Support p5 preload with ml5 functions (#288)

* support p5 preload with ml5 functions

* make withPreload object clean

Author: Yuuno, Hibiki

src/index.js

@@ -16,10 +16,14 @@ import styleTransfer from './StyleTransfer/';
 import charRNN from './CharRNN/';
 import pix2pix from './Pix2pix/';
 import SketchRNN from './SketchRNN';
+import preloadRegister from './utils/p5PreloadHelper';
 import { version } from '../package.json';
 
-module.exports = {
+const withPreload = {
   imageClassifier,
+};
+
+module.exports = Object.assign({}, preloadRegister(withPreload), {
   KNNClassifier,
   featureExtractor,
   pitchDetection,
@@ -33,4 +37,4 @@ module.exports = {
   ...imageUtils,
   tf,
   version,
-};
+});

src/utils/p5PreloadHelper.js

@@ -0,0 +1,33 @@
+// Copyright (c) 2019 ml5
+//
+// This software is released under the MIT License.
+// https://opensource.org/licenses/MIT
+
+
+/**
+ * a list to store all functions to hook p5 preload
+ * @param {obj} object or prototype to wrap with
+ * @returns obj
+ */
+export default function registerPreload(obj) {
+  if (typeof window === 'undefined') return obj;
+  if (typeof window.p5 === 'undefined') return obj;
+  if (typeof window.p5.prototype === 'undefined') return obj;
+  if (typeof window.p5.prototype.registerPreloadMethod === 'undefined') return obj;
+
+  const preloadFn = obj;
+  Object.keys(obj).forEach((key) => {
+    const fn = obj[key];
+
+    preloadFn[key] = function preloads(...args) {
+      return fn.apply(obj, [...args, function doingPreloads() {
+        const targetPreloadFn = '_decrementPreload';
+        if (window[targetPreloadFn]) return window[targetPreloadFn]();
+        return null;
+      }]);
+    };
+    window.p5.prototype.registerPreloadMethod(`${key}`, obj);
+  });
+
+  return obj;
+}