对于异步请求,React在componentDidMount中做什么?

时间:2019-05-22 02:17:11

标签: reactjs asynchronous promise axios settimeout

我所知道的setTimoutPromise晚 所以我想证明那是真的。

我在setTimeout元素中设置了script,并在React.componentDidMount中请求使用axiosasync await语法糖。 但是结果是setTimtoutconsole之前的didMount。我不知道为什么。

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <title>order?</title>
</head>

<body>
  <div id="app"></div>
  <div>
    <h2>order list</h2>
    <ol id="ol">

    </ol>
  </div>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/axios/0.18.0/axios.min.js"></script>
  <!--        <script src="https://cdn.bootcss.com/react/16.8.6/umd/react.production.min.js"></script>-->
  <!--        <script src="https://cdn.bootcss.com/react-dom/16.8.6/umd/react-dom.production.min.js"></script>-->
  <script src="https://cdn.bootcss.com/react/15.4.2/react.min.js"></script>
  <script src="https://cdn.bootcss.com/react-dom/15.4.2/react-dom.min.js"></script>

  <script>
    window.olElement = document.getElementById('ol');

    function createElement(text) {
      const element = document.createElement('li')
      element.innerText = text;
      return element;
    }

    async function getData() {
      await axios('https://www.apiopen.top/weatherApi?city=%E5%8C%97%E4%BA%AC').then(() => {
        olElement.appendChild(createElement("axios then in script element"))
      });
      olElement.appendChild(createElement("await res in script element"))
    }

    setTimeout(() => {
      olElement.appendChild(createElement("setTimeout in script element"))
    });

    getData();
  </script>

  <script>
    function _instanceof(left, right) {
      if (right != null && typeof Symbol !== "undefined" && right[Symbol.hasInstance]) {
        return right[Symbol.hasInstance](left);
      } else {
        return left instanceof right;
      }
    }

    function _typeof(obj) {
      if (typeof Symbol === "function" && typeof Symbol.iterator === "symbol") {
        _typeof = function _typeof(obj) {
          return typeof obj;
        };
      } else {
        _typeof = function _typeof(obj) {
          return obj && typeof Symbol === "function" && obj.constructor === Symbol && obj !== Symbol.prototype ? "symbol" : typeof obj;
        };
      }
      return _typeof(obj);
    }

    function _classCallCheck(instance, Constructor) {
      if (!_instanceof(instance, Constructor)) {
        throw new TypeError("Cannot call a class as a function");
      }
    }

    function _defineProperties(target, props) {
      for (var i = 0; i < props.length; i++) {
        var descriptor = props[i];
        descriptor.enumerable = descriptor.enumerable || false;
        descriptor.configurable = true;
        if ("value" in descriptor) descriptor.writable = true;
        Object.defineProperty(target, descriptor.key, descriptor);
      }
    }

    function _createClass(Constructor, protoProps, staticProps) {
      if (protoProps) _defineProperties(Constructor.prototype, protoProps);
      if (staticProps) _defineProperties(Constructor, staticProps);
      return Constructor;
    }

    function _possibleConstructorReturn(self, call) {
      if (call && (_typeof(call) === "object" || typeof call === "function")) {
        return call;
      }
      return _assertThisInitialized(self);
    }

    function _assertThisInitialized(self) {
      if (self === void 0) {
        throw new ReferenceError("this hasn't been initialised - super() hasn't been called");
      }
      return self;
    }

    function _getPrototypeOf(o) {
      _getPrototypeOf = Object.setPrototypeOf ? Object.getPrototypeOf : function _getPrototypeOf(o) {
        return o.__proto__ || Object.getPrototypeOf(o);
      };
      return _getPrototypeOf(o);
    }

    function _inherits(subClass, superClass) {
      if (typeof superClass !== "function" && superClass !== null) {
        throw new TypeError("Super expression must either be null or a function");
      }
      subClass.prototype = Object.create(superClass && superClass.prototype, {
        constructor: {
          value: subClass,
          writable: true,
          configurable: true
        }
      });
      if (superClass) _setPrototypeOf(subClass, superClass);
    }

    function _setPrototypeOf(o, p) {
      _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) {
        o.__proto__ = p;
        return o;
      };
      return _setPrototypeOf(o, p);
    }

    var A =
      /*#__PURE__*/
      function(_React$Component) {
        _inherits(A, _React$Component);

        function A() {
          _classCallCheck(this, A);

          return _possibleConstructorReturn(this, _getPrototypeOf(A).apply(this, arguments));
        }

        _createClass(A, [{
          key: "componentDidMount",
          value: async function componentDidMount() {
            olElement.appendChild(createElement("react componentDidMount"))
            var res = await axios('https://www.apiopen.top/weatherApi?city=%E5%8C%97%E4%BA%AC').then(res => {
              olElement.appendChild(createElement("axios then in react didMount"))
              return res
            });
            olElement.appendChild(createElement("await res in react didMount"))
          }
        }, {
          key: "render",
          value: function render() {
            return React.createElement("div", null, "A");
          }
        }]);

        return A;
      }(React.Component);

    ReactDOM.render(React.createElement(A, null), document.getElementById('app'));
  </script>


</body>

</html>

2 个答案:

答案 0 :(得分:0)

说setTimeout的运行时间比promise晚,这有点令人困惑,因为它们都是异步的,并且会等到他们需要完成任务的时间。

在这种情况下,立即运行setTimeout且不延迟将回调置于事件队列中。在将setTimeout从事件队列中移除并运行之前,浏览器将处理当前函数中的所有代码。

Axios向apiopen发出HTTP请求。承诺只会在请求完成后才能解决,这需要一些时间,例如大约500毫秒左右(您可以在开发工具的网络面板中检查多长时间)。您的其他代码将继续运行,包括setTimeout事件,直到http请求完成。

从本质上讲,promise是编写回调的一种更好的方法,但是真正需要时间的是您正在等待的异步任务。

MDN在using Promises上拥有大量资源。如果您有兴趣了解为什么稍后不延迟运行setTimeout,请查看Concurrency model and the Event Loop

答案 1 :(得分:0)

您的测试用例与您要测试的内容无关。通过使用axios,您实际上将HTTP请求放入Promise中,并且Promise的解析时间现在取决于请求完成的时间。

对于无偏见的测试用例,请在Promise和setTimeout内部使用相同的功能,例如this question.