Replace checks with realistic ones

osu.Game/Rulesets/Edit/Checks/CheckBackground.cs

@@ -0,0 +1,57 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System.Collections.Generic;
+using System.Linq;
+using osu.Game.Beatmaps;
+using osu.Game.Rulesets.Edit.Checks.Components;
+
+namespace osu.Game.Rulesets.Edit.Checks
+{
+    public class CheckBackground : BeatmapCheck
+    {
+        public override CheckMetadata Metadata() => new CheckMetadata
+        (
+            category: CheckMetadata.CheckCategory.Resources,
+            description: "Missing background."
+        );
+
+        public override IEnumerable<IssueTemplate> Templates() => new[]
+        {
+            templateNoneSet,
+            templateDoesNotExist
+        };
+
+        private readonly IssueTemplate templateNoneSet = new IssueTemplate
+        (
+            type: IssueTemplate.IssueType.Problem,
+            unformattedMessage: "No background has been set."
+        );
+
+        private readonly IssueTemplate templateDoesNotExist = new IssueTemplate
+        (
+            type: IssueTemplate.IssueType.Problem,
+            unformattedMessage: "The background file \"{0}\" is does not exist."
+        );
+
+        public override IEnumerable<Issue> Run(IBeatmap beatmap)
+        {
+            if (beatmap.Metadata.BackgroundFile == null)
+            {
+                yield return new Issue(templateNoneSet);
+
+                yield break;
+            }
+
+            // If the background is set, also make sure it still exists.
+
+            var set = beatmap.BeatmapInfo.BeatmapSet;
+            var file = set.Files.FirstOrDefault(f => f.Filename == beatmap.Metadata.BackgroundFile);
+
+            if (file != null)
+                yield break;
+
+            yield return new Issue(templateDoesNotExist, beatmap.Metadata.BackgroundFile);
+        }
+    }
+}