{
  "ver": "1.2.7",
  "uuid": "a8accd2e-6622-4c31-8a1e-4db5f2b568b5",
  "optimizationPolicy": "AUTO",     // prefab创建优化策略
  "asyncLoadAssets": false,         // 是否延迟加载
  "readonly": false,
  "subMetas": {}
}

{
  "9836134e-b892-4283-b6b2-78b5acf3ed45": {
    "asset": 1594351233259,
    "meta": 1594351616611,
    "relativePath": "effects"
  },
  "430eccbf-bf2c-4e6e-8c0c-884bbb487f32": {
    "asset": 1594351233254,
    "meta": 1594351616643,
    "relativePath": "effects\\__builtin-editor-gizmo-line.effect"
  },
  ...
}

{
  "__type__": "cc.Texture2D",
  "content": "0,9729,9729,33071,33071,0,0,1"
}

cc.resources.load("test assets/prefab", function (err, prefab) {
    var newNode = cc.instantiate(prefab);
    cc.director.getScene().addChild(newNode);
});

cc.assetManager.loadBundle('01_graphics', (err, bundle) => {
    bundle.load('xxx');
});

// 当复用其他项目的 Asset Bundle 时
cc.assetManager.loadBundle('https://othergame.com/remote/01_graphics', (err, bundle) => {
    bundle.load('xxx');
});

// 原生平台
cc.assetManager.loadBundle(jsb.fileUtils.getWritablePath() + '/pathToBundle/bundleName', (err, bundle) => {
    // ...
});

// 微信小游戏平台
cc.assetManager.loadBundle(wx.env.USER_DATA_PATH + '/pathToBundle/bundleName', (err, bundle) => {
    // ...
});

// 正常加载
this.pipeline = pipeline.append(preprocess).append(load);
// 预加载
this.fetchPipeline = fetchPipeline.append(preprocess).append(fetch);
// 转换资源路径
this.transformPipeline = transformPipeline.append(parse).append(combine);

// bundle类的load方法
load (paths, type, onProgress, onComplete) {
  var { type, onProgress, onComplete } = parseLoadResArgs(type, onProgress, onComplete);
  cc.assetManager.loadAny(paths, { __requestType__: RequestType.PATH, type: type, bundle: this.name }, onProgress, onComplete);
},

// assetManager的loadAny方法
loadAny (requests, options, onProgress, onComplete) {
  var { options, onProgress, onComplete } = parseParameters(options, onProgress, onComplete);
  
  options.preset = options.preset || 'default';
  let task = new Task({input: requests, onProgress, onComplete: asyncify(onComplete), options});
  pipeline.async(task);
},

let subTask = Task.create({input: task.input, options: subOptions});
task.output = task.source = transformPipeline.sync(subTask);

function parse (task) {
    // 将input转换成数组
    var input = task.input, options = task.options;
    input = Array.isArray(input) ? input : [ input ];

    task.output = [];
    for (var i = 0; i < input.length; i ++ ) {
        var item = input[i];
        var out = RequestItem.create();
        if (typeof item === 'string') {
            // 先创建object
            item = Object.create(null);
            item[options.__requestType__ || RequestType.UUID] = input[i];
        }
        if (typeof item === 'object') {
            // local options will overlap glabal options
            // 将options的属性复制到item身上，addon会复制options上有，而item没有的属性
            cc.js.addon(item, options);
            if (item.preset) {
                cc.js.addon(item, cc.assetManager.presets[item.preset]);
            }
            for (var key in item) {
                switch (key) {
                    // uuid类型资源，从bundle中取出该资源的详细信息
                    case RequestType.UUID: 
                        var uuid = out.uuid = decodeUuid(item.uuid);
                        if (bundles.has(item.bundle)) {
                            var config = bundles.get(item.bundle)._config;
                            var info = config.getAssetInfo(uuid);
                            if (info && info.redirect) {
                                if (!bundles.has(info.redirect)) throw new Error(`Please load bundle ${info.redirect} first`);
                                config = bundles.get(info.redirect)._config;
                                info = config.getAssetInfo(uuid);
                            }
                            out.config = config;
                            out.info = info;
                        }
                        out.ext = item.ext || '.json';
                        break;
                    case '__requestType__':
                    case 'ext': 
                    case 'bundle':
                    case 'preset':
                    case 'type': break;
                    case RequestType.DIR: 
                        // 解包后动态添加到input列表尾部，后续的循环会自动parse这些资源
                        if (bundles.has(item.bundle)) {
                            var infos = [];
                            bundles.get(item.bundle)._config.getDirWithPath(item.dir, item.type, infos);
                            for (let i = 0, l = infos.length; i < l; i++) {
                                var info = infos[i];
                                input.push({uuid: info.uuid, __isNative__: false, ext: '.json', bundle: item.bundle});
                            }
                        }
                        out.recycle();
                        out = null;
                        break;
                    case RequestType.PATH: 
                        // PATH类型的资源根据路径和type取出该资源的详细信息
                        if (bundles.has(item.bundle)) {
                            var config = bundles.get(item.bundle)._config;
                            var info = config.getInfoWithPath(item.path, item.type);
                            
