Add basic osu! object to object snapping

osu.Game.Rulesets.Osu/Edit/OsuHitObjectComposer.cs

@@ -15,6 +15,7 @@
 using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Objects;
 using osu.Game.Rulesets.Objects.Drawables;
+using osu.Game.Rulesets.Objects.Types;
 using osu.Game.Rulesets.Osu.Objects;
 using osu.Game.Rulesets.UI;
 using osu.Game.Screens.Edit.Compose.Components;
@@ -94,6 +95,10 @@ protected override void Update()
 
         public override SnapResult SnapScreenSpacePositionToValidTime(Vector2 screenSpacePosition)
         {
+            if (snapToVisibleBlueprints(screenSpacePosition, out var snapResult))
+                return snapResult;
+
+            // will be null if distance snap is disabled or not feasible for the current time value.
             if (distanceSnapGrid == null)
                 return base.SnapScreenSpacePositionToValidTime(screenSpacePosition);
 
@@ -102,6 +107,42 @@ public override SnapResult SnapScreenSpacePositionToValidTime(Vector2 screenSpac
             return new SnapResult(distanceSnapGrid.ToScreenSpace(pos), time, PlayfieldAtScreenSpacePosition(screenSpacePosition));
         }
 
+        private bool snapToVisibleBlueprints(Vector2 screenSpacePosition, out SnapResult snapResult)
+        {
+            // check other on-screen objects for snapping/stacking
+            var blueprints = BlueprintContainer.SelectionBlueprints.AliveChildren;
+
+            var playfield = PlayfieldAtScreenSpacePosition(screenSpacePosition);
+
+            float snapRadius =
+                playfield.GamefieldToScreenSpace(new Vector2(OsuHitObject.OBJECT_RADIUS / 5)).X -
+                playfield.GamefieldToScreenSpace(Vector2.Zero).X;
+
+            foreach (var b in blueprints)
+            {
+                if (b.IsSelected)
+                    continue;
+
+                var hitObject = b.HitObject;
+
+                Vector2 startPos = ((IHasPosition)hitObject).Position;
+
+                Vector2 objectScreenPos = playfield.GamefieldToScreenSpace(startPos);
+
+                if (Vector2.Distance(objectScreenPos, screenSpacePosition) < snapRadius)
+                {
+                    // bypasses time snapping
+                    {
+                        snapResult = new SnapResult(objectScreenPos, null, playfield);
+                        return true;
+                    }
+                }
+            }
+
+            snapResult = null;
+            return false;
+        }
+
         private void updateDistanceSnapGrid()
         {
             distanceSnapGridContainer.Clear();

osu.Game/Screens/Edit/Compose/Components/BlueprintContainer.cs

@@ -30,7 +30,7 @@ public abstract class BlueprintContainer : CompositeDrawable, IKeyBindingHandler
     {
         protected DragBox DragBox { get; private set; }
 
-        protected Container<SelectionBlueprint> SelectionBlueprints { get; private set; }
+        public Container<SelectionBlueprint> SelectionBlueprints { get; private set; }
 
         private SelectionHandler selectionHandler;