Serializing sounds and costumes into file descriptors upon 'saveProjectSb3'.

src/serialization/serialize-assets.js

@@ -0,0 +1,55 @@
+/**
+ * Serialize all the assets of the given type ('sounds' or 'costumes')
+ * in the provided runtime into an array of file descriptors.
+ * A file descriptor is an object containing the name of the file
+ * to be written and the contents of the file, the serialized asset.
+ * @param {Runtime} runtime The runtime with the assets to be serialized
+ * @param {string} assetType The type of assets to be serialized: 'sounds' | 'costumes'
+ * @returns {Array<object>} An array of file descriptors for each asset
+ */
+const serializeAssets = function (runtime, assetType) {
+    const targets = runtime.targets;
+    const assetDescs = [];
+    for (let i = 0; i < targets.length; i++) {
+        const currTarget = targets[i];
+        const currAssets = currTarget.sprite[assetType];
+        for (let j = 0; j < currAssets.length; j++) {
+            const currAsset = currAssets[j];
+            const assetId = currAsset.assetId;
+            const storage = runtime.storage;
+            const asset = storage.get(assetId);
+            assetDescs.push({
+                fileName: assetType === 'sound' ?
+                    currAsset.md5 : `${assetId}.${currAsset.dataFormat}`,
+                fileContent: asset.data});
+        }
+    }
+    return assetDescs;
+};
+
+/**
+ * Serialize all the sounds in the provided runtime into an array of file
+ * descriptors. A file descriptor is an object containing the name of the file
+ * to be written and the contents of the file, the serialized sound.
+ * @param {Runtime} runtime The runtime with the sounds to be serialized
+ * @returns {Array<object>} An array of file descriptors for each sound
+ */
+const serializeSounds = function (runtime) {
+    return serializeAssets(runtime, 'sounds');
+};
+
+/**
+ * Serialize all the costumes in the provided runtime into an array of file
+ * descriptors. A file descriptor is an object containing the name of the file
+ * to be written and the contents of the file, the serialized costume.
+ * @param {Runtime} runtime The runtime with the costumes to be serialized
+ * @returns {Array<object>} An array of file descriptors for each costume
+ */
+const serializeCostumes = function (runtime) {
+    return serializeAssets(runtime, 'costumes');
+};
+
+module.exports = {
+    serializeSounds,
+    serializeCostumes
+};

src/virtual-machine.js

@@ -15,6 +15,7 @@ const Variable = require('./engine/variable');
 
 const {loadCostume} = require('./import/load-costume.js');
 const {loadSound} = require('./import/load-sound.js');
+const {serializeSounds, serializeCostumes} = require('./serialization/serialize-assets');
 
 const RESERVED_NAMES = ['_mouse_', '_stage_', '_edge_', '_myself_', '_random_'];
 
@@ -206,7 +207,14 @@ class VirtualMachine extends EventEmitter {
      */
     saveProjectSb3 () {
         // @todo: Handle other formats, e.g., Scratch 1.4, Scratch 2.0.
-        return this.toJSON();
+        const soundDescs = serializeSounds(this.runtime);
+        const costumeDescs = serializeCostumes(this.runtime);
+
+        return {
+            projectJson: this.toJSON(),
+            sounds: soundDescs,
+            costumes: costumeDescs
+        };
     }
 
     /**