                            if (info && info.redirect) {
                                if (!bundles.has(info.redirect)) throw new Error(`you need to load bundle ${info.redirect} first`);
                                config = bundles.get(info.redirect)._config;
                                info = config.getAssetInfo(info.uuid);
                            }

                            if (!info) {
                                out.recycle();
                                throw new Error(`Bundle ${item.bundle} doesn't contain ${item.path}`);
                            }
                            out.config = config; 
                            out.uuid = info.uuid;
                            out.info = info;
                        }
                        out.ext = item.ext || '.json';
                        break;
                    case RequestType.SCENE:
                        // 场景类型，从bundle中的config调用getSceneInfo取出该场景的详细信息
                        if (bundles.has(item.bundle)) {
                            var config = bundles.get(item.bundle)._config;
                            var info = config.getSceneInfo(item.scene);
                            
                            if (info && info.redirect) {
                                if (!bundles.has(info.redirect)) throw new Error(`you need to load bundle ${info.redirect} first`);
                                config = bundles.get(info.redirect)._config;
                                info = config.getAssetInfo(info.uuid);
                            }
                            if (!info) {
                                out.recycle();
                                throw new Error(`Bundle ${config.name} doesn't contain scene ${item.scene}`);
                            }
                            out.config = config; 
                            out.uuid = info.uuid;
                            out.info = info;
                        }
                        break;
                    case '__isNative__': 
                        out.isNative = item.__isNative__;
                        break;
                    case RequestType.URL: 
                        out.url = item.url;
                        out.uuid = item.uuid || item.url;
                        out.ext = item.ext || cc.path.extname(item.url);
                        out.isNative = item.__isNative__ !== undefined ? item.__isNative__ : true;
                        break;
                    default: out.options[key] = item[key];
                }
                if (!out) break;
            }
        }
        if (!out) continue;
        task.output.push(out);
        if (!out.uuid && !out.url) throw new Error('unknown input:' + item.toString());
    }
    return null;
}

function combine (task) {
    var input = task.output = task.input;
    for (var i = 0; i < input.length; i++) {
        var item = input[i];
        // 如果item已经包含了url，则跳过，直接使用item的url
        if (item.url) continue;

        var url = '', base = '';
        var config = item.config;
        // 决定目录的前缀
        if (item.isNative) {
            base = (config && config.nativeBase) ? (config.base + config.nativeBase) : cc.assetManager.generalNativeBase;
        } 
        else {
            base = (config && config.importBase) ? (config.base + config.importBase) : cc.assetManager.generalImportBase;
        }

        let uuid = item.uuid;
            
        var ver = '';
        if (item.info) {
            if (item.isNative) {
                ver = item.info.nativeVer ? ('.' + item.info.nativeVer) : '';
            }
            else {
                ver = item.info.ver ? ('.' + item.info.ver) : '';
            }
        }

