Unity UI Image that stretch sprite to fit in RectTransform in Simple Image Type with preserve aspect toggled

PreserveAspectEnvelopImage.cs

using UnityEngine.Sprites;

namespace UnityEngine.UI {
	public class PreserveAspectEnvelopImage : Image {
		protected override void OnPopulateMesh(VertexHelper toFill) {
			if (type == Type.Simple && !useSpriteMesh && preserveAspect) {
				GenerateSimpleSprite(toFill);
			} else {
				base.OnPopulateMesh(toFill);
			}
		}

		void GenerateSimpleSprite(VertexHelper vh) {
			Vector4 v = GetDrawingDimensions();
			var activeSprite = overrideSprite;
			var uv = (activeSprite != null) ? DataUtility.GetOuterUV(activeSprite) : Vector4.zero;

			if (activeSprite) {
				var size = activeSprite.rect.size;
				var ratio = size.x / size.y;
				var rectSize = new Vector2(v.z - v.x, v.w - v.y);
				var rectRatio = rectSize.x / rectSize.y;
				if (rectRatio > ratio) {
					// Fixed Width
					var multiplier = ratio / rectRatio;
					var pivot = Mathf.Clamp(rectTransform.pivot.y, uv.y, uv.w);
					uv.y = pivot - (pivot - uv.y) * multiplier;
					uv.w = (uv.w - pivot) * multiplier + pivot;
				} else if (rectRatio < ratio) {
					// Fixed Height
					var multiplier = rectRatio / ratio;
					var pivot = Mathf.Clamp(rectTransform.pivot.x, uv.x, uv.z);
					uv.x = pivot - (pivot - uv.x) * multiplier;
					uv.z = (uv.z - pivot) * multiplier + pivot;
				}
			}

			Color32 color32 = color;
			vh.Clear();
			vh.AddVert(new Vector3(v.x, v.y), color32, new Vector2(uv.x, uv.y));
			vh.AddVert(new Vector3(v.x, v.w), color32, new Vector2(uv.x, uv.w));
			vh.AddVert(new Vector3(v.z, v.w), color32, new Vector2(uv.z, uv.w));
			vh.AddVert(new Vector3(v.z, v.y), color32, new Vector2(uv.z, uv.y));

			vh.AddTriangle(0, 1, 2);
			vh.AddTriangle(2, 3, 0);
		}

		private Vector4 GetDrawingDimensions() {
			var activeSprite = overrideSprite;
			var padding = activeSprite == null ? Vector4.zero : DataUtility.GetPadding(activeSprite);
			var size = activeSprite == null ? Vector2.zero : activeSprite.rect.size;

			Rect r = GetPixelAdjustedRect();

			int spriteW = Mathf.RoundToInt(size.x);
			int spriteH = Mathf.RoundToInt(size.y);

			var v = new Vector4(
				padding.x / spriteW,
				padding.y / spriteH,
				(spriteW - padding.z) / spriteW,
				(spriteH - padding.w) / spriteH);

			v = new Vector4(
				r.x + r.width * v.x,
				r.y + r.height * v.y,
				r.x + r.width * v.z,
				r.y + r.height * v.w
			);

			return v;
		}

#if UNITY_EDITOR
		static UnityEditor.MonoScript _script;
		[UnityEditor.MenuItem("CONTEXT/Image/Replace as Preserve Aspect Envelop Image")]
		static void ReplaceFromBuiltInImage(UnityEditor.MenuCommand command) {
			if (!_script) {
				var tmpGO = new GameObject("tempOBJ");
				var inst = tmpGO.AddComponent<PreserveAspectEnvelopImage>();
				_script = UnityEditor.MonoScript.FromMonoBehaviour(inst);
				DestroyImmediate(tmpGO);
			}
			var go = ((Component)command.context).gameObject;
			UnityEditor.Undo.RegisterCompleteObjectUndo(go, string.Format("Replace Image as Preserve Aspect Envelop Image in {0}", go.name));
			UnityEditor.SerializedObject so = new UnityEditor.SerializedObject(command.context);
			UnityEditor.SerializedProperty scriptProperty = so.FindProperty("m_Script");
			so.Update();
			scriptProperty.objectReferenceValue = _script;
			so.ApplyModifiedProperties();
		}
#endif
	}
}