add TracklistLoader for infinite scrolling tracklist

ui/widgets/tracklist.go

@@ -67,6 +67,7 @@ type Tracklist struct {
 	OnShowAlbumPage  func(albumID string)
 
 	OnColumnVisibilityMenuShown func(*widget.PopUp)
+	OnTrackShown                func(tracknum int)
 
 	visibleColumns []bool
 
@@ -113,6 +114,9 @@ func NewTracklist(tracks []*subsonic.Child) *Tracklist {
 				i = itemID + 1
 			}
 			tr.Update(t.TrackAt(itemID), itemID == t.nowPlayingIdx, i)
+			if t.OnTrackShown != nil {
+				t.OnTrackShown(itemID)
+			}
 		})
 	t.container = container.NewBorder(t.hdr, nil, nil, nil, t.list)
 	return t
@@ -203,6 +207,20 @@ func (t *Tracklist) IncrementPlayCount(trackID string) {
 	}
 }
 
+// Remove all tracks from the tracklist. Thread-safe.
+func (t *Tracklist) Clear() {
+	t.tracksMutex.Lock()
+	defer t.tracksMutex.Unlock()
+	t.Tracks = nil
+}
+
+// Append more tracks to the tracklist. Thread-safe.
+func (t *Tracklist) AppendTracks(trs []*subsonic.Child) {
+	t.tracksMutex.Lock()
+	defer t.tracksMutex.Unlock()
+	t.Tracks = append(t.Tracks, trs...)
+}
+
 func (t *Tracklist) SelectAll() {
 	t.selectionMgr.SelectAll()
 	t.list.Refresh()

ui/widgets/tracklistloader.go

@@ -0,0 +1,67 @@
+package widgets
+
+import (
+	"log"
+	"supersonic/backend"
+
+	"github.com/dweymouth/go-subsonic/subsonic"
+)
+
+// Component that manages lazily loading more tracks into a Tracklist
+// as the user scrolls near the bottom.
+type TracklistLoader struct {
+	tracklist *Tracklist
+	iter      backend.TrackIterator
+
+	trackBuffer  []*subsonic.Child
+	fetching     bool
+	done         bool
+	len          int
+	highestShown int
+}
+
+func NewTracklistLoader(tracklist *Tracklist, iter backend.TrackIterator) TracklistLoader {
+	t := TracklistLoader{
+		tracklist: tracklist,
+		iter:      iter,
+	}
+	t.tracklist.OnTrackShown = t.onTrackShown
+	t.fetching = true
+	go t.loadMoreTracks(25)
+	return t
+}
+
+func (t *TracklistLoader) onTrackShown(tracknum int) {
+	if tracknum > t.highestShown {
+		t.highestShown = tracknum
+	}
+	if t.highestShown >= t.len-25 && !t.fetching && !t.done {
+		t.fetching = true
+		go t.loadMoreTracks(25)
+	}
+}
+
+func (t *TracklistLoader) loadMoreTracks(num int) {
+	// repeat fetch task as long as user has scrolled near bottom
+	for !t.done && t.highestShown >= t.len-25 {
+		log.Println("fetching more tracks")
+		if t.trackBuffer == nil {
+			t.trackBuffer = make([]*subsonic.Child, 0, num)
+		}
+		t.trackBuffer = t.trackBuffer[:0]
+		for i := 0; i < num; i++ {
+			tr := t.iter.Next()
+			if tr == nil {
+				t.done = true
+				t.trackBuffer = nil
+				break
+			}
+			t.trackBuffer = append(t.trackBuffer, tr)
+		}
+		t.tracklist.AppendTracks(t.trackBuffer)
+		t.tracklist.Refresh()
+		log.Printf("appended %d tracks", len(t.trackBuffer))
+		t.len += len(t.trackBuffer)
+	}
+	t.fetching = false
+}