        // 拼接最终的url
        // ugly hack, WeChat does not support loading font likes 'myfont.dw213.ttf'. So append hash to directory
        if (item.ext === '.ttf') {
            url = `${base}/${uuid.slice(0, 2)}/${uuid}${ver}/${item.options.__nativeName__}`;
        }
        else {
            url = `${base}/${uuid.slice(0, 2)}/${uuid}${ver}${item.ext}`;
        }
        
        item.url = url;
    }
    return null;
}

function load (task, done) {
    if (!task.progress) {
        task.progress = {finish: 0, total: task.input.length};
    }
    
    var options = task.options, progress = task.progress;
    options.__exclude__ = options.__exclude__ || Object.create(null);
    task.output = [];
    forEach(task.input, function (item, cb) {
        // 对每个input项都创建一个子任务，并交由loadOneAssetPipeline执行
        let subTask = Task.create({ 
            input: item, 
            onProgress: task.onProgress, 
            options, 
            progress, 
            onComplete: function (err, item) {
                if (err && !task.isFinish && !cc.assetManager.force) done(err);
                task.output.push(item);
                subTask.recycle();
                cb();
            }
        });
        // 执行子任务，loadOneAssetPipeline有fetch和parse组成
        loadOneAssetPipeline.async(subTask);
    }, function () {
        // 每个input执行完成后，最后执行该函数
        options.__exclude__ = null;
        if (task.isFinish) {
            clear(task, true);
            return task.dispatch('error');
        }
        gatherAsset(task);
        clear(task, true);
        done();
    });
}

var loadOneAssetPipeline = new Pipeline('loadOneAsset', [
    function fetch (task, done) {
        var item = task.output = task.input;
        var { options, isNative, uuid, file } = item;
        var { reload } = options;
        // 如果assets里面已经加载了这个资源，则直接完成
        if (file || (!reload && !isNative && assets.has(uuid))) return done();
        // 下载文件，这是一个异步的过程，文件下载完会被放到item.file中，并执行done驱动管线
        packManager.load(item, task.options, function (err, data) {
            if (err) {
                if (cc.assetManager.force) {
                    err = null;
                } else {
                    cc.error(err.message, err.stack);
                }
                data = null;
            }
            item.file = data;
            done(err);
        });
    },
    // 将资源文件转换成资源对象的过程
    function parse (task, done) {
        var item = task.output = task.input, progress = task.progress, exclude = task.options.__exclude__;
        var { id, file, options } = item;

        if (item.isNative) {
            // 对于原生资源，调用parser.parse进行处理，将处理完的资源放到item.content中，并结束流程
            parser.parse(id, file, item.ext, options, function (err, asset) {
                if (err) {
                    if (!cc.assetManager.force) {
                        cc.error(err.message, err.stack);
                        return done(err);
                    }
                }
                item.content = asset;
                task.dispatch('progress', ++progress.finish, progress.total, item);
                files.remove(id);
                parsed.remove(id);
                done();
            });
        } else {
            var { uuid } = item;
            // 非原生资源，如果在task.options.__exclude__中，直接结束
            if (uuid in exclude) {
                var { finish, content, err, callbacks } = exclude[uuid];
                task.dispatch('progress', ++progress.finish, progress.total, item);
    
                if (finish || checkCircleReference(uuid, uuid, exclude) ) {
                    content && content.addRef();
                    item.content = content;
                    done(err);
                } else {
                    callbacks.push({ done, item });
                }
            } else {
                // 如果不是reload，且asset中包含了该uuid
                if (!options.reload && assets.has(uuid)) {
                    var asset = assets.get(uuid);
                    // 开启了options.__asyncLoadAssets__，或asset.__asyncLoadAssets__为false，直接结束，不加载依赖
                    if (options.__asyncLoadAssets__ || !asset.__asyncLoadAssets__) {
                        item.content = asset.addRef();
                        task.dispatch('progress', ++progress.finish, progress.total, item);
                        done();
                    }
                    else {
                        loadDepends(task, asset, done, false);
                    }
                } else {
                    // 如果是reload，或者assets中没有，则进行解析，并加载依赖
                    parser.parse(id, file, 'import', options, function (err, asset) {
                        if (err) {
                            if (cc.assetManager.force) {
                                err = null;
                            }
                            else {
                                cc.error(err.message, err.stack);
                            }
                            return done(err);
                        }
                        
