Add SpawnIfUnobstructed IEntityManager extension method (#3165)

* Add SpawnIfUnobstructed IEntityManager extension method

* Clarify test comments

* Use GetCollidingEntities

* Enable nullable in test

Content.IntegrationTests/Tests/Utility/EntitySystemExtensionsTest.cs

@@ -0,0 +1,82 @@
+#nullable enable
+using System.Threading.Tasks;
+using Content.Shared.Physics;
+using Content.Shared.Utility;
+using NUnit.Framework;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Map;
+
+namespace Content.IntegrationTests.Tests.Utility
+{
+    [TestFixture]
+    [TestOf(typeof(EntitySystemExtensions))]
+    public class EntitySystemExtensionsTest : ContentIntegrationTest
+    {
+        private const string BlockerDummyId = "BlockerDummy";
+
+        private static readonly string Prototypes = $@"
+- type: entity
+  id: {BlockerDummyId}
+  name: {BlockerDummyId}
+  components:
+  - type: Physics
+    shapes:
+    - !type:PhysShapeAabb
+      bounds: ""-0.49,-0.49,0.49,0.49""
+      mask:
+      - Impassable
+";
+
+        [Test]
+        public async Task Test()
+        {
+            var serverOptions = new ServerContentIntegrationOption {ExtraPrototypes = Prototypes};
+            var server = StartServer(serverOptions);
+
+            await server.WaitIdleAsync();
+
+            var sMapManager = server.ResolveDependency<IMapManager>();
+            var sEntityManager = server.ResolveDependency<IEntityManager>();
+
+            await server.WaitAssertion(() =>
+            {
+                var grid = sMapManager.GetGrid(new GridId(1));
+                var entityCoordinates = new EntityCoordinates(grid.GridEntityId, 0, 0);
+
+                // Nothing blocking it, only entity is the grid
+                Assert.NotNull(sEntityManager.SpawnIfUnobstructed(null, entityCoordinates, CollisionGroup.Impassable));
+                Assert.True(sEntityManager.TrySpawnIfUnobstructed(null, entityCoordinates, CollisionGroup.Impassable, out var entity));
+                Assert.NotNull(entity);
+
+                var mapId = new MapId(1);
+                var mapCoordinates = new MapCoordinates(0, 0, mapId);
+
+                // Nothing blocking it, only entity is the grid
+                Assert.NotNull(sEntityManager.SpawnIfUnobstructed(null, mapCoordinates, CollisionGroup.Impassable));
+                Assert.True(sEntityManager.TrySpawnIfUnobstructed(null, mapCoordinates, CollisionGroup.Impassable, out entity));
+                Assert.NotNull(entity);
+
+                // Spawn a blocker with an Impassable mask
+                sEntityManager.SpawnEntity(BlockerDummyId, entityCoordinates);
+
+                // Cannot spawn something with an Impassable layer
+                Assert.Null(sEntityManager.SpawnIfUnobstructed(null, entityCoordinates, CollisionGroup.Impassable));
+                Assert.False(sEntityManager.TrySpawnIfUnobstructed(null, entityCoordinates, CollisionGroup.Impassable, out entity));
+                Assert.Null(entity);
+
+                Assert.Null(sEntityManager.SpawnIfUnobstructed(null, mapCoordinates, CollisionGroup.Impassable));
+                Assert.False(sEntityManager.TrySpawnIfUnobstructed(null, mapCoordinates, CollisionGroup.Impassable, out entity));
+                Assert.Null(entity);
+
+                // Other layers are fine
+                Assert.NotNull(sEntityManager.SpawnIfUnobstructed(null, entityCoordinates, CollisionGroup.MobImpassable));
+                Assert.True(sEntityManager.TrySpawnIfUnobstructed(null, entityCoordinates, CollisionGroup.MobImpassable, out entity));
+                Assert.NotNull(entity);
+
+                Assert.NotNull(sEntityManager.SpawnIfUnobstructed(null, mapCoordinates, CollisionGroup.MobImpassable));
+                Assert.True(sEntityManager.TrySpawnIfUnobstructed(null, mapCoordinates, CollisionGroup.MobImpassable, out entity));
+                Assert.NotNull(entity);
+            });
+        }
+    }
+}