Add BeatmapSetDownloadButton.

osu.Game/Graphics/UserInterface/BeatmapSetDownloadButton.cs

@@ -0,0 +1,83 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System;
+using System.Linq;
+using osu.Framework.Allocation;
+using osu.Framework.Configuration;
+using osu.Framework.Input;
+using osu.Game.Beatmaps;
+using osu.Game.Graphics.Containers;
+using osu.Game.Online.API;
+
+namespace osu.Game.Graphics.UserInterface
+{
+    public abstract class BeatmapSetDownloadButton : OsuClickableContainer
+    {
+        private readonly BeatmapSetInfo set;
+        private readonly bool noVideo;
+        private readonly BindableBool downloaded = new BindableBool();
+
+        private Action action;
+        public Action Action
+        {
+            get => action;
+            set => action = value;
+        }
+
+        protected BeatmapSetDownloadButton(BeatmapSetInfo set, bool noVideo = false)
+        {
+            this.set = set;
+            this.noVideo = noVideo;
+
+            downloaded.ValueChanged += e =>
+            {
+                if (e)
+                    Disable();
+                else
+                    Enable();
+            };
+        }
+
+        [BackgroundDependencyLoader]
+        private void load(BeatmapManager beatmaps, APIAccess api)
+        {
+            beatmaps.ItemAdded += s =>
+            {
+                if (s.OnlineBeatmapSetID == set.OnlineBeatmapSetID)
+                    downloaded.Value = true;
+            };
+
+            beatmaps.ItemRemoved += s =>
+            {
+                if (s.OnlineBeatmapSetID == set.OnlineBeatmapSetID)
+                    downloaded.Value = false;
+            };
+
+            // initial downloaded value
+            downloaded.Value = beatmaps.QueryBeatmapSets(s => s.OnlineBeatmapSetID == set.OnlineBeatmapSetID && !s.DeletePending).Count() != 0;
+
+            Action = () =>
+            {
+                if (beatmaps.GetExistingDownload(set) != null)
+                {
+                    AlreadyDownloading();
+                    return;
+                }
+
+                beatmaps.Download(set, noVideo);
+            };
+        }
+
+        protected override bool OnClick(InputState state)
+        {
+            if (!downloaded.Value)
+                Action?.Invoke();
+            return true;
+        }
+
+        protected abstract void Enable();
+        protected abstract void Disable();
+        protected abstract void AlreadyDownloading();
+    }
+}