                        asset._uuid = uuid;
                        loadDepends(task, asset, done, true);
                    });
                }
            }
        }
    }
]);

// packManager.load的实现
load (item, options, onComplete) {
  // 如果资源没有被打包，则直接调用downloader.download下载（download内部也有已下载和加载中的判断）
  if (item.isNative || !item.info || !item.info.packs) return downloader.download(item.id, item.url, item.ext, item.options, onComplete);
  // 如果文件已经下载过了，则直接返回
  if (files.has(item.id)) return onComplete(null, files.get(item.id));

  var packs = item.info.packs;
  // 如果pack已经在加载中，则将回调添加到_loading队列，等加载完成后触发回调
  var pack = packs.find(isLoading);
  if (pack) return _loading.get(pack.uuid).push({ onComplete, id: item.id });

  // 下载一个新的pack
  pack = packs[0];
  _loading.add(pack.uuid, [{ onComplete, id: item.id }]);
  let url = cc.assetManager._transform(pack.uuid, {ext: pack.ext, bundle: item.config.name});
  // 下载pack并解包，
  downloader.download(pack.uuid, url, pack.ext, item.options, function (err, data) {
      files.remove(pack.uuid);
      if (err) {
          cc.error(err.message, err.stack);
      }
      // unpack package，内部实现包含2种解包，一种针对prefab、图集等json数组的分割解包，另一种针对Texture2D的content进行解包
      packManager.unpack(pack.packs, data, pack.ext, item.options, function (err, result) {
          if (!err) {
              for (var id in result) {
                  files.add(id, result[id]);
              }
          }
          var callbacks = _loading.remove(pack.uuid);
          for (var i = 0, l = callbacks.length; i < l; i++) {
              var cb = callbacks[i];
              if (err) {
                  cb.onComplete(err);
                  continue;
              }

              var data = result[cb.id];
              if (!data) {
                  cb.onComplete(new Error('can not retrieve data from package'));
              }
              else {
                  cb.onComplete(null, data);
              }
          }
      });
  });
}

download (id, url, type, options, onComplete) {
  // 取出downloaders中对应类型的下载回调
  let func = downloaders[type] || downloaders['default'];
  let self = this;
  // 避免重复下载
  let file, downloadCallbacks;
  if (file = files.get(id)) {
      onComplete(null, file);
  }
  // 如果在下载中，添加到队列
  else if (downloadCallbacks = _downloading.get(id)) {
      downloadCallbacks.push(onComplete);
      for (let i = 0, l = _queue.length; i < l; i++) {
          var item = _queue[i];
          if (item.id === id) {
              var priority = options.priority || 0;
              if (item.priority < priority) {
                  item.priority = priority;
                  _queueDirty = true;
              } 
              return;
          }
      } 
  }
  else {
      // 进行下载，并设置好下载失败的重试
      var maxRetryCount = options.maxRetryCount || this.maxRetryCount;
      var maxConcurrency = options.maxConcurrency || this.maxConcurrency;
      var maxRequestsPerFrame = options.maxRequestsPerFrame || this.maxRequestsPerFrame;

      function process (index, callback) {
          if (index === 0) {
              _downloading.add(id, [onComplete]);
          }
          if (!self.limited) return func(urlAppendTimestamp(url), options, callback);
          updateTime();

          function invoke () {
              func(urlAppendTimestamp(url), options, function () {
                  // when finish downloading, update _totalNum
                  _totalNum--;
                  if (!_checkNextPeriod && _queue.length > 0) {
                      callInNextTick(handleQueue, maxConcurrency, maxRequestsPerFrame);
                      _checkNextPeriod = true;
                  }
                  callback.apply(this, arguments);
              });
          }

          if (_totalNum < maxConcurrency && _totalNumThisPeriod < maxRequestsPerFrame) {
              invoke();
              _totalNum++;
              _totalNumThisPeriod++;
          }
          else {
              // when number of request up to limitation, cache the rest
              _queue.push({ id, priority: options.priority || 0, invoke });
              _queueDirty = true;

              if (!_checkNextPeriod && _totalNum < maxConcurrency) {
                  callInNextTick(handleQueue, maxConcurrency, maxRequestsPerFrame);
                  _checkNextPeriod = true;
              }
          }
      }

