统一多实例的插件创建方式，文档化提供最佳实践 about egg HOT 3 CLOSED

class Sigleton {
  constructor(opts) {
    this.clients = new Map();

    if (opts.clients) {
      for (const id in opts.clients) {
        this.set(id, this.create(opts.clients[id]));
      }
      return;
    }
  }

  get(id) {
    return this.clients.get(id);
  }

  set(id, client) {
  return this.clients.set(id, client);
  }

  create(config) {
    // 子类需要实现 create 方法    
  }
}

// lib/oss.js
const oss = require('ali-oss');
const Sigleton = require('egg-sigleton').;

class Oss extends Sigleton {
  constructor(opts) {
    super(opts);
  }

  create(config) {
    return oss(config);
  }
}

// app.js
const oss = require('./lib/oss');

module.exports = function (app) {
  app.oss = new Oss(app.config.oss);
  app.oss.set('cdn', app.oss.create(app.config.cdnOss));
  // or app.cdnOss = app.oss.create(app.config.cdnOss);
}

// services 中
get cdnOss() {
  return this.app.oss.get('cdnOss');
}

* upload(name, file) {
  yield this.cdnOss.upload(name, file);
}