Delay for a XMLHTTP Request

问题:

Ιs there any convenient way to make XMLHTTP Request in Javascript? Wait for example 3 seconds before sending? I have an array full names

var items = [
    { url: "www.google.com/getChocolate", name: "Chocholate"},
    { url: "www.google.com/getCake", name: "Cake"},
    { url: "www.google.com/getCookie", name: "Cookie"},
];

for (var i = 0; i < items.length; i++) {
    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function () {
        if (this.readyState == 4 && this.status == 200) {
            var data;
            data = JSON.parse(xhr.responseText);
            // processing data here
        }
    };
    xhr.open("GET", items[i].url, false);
    xhr.send();
    // I somehow have to force to send the request for each item only every 3 seconds
}

And for each of them I want to receive JSON response from a server, but it bans me for sometime if i send requests too often, so I need to send them like every 3 seconds, wait for response, process the response and start a new one. I guess I'll have to make it synchronous, so I already put there false argument in xhr.open.

回答1:

H i friend,

I just saw your post and I understand that you want to do a request queue - send the first request after 3 seconds and wait it to complete then send next and next till the end of the queue.

I made a very simple class Request and RequestManager which will do this for you.

Have a look on the code and let me know if something is unclear to you. Try to read the comments also.

var items = [{
    url: "www.google.com/getChocolate",
    name: "Chocholate"
  },
  {
    url: "http://en.wikipedia.org/w/api.php?action=opensearch&format=json&limit=15&origin=*&search=cake",
    name: "Cake"
  },
  {
    url: "http://en.wikipedia.org/w/api.php?action=opensearch&format=json&limit=15&origin=*&search=cookie",
    name: "Cookie"
  },
];

/* First prepare array of requests that later will be send */
var requests = [];
for (var i = 0; i < items.length; i++) {
  requests.push(new Request(items[i].url));
}

/* Initialize the object that will be responsible for 
 * request sending and process the requests  */
var manager = new RequestManager(requests);
manager.process();
manager.onSend(function(url, response) {
  /* this code will fire once a request is completed, no matter if success of failed */
  console.log('request to ' + url + ' completed ....');
  console.log('----------------------------------------');
})


/**
 * This class is a wrapper that will hold the request
 * and will be responsible to send it with a delay of 3 seconds
 * 
 * @param {string} url - this is the url which is going to be requested
 * @returns {Request} - new Request object
 */
function Request(url) {
  var that = this, resolve, reject;
  that.url = url;

  that.promise = new Promise((res, rej) => {
    resolve = res;
    reject = rej;
  });

  that.xhr = new XMLHttpRequest();
  that.xhr.onreadystatechange = function() {
    if (that.xhr.readyState == 4) {
      if (that.xhr.status == 200) {
        var data = null;
        try {
          data = JSON.parse(that.xhr.responseText);
          resolve(data);
        } catch (e) {
          reject(e);
        }
      } else {
        reject({
          statusText: that.xhr.statusText,
          response: that.xhr.response
        });
      }
    }
  };

  that.send = function() {
    /* send request after 3 seconds */
    setTimeout(function() {
      that.xhr.open("GET", that.url, true);
      that.xhr.send();
    }, 3000)

    return this;
  }
}


/**
 * This class is responsible for processing all the request
 * The main role is to call the Request's send method, 
 * wait the request to complete and then call the next request from the queue
 * until all the requests are processed
 * 
 * @param {Array} requests - this is an array of Request objects
 * @returns {RequestManager} new RequestManager object
 */
function RequestManager(requests) {
  var that = this,
    callback;
  that.requests = requests;

  that.onSend = function(cb) {
    callback = cb;
  }

  that.process = function() {
    console.log('Starting requests sending .......');
    doRequestRecursive(that.requests.pop());
  }

  function doRequestRecursive(request) {
    request.send().promise.then(function(data) {
      console.log('request ' + request.url + ' success ...');
      callback(request.url, data);
    }, function(err) {
      console.log('request ' + request.url + ' failed ...');
      callback(request.url, err);
    }).then(function() {
      var nextRequest = that.requests.pop();
      if (nextRequest) {
        doRequestRecursive(nextRequest);
      }
    });
  }
}

The sample code snippet is sending requests to Wikipedia to demonstrate that they are succeeding because your links are not working ones.