      // retry完成后，将文件添加到files缓存中，从_downloading队列中移除，并执行callbacks回调
      // when retry finished, invoke callbacks
      function finale (err, result) {
          if (!err) files.add(id, result);
          var callbacks = _downloading.remove(id);
          for (let i = 0, l = callbacks.length; i < l; i++) {
              callbacks[i](err, result);
          }
      }

      retry(process, maxRetryCount, this.retryInterval, finale);
  }
}

function downloadFile (url, options, onProgress, onComplete) {
    var { options, onProgress, onComplete } = parseParameters(options, onProgress, onComplete);
    var xhr = new XMLHttpRequest(), errInfo = 'download failed: ' + url + ', status: ';
    xhr.open('GET', url, true);
    
    if (options.responseType !== undefined) xhr.responseType = options.responseType;
    if (options.withCredentials !== undefined) xhr.withCredentials = options.withCredentials;
    if (options.mimeType !== undefined && xhr.overrideMimeType ) xhr.overrideMimeType(options.mimeType);
    if (options.timeout !== undefined) xhr.timeout = options.timeout;

    if (options.header) {
        for (var header in options.header) {
            xhr.setRequestHeader(header, options.header[header]);
        }
    }

    xhr.onload = function () {
        if ( xhr.status === 200 || xhr.status === 0 ) {
            onComplete && onComplete(null, xhr.response);
        } else {
            onComplete && onComplete(new Error(errInfo + xhr.status + '(no response)'));
        }

    };

    if (onProgress) {
        xhr.onprogress = function (e) {
            if (e.lengthComputable) {
                onProgress(e.loaded, e.total);
            }
        };
    }

    xhr.onerror = function(){
        onComplete && onComplete(new Error(errInfo + xhr.status + '(error)'));
    };
    xhr.ontimeout = function(){
        onComplete && onComplete(new Error(errInfo + xhr.status + '(time out)'));
    };
    xhr.onabort = function(){
        onComplete && onComplete(new Error(errInfo + xhr.status + '(abort)'));
    };

    xhr.send(null);
    return xhr;
}

downloader.register({
    // JS
    '.js' : downloadScript,
    '.jsc' : downloadScript,

    // Images
    '.png' : downloadAsset,
    '.jpg' : downloadAsset,
    ...
});

// func传入的是下载完成之后的处理，比如脚本下载完成后需要执行，此时会调用window.require
// 如果说要下载的是json资源之类的，传入的func是doNothing，也就是直接调用onComplete方法
function download (url, func, options, onFileProgress, onComplete) {
    var result = transformUrl(url, options);
    // 如果是本地文件，直接指向func
    if (result.inLocal) {
        func(result.url, options, onComplete);
    }
    // 如果在缓存中，更新资源的最后使用时间（lru）
    else if (result.inCache) {
        cacheManager.updateLastTime(url)
        func(result.url, options, function (err, data) {
            if (err) {
                cacheManager.removeCache(url);
            }
            onComplete(err, data);
        });
    }
    else {
        // 未下载的网络资源，调用downloadFile进行下载
        var time = Date.now();
        var storagePath = '';
        if (options.__cacheBundleRoot__) {
            storagePath = `${cacheManager.cacheDir}/${options.__cacheBundleRoot__}/${time}${suffix++}${cc.path.extname(url)}`;
        }
        else {
            storagePath = `${cacheManager.cacheDir}/${time}${suffix++}${cc.path.extname(url)}`;
        }
        // 使用downloadFile下载并缓存
        downloadFile(url, storagePath, options.header, onFileProgress, function (err, path) {
            if (err) {
                onComplete(err, null);
                return;
            }
            func(path, options, function (err, data) {
                if (!err) {
                    cacheManager.cacheFile(url, storagePath, options.__cacheBundleRoot__);
                }
                onComplete(err, data);
            });
        });
    }
}

