fix race condition with async image fetches

ui/albumcard.go

@@ -1,6 +1,7 @@
 package ui
 
 import (
+	"context"
 	"image"
 	"log"
 
@@ -39,9 +40,15 @@ type AlbumCard struct {
 	albumID   string
 	title     *widget.Label
 	artist    *widget.Label
-	Cover     *TappableImage
 	container *fyne.Container
 
+	// updated by AlbumGrid
+	Cover *TappableImage
+
+	// these fields are used by AlbumGrid to track async update tasks
+	PrevAlbumID   string
+	ImgLoadCancel context.CancelFunc
+
 	OnTapped func()
 }
 

ui/albumgrid.go

@@ -1,6 +1,7 @@
 package ui
 
 import (
+	"context"
 	"gomuse/backend"
 
 	"fyne.io/fyne/v2"
@@ -41,15 +42,33 @@ func NewAlbumGrid(iter backend.AlbumIterator, pm *backend.PlaybackManager, im *b
 		// update func
 		func(itemID int, obj fyne.CanvasObject) {
 			ac := obj.(*AlbumCard)
-			ac.Update(ag.albums[itemID])
+			album := ag.albums[itemID]
+			if ac.PrevAlbumID == album.ID {
+				// nothing to do
+				return
+			}
+			ac.Update(album)
+			ac.PrevAlbumID = album.ID
 			// TODO: set image to a placeholder before spinning off async fetch
-			go func() {
+			// cancel any previous image fetch
-				i, err := im.GetAlbumThumbnail(ag.albums[itemID].ID)
+			if ac.ImgLoadCancel != nil {
+				ac.ImgLoadCancel()
+				ac.ImgLoadCancel = nil
+			}
+			ctx, cancel := context.WithCancel(context.Background())
+			go func(ctx context.Context) {
+				i, err := im.GetAlbumThumbnail(album.ID)
+				select {
+				case <-ctx.Done():
+					return
+				default:
 					if err == nil {
 						ac.Cover.SetImage(i)
 						ac.Refresh()
 					}
-			}()
+				}
+			}(ctx)
+			ac.ImgLoadCancel = cancel
 
 			// TODO: remove magic number 10
 			if !ag.done && !ag.fetching && itemID > len(ag.albums)-10 {

ui/nowplayingcard.go

@@ -37,7 +37,7 @@ func NewNowPlayingCard() *NowPlayingCard {
 	n.cover.SetMinSize(fyne.NewSize(100, 100))
 	n.cover.FillMode = canvas.ImageFillContain
 
-	n.c = container.NewBorder(nil, nil, n.cover, nil, container.New(&layout.VboxCustomPadding{-10}, n.trackName, n.artistName, n.albumName))
+	n.c = container.NewBorder(nil, nil, n.cover, nil, container.New(&layout.VboxCustomPadding{ExtraPad: -10}, n.trackName, n.artistName, n.albumName))
 	return n
 }