FIX: Draco decoding in GLTFLoader

 - Added a GLTFParser.setDependency() method to be able to store a Promise to a dependency in the GLTFRegistry cache. This allows the Draco decoder extension to asynchronously decode the compressed meshes and attributes, without changing anything in the rest of the GLTF parsing code.
 - Update TODO comments in Draco extension, as additional uncompressed attributes is now supported, and passing primitive.attributes to DRACOLoader is not necessary with the new design (from previous commits).

lib/GLTFLoader.js

@@ -486,9 +486,7 @@ THREE.GLTFLoader = ( function () {
 	 * Specification: https://github.com/KhronosGroup/glTF/pull/874
 	 *
 	 * TODO:
-	 * - Support additional uncompressed attributes alongside those provided by Draco.
-	 * - Support fallback to uncompressed data if decoder is not unavailable.
-	 * - `primitive.attributes` should be passed to DRACOLoader, but isn't yet supported.
+	 * - Support fallback to uncompressed data if decoder is not available.
 	 */
 	function GLTFDracoMeshCompressionExtension ( dracoLoader ) {
 
@@ -502,24 +500,22 @@ THREE.GLTFLoader = ( function () {
 		this.dracoLoader = dracoLoader;
 	}
 
-	GLTFDracoMeshCompressionExtension.prototype.decodeAccessors = function ( parser, accessors ) {
+	GLTFDracoMeshCompressionExtension.prototype.decodeAccessors = function ( parser ) {
 
-		var scope = this;
-		return _each( parser.json.meshes, function( mesh ) {
-			return _each( mesh.primitives, function( primitive ) {
-				if ( primitive.extensions && primitive.extensions[ EXTENSIONS.KHR_DRACO_MESH_COMPRESSION ] ) {
-					return scope.decodePrimitiveAccessors( primitive, parser, accessors );
-				} else {
-					return {};
+		var meshes = parser.json.meshes;
+		for ( var i = 0, il = meshes.length; i < il; i ++ ) {
+			var primitives = meshes[i].primitives;
+			for ( var j = 0, jl = primitives.length; j < jl; j ++ ) {
+				var primitiveDef = primitives[j];
+				if ( primitiveDef.extensions && primitiveDef.extensions[ EXTENSIONS.KHR_DRACO_MESH_COMPRESSION ] ) {
+					this.decodePrimitiveAccessors( primitiveDef, parser );
 				}
-			} );
-		} ).then( function () {
-			return accessors;
-		} );
+			}
+		}
 
 	};
 
-	GLTFDracoMeshCompressionExtension.prototype.decodePrimitiveAccessors = function ( primitive, parser, accessors ) {
+	GLTFDracoMeshCompressionExtension.prototype.decodePrimitiveAccessors = function ( primitive, parser ) {
 
 		var dracoLoader = this.dracoLoader;
 		var jsonAccessors = parser.json.accessors;
@@ -548,35 +544,32 @@ THREE.GLTFLoader = ( function () {
 			}
 		}
 
-		return parser.getDependency( 'bufferView', compressedBufferViewIndex ).then( function ( bufferView ) {
-
+		var decodePromise = parser.getDependency( 'bufferView', compressedBufferViewIndex ).then( function ( bufferView ) {
 			return new Promise( function ( resolve ) {
-
 				dracoLoader.decodeDracoFileData( bufferView, resolve, dracoAttributesToArrayType );
+			} );
+		} );
 
-			} ).then( function ( geometryBuffer ) {
-
-				// indices
-				{
-					var array = geometryBuffer.indices;
-					var accessorId = primitive.indices;
-					var itemSize = WEBGL_TYPE_SIZES[ jsonAccessors[accessorId].type ];
-					accessors[accessorId] = new THREE.BufferAttribute( array, itemSize, false );
-				}
-				// attributes and morph targets
-				for (var dracoId in geometryBuffer.attributes) {
-					var array = geometryBuffer.attributes[dracoId];
-					var accessorId = dracoAttributesToAccessorId[dracoId];
-					var itemSize = WEBGL_TYPE_SIZES[ jsonAccessors[accessorId].type ];
-					var normalized = jsonAccessors[accessorId].normalized === true;
-					accessors[accessorId] = new THREE.BufferAttribute( array, itemSize, normalized );
-				}
-
-				return accessors;
+		// indices accessor
+		parser.setDependency('accessor', primitive.indices, decodePromise.then( function ( geometryBuffer ) {
+			var array = geometryBuffer.indices;
+			var accessorId = primitive.indices;
+			var itemSize = WEBGL_TYPE_SIZES[ jsonAccessors[accessorId].type ];
+			return new THREE.BufferAttribute( array, itemSize, false );
+		} ) );
 
-			} );
+		// attributes and morph targets
+		for (let dracoId in dracoAttributesToAccessorId) {
+
+			let accessorId = dracoAttributesToAccessorId[dracoId];
+			parser.setDependency('accessor', accessorId, decodePromise.then( function ( geometryBuffer ) {
+				var array = geometryBuffer.attributes[dracoId];
+				var itemSize = WEBGL_TYPE_SIZES[ jsonAccessors[accessorId].type ];
+				var normalized = jsonAccessors[accessorId].normalized === true;
+				return new THREE.BufferAttribute( array, itemSize, normalized );
+			} ) );
 
-		} );
+		}
 
 	};
 
@@ -1347,6 +1340,11 @@ THREE.GLTFLoader = ( function () {
 		// Mark the special nodes/meshes in json for efficient parse
 		this.markDefs();
 
+		// Async decoding of compressed meshes
+		if ( parser.extensions[ EXTENSIONS.KHR_DRACO_MESH_COMPRESSION ] !== undefined) {
+			parser.extensions[ EXTENSIONS.KHR_DRACO_MESH_COMPRESSION ].decodeAccessors( parser );
+		}
+
 		// Fire the callback on complete
 		this.getMultiDependencies( [
 
@@ -1430,6 +1428,19 @@ THREE.GLTFLoader = ( function () {
 
 	};
 
+	/**
+	 * Store a dependency value as a Promise in the cache.
+	 * @param {string} type
+	 * @param {number} index
+	 * @param {Promise<Object>} value
+	 */
+	GLTFParser.prototype.setDependency = function ( type, index, value ) {
+
+		var cacheKey = type + ':' + index;
+		this.cache.add( cacheKey, value );
+
+	};
+
 	/**
 	 * Requests the specified dependency asynchronously, with caching.
 	 * @param {string} type
@@ -1685,14 +1696,6 @@ THREE.GLTFLoader = ( function () {
 
 			return bufferAttribute;
 
-		} ).then( function ( accessors ) { // TODO: THIS CODE NEEDS TO BE CHANGED DUE TO DEPENDENCIES REFACTORING
-
-			if ( parser.extensions[ EXTENSIONS.KHR_DRACO_MESH_COMPRESSION ] !== undefined) {
-				return parser.extensions[ EXTENSIONS.KHR_DRACO_MESH_COMPRESSION ].decodeAccessors( parser, accessors );
-			} else {
-				return accessors;
-			}
-
 		} );
 
 	};