Javascript setInterval和`this`解决方案 [英] Javascript setInterval and `this` solution

问题描述

我需要从我的 setInterval 处理程序访问此

I need to access this from my setInterval handler

prefs: null,
startup : function()
    {
        // init prefs
        ...
        this.retrieve_rate();
        this.intervalID = setInterval(this.retrieve_rate, this.INTERVAL);
    },

retrieve_rate : function()
    {
        var ajax = null;
        ajax = new XMLHttpRequest();
        ajax.open('GET', 'http://xyz.com', true);
        ajax.onload = function()
        {
            // access prefs here
        }
    }

如何在 ajax.onload 中访问this.prefs？

How can I access this.prefs in ajax.onload ?

推荐答案

setInterval行应如下所示： -

The setInterval line should look like this:-

 this.intervalID = setInterval(
     (function(self) {         //Self-executing func which takes 'this' as self
         return function() {   //Return a function in the context of 'self'
             self.retrieve_rate(); //Thing you wanted to run as non-window 'this'
         }
     })(this),
     this.INTERVAL     //normal interval, 'this' scope not impacted here.
 ); 

编辑：同样的原则适用于 onload 。在这种情况下，它对于外部代码的共同作用很少，它只是设置请求然后发送它。在这种情况下，额外的开销和上述代码中的附加功能是不必要的。你的retrieve_rate应该更像这样： -

Edit: The same principle applies to the " onload ". In this case its common for the "outer" code to do little, it just sets up the request an then sends it. In this case the extra overhead an additinal function as in the above code is unnecessary. Your retrieve_rate should look more like this:-

retrieve_rate : function()
{
    var self = this;
    var ajax = new XMLHttpRequest();
    ajax.open('GET', 'http://xyz.com', true);
    ajax.onreadystatechanged= function()
    {
        if (ajax.readyState == 4 && ajax.status == 200)
        {
            // prefs available as self.prefs
        }
    }
    ajax.send(null);
}

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