function transformUrl (url, options) {
    var inLocal = false;
    var inCache = false;
    // 通过正则匹配是不是URL
    if (REGEX.test(url)) {
        if (options.reload) {
            return { url };
        }
        else {
            // 检查是否在缓存中（本地磁盘缓存）
            var cache = cacheManager.cachedFiles.get(url);
            if (cache) {
                inCache = true;
                url = cache.url;
            }
        }
    }
    else {
        inLocal = true;
    }
    return { url, inLocal, inCache };
}

downloadFile (remoteUrl, filePath, header, onProgress, onComplete) {
  downloading.add(remoteUrl, { onProgress, onComplete });
  var storagePath = filePath;
  if (!storagePath) storagePath = tempDir + '/' + performance.now() + cc.path.extname(remoteUrl);
  jsb_downloader.createDownloadFileTask(remoteUrl, storagePath, header);
},

parse (id, file, type, options, onComplete) {
  let parsedAsset, parsing, parseHandler;
  if (parsedAsset = parsed.get(id)) {
      onComplete(null, parsedAsset);
  }
  else if (parsing = _parsing.get(id)){
      parsing.push(onComplete);
  }
  else if (parseHandler = parsers[type]){
      _parsing.add(id, [onComplete]);
      parseHandler(file, options, function (err, data) {
          if (err) {
              files.remove(id);
          } 
          else if (!isScene(data)){
              parsed.add(id, data);
          }
          let callbacks = _parsing.remove(id);
          for (let i = 0, l = callbacks.length; i < l; i++) {
              callbacks[i](err, data);
          }
      });
  }
  else {
      onComplete(null, file);
  }
}

// 映射图片格式到解析方法
var parsers = {
  '.png' : parser.parseImage,
  '.jpg' : parser.parseImage,
  '.bmp' : parser.parseImage,
  '.jpeg' : parser.parseImage,
  '.gif' : parser.parseImage,
  '.ico' : parser.parseImage,
  '.tiff' : parser.parseImage,
  '.webp' : parser.parseImage,
  '.image' : parser.parseImage,
  '.pvr' : parser.parsePVRTex,
  '.pkm' : parser.parsePKMTex,
  // Audio
  '.mp3' : parser.parseAudio,
  '.ogg' : parser.parseAudio,
  '.wav' : parser.parseAudio,
  '.m4a' : parser.parseAudio,

  // plist
  '.plist' : parser.parsePlist,
  'import' : parser.parseImport
};

// 图片并不会解析成Asset对象，而是解析成对应的图片对象
parseImage (file, options, onComplete) {
  if (capabilities.imageBitmap && file instanceof Blob) {
      let imageOptions = {};
      imageOptions.imageOrientation = options.__flipY__ ? 'flipY' : 'none';
      imageOptions.premultiplyAlpha = options.__premultiplyAlpha__ ? 'premultiply' : 'none';
      createImageBitmap(file, imageOptions).then(function (result) {
          result.flipY = !!options.__flipY__;
          result.premultiplyAlpha = !!options.__premultiplyAlpha__;
          onComplete && onComplete(null, result);
      }, function (err) {
          onComplete && onComplete(err, null);
      });
  }
  else {
      onComplete && onComplete(null, file);
  }
},

// Asset对象的解析，通过deserialize实现，大致流程是解析json然后找到对应的class，并调用对应class的_deserialize方法拷贝数据、初始化变量，并将依赖资源放到asset.__depends
parseImport (file, options, onComplete) {
  if (!file) return onComplete && onComplete(new Error('Json is empty'));
  var result, err = null;
  try {
      result = deserialize(file, options);
  }
  catch (e) {
      err = e;
  }
  onComplete && onComplete(err, result);
},

parser.register({
    '.png' : downloader.downloadDomImage,
    '.binary' : parseArrayBuffer,
    '.txt' : parseText,
    '.plist' : parsePlist,
    '.font' : loadFont,
    '.ExportJson' : parseJson,
    ...
});

var Texture2D = cc.Class({
    name: 'cc.Texture2D',
    extends: require('../assets/CCAsset'),
    mixins: [EventTarget],

    properties: {
        _nativeAsset: {
            get () {
                // maybe returned to pool in webgl
                return this._image;
            },
            set (data) {
                if (data._data) {
                    this.initWithData(data._data, this._format, data.width, data.height);
                }
                else {
                    this.initWithElement(data);
                }
            },
            override: true
        },

