some work on the quick search dialog + image load refactoring

backend/imagemanager.go

@@ -188,6 +188,55 @@ func (i *ImageManager) RefreshCachedArtistImageIfExpired(artistID string, imgURL
 	return err
 }
 
+// ThumbnailLoader is a utility type that exposes a single API to load
+// a cover thumbnail by ID. If the image is immediately available in
+// the cache, OnLoaded will be called immediately. If it is not,
+// OnBeforeLoad will be called first, then OnLoaded will be called async
+// once the image is available.
+// Any subsequent calls to Load will cancel the previous load if not yet completed.
+type ThumbnailLoader struct {
+	prevLoadCancel context.CancelFunc
+	im             *ImageManager
+
+	OnBeforeLoad func()
+	OnLoaded     func(image.Image)
+}
+
+func (i *ImageManager) NewThumbnailLoader(onLoaded func(image.Image)) ThumbnailLoader {
+	return ThumbnailLoader{im: i, OnLoaded: onLoaded}
+}
+
+func (i *ThumbnailLoader) Load(coverID string) {
+	if i.prevLoadCancel != nil {
+		i.prevLoadCancel()
+	}
+	if coverID == "" {
+		i.callOnLoaded(nil)
+		return
+	}
+	if img, ok := i.im.GetCoverThumbnailFromCache(coverID); ok {
+		i.callOnLoaded(img)
+		return
+	}
+	if i.OnBeforeLoad != nil {
+		i.OnBeforeLoad()
+	}
+	i.prevLoadCancel = i.im.GetCoverThumbnailAsync(coverID, func(img image.Image, err error) {
+		if err != nil {
+			log.Printf("Error loading cover image: %s", err.Error())
+		} else {
+			i.callOnLoaded(img)
+		}
+		i.prevLoadCancel() // Done. Release resources associated with un-cancelled ctx
+	})
+}
+
+func (i *ThumbnailLoader) callOnLoaded(im image.Image) {
+	if i.OnLoaded != nil {
+		i.OnLoaded(im)
+	}
+}
+
 func (i *ImageManager) ensureCoverCacheDir() string {
 	// if user logged out with pending fetches in progress,
 	// make sure we don't write to nil (00000000-*0) cache directory