How to use handwritten Promise handwriting to implement asynchronous operations in Vue

Vue is a very popular front-end framework. Its core idea is data-driven views. Vue greatly simplifies the front-end development workflow through data-responsive mechanism and componentization ideas, providing developers with a more efficient and convenient development experience. In Vue, we often need to perform asynchronous operations, such as requesting server data, timers, etc. Promise, as a commonly used asynchronous operation method in JavaScript, is also widely used in Vue projects. This article will introduce in detail how to implement asynchronous operations by handwriting Promise in Vue.

Basic usage of Promise

In Vue, we usually obtain server data through AJAX requests, and AJAX is an asynchronous operation, so it needs to be encapsulated with Promise. Common usage of Promise is as follows:

let promise = new Promise((resolve, reject) => {
  // 异步操作
  setTimeout(() => {
    // 模拟异步操作结束
    let result = 'This is data requested from server.';
    // 将数据传递给 then 方法
    resolve(result);
  }, 1000);
});

promise.then(data => {
  console.log(data);
}).catch(error => {
  console.log(error);
});

In the above code, we first create a Promise object. The Promise constructor receives a function as a parameter. The function contains two parameters, resolve and reject, which represent the success and rejection of the asynchronous operation respectively. Failure callback function. After the asynchronous operation ends, we pass the data to the callback function in the then method by calling the resolve function. If an error occurs during an asynchronous operation, we can pass the error information to the callback function in the catch method through the reject function.

Handwritten Promise to implement asynchronous operations

In Vue, we often need to encapsulate certain tool methods or Vue instance methods and expose them for global use. Therefore, handwriting Promise has become an essential skill. Below we will demonstrate how to use JavaScript handwritten Promise to implement asynchronous operations.

First, we need to define a function that encapsulates asynchronous operations, and pass in two parameters, resolve and reject, to control the success or failure callback function of the asynchronous operation. The code is as follows:

function myPromise(fn) {
  let self = this;
  self.value = null;
  self.error = null;
  self.onFulfilled = null;
  self.onRejected = null;
  
  function resolve(value) {
    self.value = value;
    self.onFulfilled(self.value);
  }
  
  function reject(error) {
    self.error = error;
    self.onRejected(self.error);
  }

  fn(resolve, reject);
}

In the above code, we define the myPromise function and initialize some variables, such as value and error. In the function, we define two callback functions resolve and reject and call them with the passed parameter fn. The resolve callback function receives a value value, which is used to pass data to the then method after the asynchronous operation is successful. The reject callback function receives an error message, which is used to pass the error message to the catch method after the asynchronous operation fails.

Next, we need to implement the then method to handle the callback function after the asynchronous operation is successful. The code is as follows:

myPromise.prototype.then = function(onFulfilled, onRejected) {
  let self = this;
  self.onFulfilled = onFulfilled;
  self.onRejected = onRejected;
};

In the above code, we implement the then method on the myPromise prototype and pass in the two callback functions onFulfilled and onRejected as parameters. In the method, we store these two callback functions in the Self object, call the onFulfilled function after the asynchronous operation is successful, and pass the data to the callback function in the then method. After the asynchronous operation fails, call the onRejected function and pass the error information to the catch method.

Finally, we need to implement the catch method to handle the callback function after the asynchronous operation fails. The code is as follows:

myPromise.prototype.catch = function(onRejected) {
  let self = this;
  self.onRejected = onRejected;
};

In the above code, we implement the catch method on the myPromise prototype and pass in the onRejected callback function as a parameter. In the method, we store the onRejected function in the Self object and pass the error message to the onRejected callback function after the asynchronous operation fails.

Comprehensive application

Next, we will implement asynchronous operations based on Vue and combined with myPromise. In a Vue instance, we define an asynchronous method fetchData for requesting server data. In the fetchData method, we hand-write a Promise object to request data, and then store the data in the data attribute.

new Vue({
  el: '#app',
  data: {
    data: null,
    error: null
  },
  methods: {
    fetchData: function() {
      let self = this;
      return new myPromise(function(resolve, reject) {
        let xhr = new XMLHttpRequest();
        xhr.open('GET', '/api/data', true);
        xhr.onreadystatechange = function() {
          if(xhr.readyState === 4) {
            if(xhr.status === 200) {
              resolve(JSON.parse(xhr.responseText));
            } else {
              reject(xhr.statusText);
            }
          }
        };
        xhr.onerror = function() {
          reject(xhr.statusText);
        };
        xhr.send();
      });
    }
  },
  mounted: function() {
    let self = this;
    self.fetchData().then(function(data) {
      self.data = data;
    }).catch(function(error) {
      self.error = error;
    });
  }
});

In the above code, we first define a Vue instance, and then initialize the variables used to store data and error information in the data attribute. In the fetchData method, we hand-write a Promise object, create an AJAX request object, and pass the callback functions for success and failure of the data request to the resolve and reject functions respectively. After successfully executing the asynchronous operation, we obtain the data returned from the server and store it in the data attribute. After the asynchronous operation fails, we get the error information and store it in the error attribute. In the mounted hook function, we obtain the server data by calling the fetchData method, and handle the success and failure of the asynchronous operation through the then and catch methods respectively. After the asynchronous operation is successful, we obtain the data returned from the server and assign it to the global variable self.data. After the asynchronous operation fails, we get the error message and store it in the global variable self.error.

By handwriting Promise, we can better understand the principles and usage of Promise, and can encapsulate our own tool methods or Vue instance methods. In actual development, we should choose appropriate asynchronous processing methods based on specific needs and business scenarios to improve the development efficiency and user experience of the project.

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