如何获得JavaScript类方法以等待返回数据，直到加载了构造函数中的数据？

Question

如何让list()方法等待将数据加载到构造函数中，然后它才能将其承诺返回给调用方？

import fetch from 'node-fetch';

class Employees {
    constructor() {
        if (Employees._instance) {
            return Employees._instance
        }
        Employees._instance = this;

        this.employees = [];
        this.dataLoaded = false;

        this.url = 'https://raw.githubusercontent.com/graphql-compose/graphql-compose-examples/master/examples/northwind/data/json/employees.json';

        (async () => {
            const response = await fetch(this.url);
            this.employees = await response.json();
            this.dataLoaded = true;
            console.log(`work done: got ${this.employees.length} employees`);
        })();
    }

    list() {
        return new Promise((resolve) => {
            resolve(this.employees.map(m => `${m.firstName} ${m.lastName} (${m.id})`));
        });
    }

}

const employees = new Employees();

(async () => {
    console.log(await employees.list());
})();

Answer 1

我建议构造函数将数据加载到this中的承诺保存到list上，然后list可以等待这个承诺：

class Employees() {
  constructor() {
    if (Employees._instance) {
      return Employees._instance
    }
    Employees._instance = this;

    this.employees = [];
    this.dataLoaded = false;

    this.url = 'https://raw.githubusercontent.com/graphql-compose/graphql-compose-examples/master/examples/northwind/data/json/employees.json';
    
    this.initPromise = (async () => {
      const response = await fetch(this.url);
      this.employees = await response.json();
      this.dataLoaded = true;
      console.log(`work done: got ${this.employees.length} employees`);
    })();
  }

  async list() {
    await this.initPromise;
    return this.employees.map(m => `${m.firstName} ${m.lastName} (${m.id})`));
  }
}

如果加载尚未完成，那么await将导致list等待无论需要多长时间。如果加载已经完成，那么initPromise处于解析状态，list将或多或少地立即恢复(当微任务队列执行时)。

Answer 2

使用Deferred对象。

延迟表示尚未完成的工作

​

// fetch mock with 3 seconds wait
fetch = () => {
     return new Promise((resolve) => {
         setTimeout(() => {
             resolve({
                 json: () => new Promise((r) => r([
                        {firstName: 'Arthur', lastName: 'Pym', id: 1},
                        {firstName: 'August', lastName: 'Barnard', id: 2},
                        {firstName: 'M.', lastName: 'Poe', id: 3}
                 ]))
             });
         }, 3000);
     });
}

class MyDeferred {
    resolve = null;
    reject = null;
    promise = null;
    constructor() {
        this.promise = new Promise((res, rej) => {
            this.resolve = res;
            this.reject = rej;
        });
    }
}

class Employees {
    deferred = new MyDeferred();
    constructor() {
        if (Employees._instance) {
            return Employees._instance
        }
        Employees._instance = this;

        this.employees = [];
        this.dataLoaded = false;

        this.url = 'https://raw.githubusercontent.com/graphql-compose/graphql-compose-examples/master/examples/northwind/data/json/employees.json';

        (async () => {
            const response = await fetch(this.url);
            this.employees = await response.json();
            this.dataLoaded = true;
            this.deferred.resolve();
            console.log(`work done: got ${this.employees.length} employees`);
        })();
    }

    list() {
        return new Promise((resolve) => {
            this.deferred.promise.then(() => {
                resolve(this.employees.map(m => `${m.firstName} ${m.lastName} (${m.id})`));
            });
        });
    }

}

const employees = new Employees();

(async () => {
    console.log(await employees.list());
})();

​