Add API for loading meshes on their own

UnityGLTF/Assets/UnityGLTF/Scripts/GLTFSceneImporter.cs

@@ -330,38 +330,11 @@ public IEnumerator LoadScene(int sceneIndex = -1, bool showSceneObj = true, Acti
 		/// <returns></returns>
 		public async Task LoadNodeAsync(int nodeIndex, CancellationToken cancellationToken)
 		{
-			try
+			await SetupLoad(async () =>
 			{
-				lock (this)
-				{
-					if (_isRunning)
-					{
-						throw new GLTFLoadException("Cannot call LoadNode while GLTFSceneImporter is already running");
-					}
-
-					_isRunning = true;
-				}
-
-				if (_gltfRoot == null)
-				{
-					await LoadJson(_gltfFileName);
-				}
-
-				if (_assetCache == null)
-				{
-					_assetCache = new AssetCache(_gltfRoot);
-				}
-
 				CreatedObject = await GetNode(nodeIndex, cancellationToken);
 				InitializeGltfTopLevelObject();
-			}
-			finally
-			{
-				lock (this)
-				{
-					_isRunning = false;
-				}
-			}
+			});
 		}
 
 		/// <summary>
@@ -371,49 +344,45 @@ public async Task LoadNodeAsync(int nodeIndex, CancellationToken cancellationTok
 		/// <returns></returns>
 		public virtual async Task<Material> LoadMaterialAsync(int materialIndex)
 		{
-			try
+			await SetupLoad(async () =>
 			{
-				lock (this)
-				{
-					if (_isRunning)
-					{
-						throw new GLTFLoadException("Cannot CreateTexture while GLTFSceneImporter is already running");
-					}
-
-					_isRunning = true;
-				}
-
-				if (_gltfRoot == null)
-				{
-					await LoadJson(_gltfFileName);
-				}
-
 				if (materialIndex < 0 || materialIndex >= _gltfRoot.Materials.Count)
 				{
 					throw new ArgumentException($"There is no material for index {materialIndex}");
 				}
 
-				if (_assetCache == null)
-				{
-					_assetCache = new AssetCache(_gltfRoot);
-				}
-
 				if (_assetCache.MaterialCache[materialIndex] == null)
 				{
 					var def = _gltfRoot.Materials[materialIndex];
 					await ConstructMaterialImageBuffers(def);
 					await ConstructMaterial(def, materialIndex);
 				}
-			}
-			finally
+			});
+			return _assetCache.MaterialCache[materialIndex].UnityMaterialWithVertexColor;
+		}
+
+		/// <summary>
+		/// Load a Mesh from the glTF by index
+		/// </summary>
+		/// <param name="meshIndex"></param>
+		/// <returns></returns>
+		public virtual async Task<Mesh> LoadMeshAsync(int meshIndex, CancellationToken cancellationToken)
+		{
+			await SetupLoad(async () =>
 			{
-				lock (this)
+				if (meshIndex < 0 || meshIndex >= _gltfRoot.Meshes.Count)
 				{
-					_isRunning = false;
+					throw new ArgumentException($"There is no mesh for index {meshIndex}");
 				}
-			}
 
-			return _assetCache.MaterialCache[materialIndex].UnityMaterialWithVertexColor;
+				if (_assetCache.MeshCache[meshIndex] == null)
+				{
+					var def = _gltfRoot.Meshes[meshIndex];
+					await ConstructMeshAttributes(def, new MeshId() { Id = meshIndex, Root = _gltfRoot });
+					await ConstructMesh(def, meshIndex, cancellationToken);
+				}
+			});
+			return _assetCache.MeshCache[meshIndex].LoadedMesh;
 		}
 
 		/// <summary>
@@ -1589,7 +1558,7 @@ protected virtual async Task ConstructMesh(GLTFMesh mesh, int meshIndex, Cancell
 
 			if (_assetCache.MeshCache[meshIndex] == null)
 			{
-				throw new Exception("Cannot generate mesh before ConstructBufferData is called!");
+				throw new Exception("Cannot generate mesh before ConstructMeshAttributes is called!");
 			}
 
 			var totalVertCount = mesh.Primitives.Aggregate((uint)0, (sum, p) => sum + p.Attributes[SemanticProperties.POSITION].Value.Count);
@@ -2107,6 +2076,11 @@ public virtual async Task LoadTextureAsync(GLTFTexture texture, int textureIndex
 					_isRunning = true;
 				}
 
+				if (_options.ThrowOnLowMemory)
+				{
+					_memoryChecker = new MemoryChecker();
+				}
+
 				if (_gltfRoot == null)
 				{
 					await LoadJson(_gltfFileName);
@@ -2340,5 +2314,49 @@ private void Cleanup()
 				_assetCache = null;
 			}
 		}
+
+		private async Task SetupLoad(Func<Task> callback)
+		{
+			try
+			{
+				lock (this)
+				{
+					if (_isRunning)
+					{
+						throw new GLTFLoadException("Cannot start a load while GLTFSceneImporter is already running");
+					}
+
+					_isRunning = true;
+				}
+
+				if (_options.ThrowOnLowMemory)
+				{
+					_memoryChecker = new MemoryChecker();
+				}
+
+				if (_gltfRoot == null)
+				{
+					await LoadJson(_gltfFileName);
+				}
+
+				if (_assetCache == null)
+				{
+					_assetCache = new AssetCache(_gltfRoot);
+				}
+
+				await callback();
+			}
+			catch
+			{
+				Cleanup();
+			}
+			finally
+			{
+				lock (this)
+				{
+					_isRunning = false;
+				}
+			}
+		}
 	}
 }