Add initial implementation of juice stream placement

osu.Game.Rulesets.Catch/Edit/Blueprints/Components/PlacementEditablePath.cs

@@ -0,0 +1,44 @@
+// 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;
+using osu.Game.Rulesets.Catch.Objects;
+using osuTK;
+
+namespace osu.Game.Rulesets.Catch.Edit.Blueprints.Components
+{
+    public class PlacementEditablePath : EditablePath
+    {
+        private JuiceStreamPathVertex originalNewVertex;
+
+        public PlacementEditablePath(Func<float, double> positionToDistance)
+            : base(positionToDistance)
+        {
+        }
+
+        public void AddNewVertex()
+        {
+            var endVertex = Vertices[^1];
+            int index = AddVertex(endVertex.Distance, endVertex.X);
+
+            for (int i = 0; i < VertexCount; i++)
+            {
+                VertexStates[i].IsSelected = i == index;
+                VertexStates[i].VertexBeforeChange = Vertices[i];
+            }
+
+            originalNewVertex = Vertices[index];
+        }
+
+        /// <summary>
+        /// Move the vertex added by <see cref="AddNewVertex"/> in the last time.
+        /// </summary>
+        public void MoveLastVertex(Vector2 screenSpacePosition)
+        {
+            Vector2 position = ToRelativePosition(screenSpacePosition);
+            double distanceDelta = PositionToDistance(position.Y) - originalNewVertex.Distance;
+            float xDelta = position.X - originalNewVertex.X;
+            MoveSelectedVertices(distanceDelta, xDelta);
+        }
+    }
+}