImageData where
     show (Unloaded fileID) = "Unloaded Image with fileID " <> fileID
+    show Loading = "Image loading..."
     show (ILoaded url) = "image: " <> url
     show (IError e) = "error loading image: " <> (show e)
 

src/UI.purs

@@ -64,7 +64,8 @@ renderThumbnail pos image =
             maybe [] (backgroundUrl >>> HP.style >>> pure) (image.blurhashUrl)
         )
         case image.thumb of
-            Unloaded _ ->
+            Unloaded _ -> []
+            Loading ->
                 [ HH.span
                     [ HP.class_ $ ClassName "loading-msg"
                     ]
@@ -124,7 +125,8 @@ renderFocused zoom image =
             , HP.class_ $ ClassName "blurhash-frame"
             ]
             case image.full of
-                Unloaded _ ->
+                Unloaded _ -> []
+                Loading ->
                     [ HH.span
                         [ HP.class_ $ ClassName "loading-msg"
                         ]
@@ -218,11 +220,15 @@ update' (Pan right) (GLoaded gal@{images, focus: Just { imageIndex }}) =
     Both
         [ pure $ DownloadImage $ (offset + newImageIndex) ]
         (GLoaded gal{ focus = Just {imageIndex: newImageIndex, zoom: false }})
-update' (DownloadImage indx) (GLoaded { images }) =
+update' (DownloadImage indx) (GLoaded gal@{ images }) =
     let revisedIndex = mod indx (length images)
         maybeImageData = index images revisedIndex in
     case maybeImageData of
-        Just imageData -> Affect [fetchFullAction revisedIndex imageData]
+        Just imageData -> case modifyAt revisedIndex _{full = Loading} images of
+            Just newImages -> Both
+                [fetchFullAction revisedIndex imageData]
+                (GLoaded gal{ images = newImages })
+            Nothing -> Modify $ GError $ UnexpectedError "Valid position asserted by index and module declared invalid by index"
         Nothing -> Modify $ GError $ UnexpectedError "Despite taking the modulo, still invalid index"
 update' _ modl = Modify modl