// 在jsb-loader.js文件中
function parseText (url, options, onComplete) {
    readText(url, onComplete);
}

function parseArrayBuffer (url, options, onComplete) {
    readArrayBuffer(url, onComplete);
}

function parseJson (url, options, onComplete) {
    readJson(url, onComplete);
}

// 在jsb-fs-utils.js文件中
    readText (filePath, onComplete) {
        fsUtils.readFile(filePath, 'utf8', onComplete);
    },

    readArrayBuffer (filePath, onComplete) {
        fsUtils.readFile(filePath, '', onComplete);
    },

    readJson (filePath, onComplete) {
        fsUtils.readFile(filePath, 'utf8', function (err, text) {
            var out = null;
            if (!err) {
                try {
                    out = JSON.parse(text);
                }
                catch (e) {
                    cc.warn('Read json failed: ' + e.message);
                    err = new Error(e.message);
                }
            }
            onComplete && onComplete(err, out);
        });
    },

// 加载指定asset的依赖项
function loadDepends (task, asset, done, init) {

    var item = task.input, progress = task.progress;
    var { uuid, id, options, config } = item;
    var { __asyncLoadAssets__, cacheAsset } = options;

    var depends = [];
    // 增加引用计数来避免加载依赖的过程中资源被释放，调用getDepends获取依赖资源
    asset.addRef && asset.addRef();
    getDepends(uuid, asset, Object.create(null), depends, false, __asyncLoadAssets__, config);
    task.dispatch('progress', ++progress.finish, progress.total += depends.length, item);

    var repeatItem = task.options.__exclude__[uuid] = { content: asset, finish: false, callbacks: [{ done, item }] };

    let subTask = Task.create({ 
        input: depends, 
        options: task.options, 
        onProgress: task.onProgress, 
        onError: Task.prototype.recycle, 
        progress, 
        onComplete: function (err) {
            // 在所有依赖项加载完成之后回调
            asset.decRef && asset.decRef(false);
            asset.__asyncLoadAssets__ = __asyncLoadAssets__;
            repeatItem.finish = true;
            repeatItem.err = err;

            if (!err) {
                var assets = Array.isArray(subTask.output) ? subTask.output : [subTask.output];
                // 构造一个map，记录uuid到asset的映射
                var map = Object.create(null);
                for (let i = 0, l = assets.length; i < l; i++) {
                    var dependAsset = assets[i];
                    dependAsset && (map[dependAsset instanceof cc.Asset ? dependAsset._uuid + '@import' : uuid + '@native'] = dependAsset);
                }

                // 调用setProperties将对应的依赖资源设置到asset的成员变量中
                if (!init) {
                    if (asset.__nativeDepend__ && !asset._nativeAsset) {
                        var missingAsset = setProperties(uuid, asset, map);
                        if (!missingAsset) {
                            try {
                                asset.onLoad && asset.onLoad();
                            }
                            catch (e) {
                                cc.error(e.message, e.stack);
                            }
                        }
                    }
                }
                else {
                    var missingAsset = setProperties(uuid, asset, map);
                    if (!missingAsset) {
                        try {
                            asset.onLoad && asset.onLoad();
                        }
                        catch (e) {
                            cc.error(e.message, e.stack);
                        }
                    }
                    files.remove(id);
                    parsed.remove(id);
                    cache(uuid, asset, cacheAsset !== undefined ? cacheAsset : cc.assetManager.cacheAsset); 
                }
                subTask.recycle();
            }
            
