带有xmlhttp的通用回调函数如何在“this”更改时发送对函数的引用

我正在轮询数据库。 请参阅下面代码中的“<------------”。

这项工作:

class MultiplayerGame{
...

...
callPhpWithAjax(url){
    var xmlhttp = new XMLHttpRequest();
    var instance = this;
    xmlhttp.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
            instance.pollResponse(this.responseText); <-----------works, pollResponse is called and I can access the object with "this"
        }
    };
    xmlhttp.open("GET", url, true);
    xmlhttp.send();
}

poll(){
    var url = "myurl.com"
    this.callPhpWithAjax(url);
}

pollResponse(response){
    this.variable = response;
}

}

当我尝试将其实现得更通用一些时,它不起作用:

class MultiplayerGame{
...
callPhpWithAjaxGenericNOTWORKING(url,callbackfunction){
    var xmlhttp = new XMLHttpRequest();
    xmlhttp.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
            callbackfunction(this.responseText); <----------callbackfunction not calling what I wanted.
        }
    };
    xmlhttp.open("GET", url, true);
    xmlhttp.send();
}

poll(){
    var url = "myurl.com"
    this.callPhpWithAjaxGenericNOTWORKING(url, this.pollResponse);  <---------------  it seems like this.pollResponse is not ok. I don't know how to do it.
}

pollResponse(response){
    this.variable = response;  <----- this.variable is undefined. (this is not the class instance i was hoping) 
}

当我用callbackfunction调用函数时,我使用“this”,但显然,它并不引用同一个对象。我在这里感到困惑。

如何将callbackfunction作为参数正确发送?