Preload images adjacent to the currently viewed image

spago.dhall

@@ -34,6 +34,7 @@ to generate this file without the comments in this block.
   , "strings"
   , "subtlecrypto"
   , "transformers"
+  , "tuples"
   , "web-file"
   , "web-html"
   , "web-uievents"

src/UI.purs

@@ -10,8 +10,9 @@ import Aviary.Model ( GalleryError(..)
                     )
 
 import Control.Parallel (parSequence_)
-import Data.Array (index, length, mapWithIndex, modifyAt)
+import Data.Array (index, length, mapMaybe, mapWithIndex, modifyAt)
 import Data.Maybe (fromMaybe, maybe, Maybe(..))
+import Data.Tuple (uncurry, Tuple(..))
 import Effect.Aff (Aff)
 import Halogen as H
 import Halogen.HTML as HH
@@ -33,7 +34,7 @@ data Event = LoadThumbs
     | Zoom
     | Unzoom
     | Pan Boolean -- True if right
-    | DownloadFocused
+    | DownloadImages (Array Int)
     | Init
     | RegisterListeners
 
@@ -174,6 +175,10 @@ eventByKey ev = case KE.key ev of
     "1" -> Just $ Focus 0
     _ -> Nothing
 
+focusedIndex :: Model -> Maybe Int
+focusedIndex (GLoaded { focus: Just { imageIndex } }) = Just imageIndex
+focusedIndex _ = Nothing
+
 update :: Event -> H.HalogenM Model Event () Event Aff Unit
 update Init = update RegisterListeners <> update LoadThumbs
 update RegisterListeners = do
@@ -194,7 +199,8 @@ update (Focus imageIndex) =  do
     _ <- H.modify_ \model -> case model of
         GError e -> GError e
         GLoaded gal -> GLoaded gal{ focus = Just { imageIndex, zoom: false } }
-    update DownloadFocused
+    _ <- update $ DownloadImages [imageIndex]
+    update $ DownloadImages [imageIndex - 1, imageIndex + 1]
 update Unfocus = H.modify_ \model -> case model of
     GError e -> GError e
     GLoaded gal -> GLoaded gal { focus = Nothing }
@@ -204,25 +210,28 @@ update Zoom = H.modify_ \model -> case model of
 update Unzoom = H.modify_ \model -> case model of
     GError e -> GError e
     GLoaded gal -> GLoaded gal { focus = gal.focus <#> _{ zoom = false } }
-update (Pan right) = do
+update (Pan right) = let offset = if right then 1 else -1 in do
     _ <- H.modify_ \model -> case model of
         GError e -> GError e
         GLoaded gal -> GLoaded gal
             { focus = gal.focus <#> \foc ->
                 foc{ imageIndex =
-                    mod (foc.imageIndex + (if right then 1 else -1)) (length gal.images)
+                    mod (foc.imageIndex + offset) (length gal.images)
                 }
             }
-    update DownloadFocused
-update DownloadFocused = do
+    focus <- H.get <#> focusedIndex
+    let generateIndicies :: Int -> Array Int
+        generateIndicies = pure >>> (<*>) [identity, (+) offset]
+    let maybeEvent = focus <#> generateIndicies <#> DownloadImages
+    maybe (pure unit) update maybeEvent
+update (DownloadImages indicies) = do
     model <- H.get
     case model of
-        GLoaded {images, focus: Just {imageIndex}} ->
-            let focusedImage = index images imageIndex
-            in case focusedImage of
-                Just focusedImage' -> fetchFullAction imageIndex focusedImage'
-                Nothing ->
-                    H.put $ GError $ UnexpectedError "Focus event raised with an out of bounds index!"
+        GLoaded { images } ->
+            parSequence_ $ indicies
+                # (<$>) (\i -> mod i (length images))
+                # mapMaybe (\i -> index images i <#> Tuple i)
+                <#> uncurry fetchFullAction
         _ -> pure unit