            // 这个repeatItem可能有很多个地方都加载了它，要通知所有回调加载完成
            var callbacks = repeatItem.callbacks;
            for (var i = 0, l = callbacks.length; i < l; i++) {
                var cb = callbacks[i];
                asset.addRef && asset.addRef();
                cb.item.content = asset;
                cb.done(err);
            }
            callbacks.length = 0;
        }
    });

    pipeline.async(subTask);
}

getDepends (uuid, data, exclude, depends, preload, asyncLoadAssets, config) {
  var err = null;
  try {
      var info = dependUtil.parse(uuid, data);
      var includeNative = true;
      if (data instanceof cc.Asset && (!data.__nativeDepend__ || data._nativeAsset)) includeNative = false; 
      if (!preload) {
          asyncLoadAssets = !CC_EDITOR && (!!data.asyncLoadAssets || (asyncLoadAssets && !info.preventDeferredLoadDependents));
          for (let i = 0, l = info.deps.length; i < l; i++) {
              let dep = info.deps[i];
              if (!(dep in exclude)) {
                  exclude[dep] = true;
                  depends.push({uuid: dep, __asyncLoadAssets__: asyncLoadAssets, bundle: config && config.name});
              }
          }

          if (includeNative && !asyncLoadAssets && !info.preventPreloadNativeObject && info.nativeDep) {
              config && (info.nativeDep.bundle = config.name);
              depends.push(info.nativeDep);
          }
          
      } else {
          for (let i = 0, l = info.deps.length; i < l; i++) {
              let dep = info.deps[i];
              if (!(dep in exclude)) {
                  exclude[dep] = true;
                  depends.push({uuid: dep, bundle: config && config.name});
              }
          }
          if (includeNative && info.nativeDep) {
              config && (info.nativeDep.bundle = config.name);
              depends.push(info.nativeDep);
          }
      }
  }
  catch (e) {
      err = e;
  }
  return err;
},

// 根据json信息获取其资源依赖列表，实际上json信息就是asset对象
parse (uuid, json) {
  var out = null;
  // 如果是场景或者Prefab，data会是一个数组，scene or prefab
  if (Array.isArray(json)) {
      // 如果已经解析过了，在_depends中有依赖列表，则直接返回
      if (this._depends.has(uuid)) return this._depends.get(uuid)
      out = {
          // 对于Prefab或场景，直接使用_parseDepsFromJson方法返回
          deps: cc.Asset._parseDepsFromJson(json),
          asyncLoadAssets: json[0].asyncLoadAssets
      };
  }
  // 如果包含__type__，获取其构造函数，并从json中查找依赖资源 get deps from json
  // 实际测试，预加载的资源会走下面这个分支，预加载的资源并没有把json反序列化成Asset对象
  else if (json.__type__) {
      if (this._depends.has(uuid)) return this._depends.get(uuid);
      var ctor = js._getClassById(json.__type__);
      // 部分资源重写了_parseDepsFromJson和_parseNativeDepFromJson方法
      // 比如cc.Texture2D
      out = {
          preventPreloadNativeObject: ctor.preventPreloadNativeObject,
          preventDeferredLoadDependents: ctor.preventDeferredLoadDependents,
          deps: ctor._parseDepsFromJson(json),
          nativeDep: ctor._parseNativeDepFromJson(json)
      };
      out.nativeDep && (out.nativeDep.uuid = uuid);
  }
  // get deps from an existing asset 
  // 如果没有__type__字段，则无法找到它对应的ctor，从asset的__depends__字段中取出依赖
  else {
      if (!CC_EDITOR && (out = this._depends.get(uuid)) && out.parsedFromExistAsset) return out;
      var asset = json;
      out = {
          deps: [],
          parsedFromExistAsset: true,
          preventPreloadNativeObject: asset.constructor.preventPreloadNativeObject,
          preventDeferredLoadDependents: asset.constructor.preventDeferredLoadDependents
      };
      let deps = asset.__depends__;
      for (var i = 0, l = deps.length; i < l; i++) {
          var dep = deps[i].uuid;
          out.deps.push(dep);
      }
  
      if (asset.__nativeDepend__) {
          // asset._nativeDep会返回类似这样的对象 {__isNative__: true, uuid: this._uuid, ext: this._native}
          out.nativeDep = asset._nativeDep;
      }
  }
  // 第一次找到依赖，直接放到_depends列表中，cache dependency list
  this._depends.add(uuid, out);
  return out;
}