Javascript OOP - 在异步回调中丢失了这个 [英] Javascript OOP - lost this in asynchronous callback

问题描述

我在 js oop 上有问题仍然困扰着我 - 我确定我做得不好，但我不知道如何正确地做.

I have problem which still bothers me on js oop - I'm sure I'm doing it bad, but I cant get how to do it right.

例如，我有这个代码

Auth.prototype.auth = function () {
    var request = new XMLHttpRequest();

    request.open('GET', this.getAuthServerURL() + '/token', true);
    request.send();

    request.onloadend = function () {
      var response = JSON.parse(request.responseText);

      console.log(response);
      if(response.result == 'found') {
        var token = response.token;

        this.setToken(token);
        this.isSigned = true;
      } else {
        console.log('Not logged yet.');
      }
    }
}

问题是我无法从request.onloadend"函数的上下文访问函数 setToken - 这可能是因为我丢失了对this"的引用.

The problem is that I cant access to function setToken from context of "request.onloadend" function - its probably because I lost reference to "this".

这个问题的解决方案是什么?我可以以某种方式将这个"变量传递给这个函数的上下文吗?

Whats a solution of this problem? Can I somehow pass the "this" var to context of this function?

谢谢！

推荐答案

有几种方法可以做到这一点.最直接的就是简单的保存一份你需要的值:

There are a couple of ways to do this. The most direct is to simply save a copy of the value you need:

Auth.prototype.auth = function () {
    var request = new XMLHttpRequest();
    var self = this; // save "this" value

    request.open('GET', this.getAuthServerURL() + '/token', true);
    request.send();

    request.onloadend = function () {
      var response = JSON.parse(request.responseText);

      console.log(response);
      if(response.result == 'found') {
        var token = response.token;

        self.setToken(token); // use saved "this" value
        self.isSigned = true;
      } else {
        console.log('Not logged yet.');
      }
    }
}

另一种方法是使用 bind:

Another way is to use bind:

request.onloadend = (function () {
  var response = JSON.parse(request.responseText);

  console.log(response);
  if(response.result == 'found') {
    var token = response.token;

    this.setToken(token); // use saved "this" value
    this.isSigned = true;
  } else {
    console.log('Not logged yet.');
  }
}).bind(this);

第二种方法更干净"，但存在浏览器兼容性问题(IE <9 不支持).

The second approach is "cleaner", but it has browser compatibility issues (IE < 9 does not support it).

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