javascript - 如何向 javascript 类添加回调函数？

Question

以下 JavaScript 代码给出了错误“this.callback 不是函数

function ajaxRequest()
{
    var httpObject;

    this.open = open;
    this.callback = function(){};

    function getHTTPObject()
    {
        if (window.ActiveXObject) 
            return new ActiveXObject("Microsoft.XMLHTTP");
        else if (window.XMLHttpRequest)
            return new XMLHttpRequest();
        else 
        {
            alert("Your browser does not support AJAX.");
            return null;
        }
    }

    function onstatechange()
    {
        if(httpObject.readyState == 4)
        {
            this.callback(httpObject.responseText);
        }

    }


    function open(url, callback)
    {
        httpObject = getHTTPObject();
        if (httpObject != null) 
        {
            httpObject.open("GET", url, true);
            httpObject.send(null);
            this.callback = callback;
            httpObject.onreadystatechange = onstatechange;
        }
    }
}

为什么open方法不把回调参数当作函数处理？

如果是的话，为什么我不能在 onstatechange 函数中调用它？

我该如何完成这项工作？

Answer 1

原因是因为 onstatechange 被作为事件处理程序调用，并且 this 指针可能指向触发事件的对象，而不是 ajaxRequest 对象，如您所料。

下面的重写将 this 变量存储在 onstatechange() 函数有权访问的执行上下文中名为 that 的变量中。这应该可以解决问题。

总而言之，如果您没有彻底理解 Javascript 闭包和执行上下文，即使您了解，使用框架来执行 AJAX 请求也会好得多。 jQuery 和 Prototype 是不错的选择。

function ajaxRequest()
{
    var httpObject;

    this.open = open;
    this.callback = function(){};
    var that = this;

    function getHTTPObject()
    {
        if (window.ActiveXObject) 
                return new ActiveXObject("Microsoft.XMLHTTP");
        else if (window.XMLHttpRequest)
                return new XMLHttpRequest();
        else 
        {
                alert("Your browser does not support AJAX.");
                return null;
        }
    }

    function onstatechange()
    {
        if(httpObject.readyState == 4)
        {
                that.callback(httpObject.responseText);
        }

    }


    function open(url, callback)
    {
        httpObject = getHTTPObject();
        if (httpObject != null) 
        {
                httpObject.open("GET", url, true);
                httpObject.send(null);
                this.callback = callback;
                httpObject.onreadystatechange = onstatechange;
        }
    }
}