问题描述
我要的addEventListener 来听进度事件 开场中的XMLHTTP连接之前（即 xhr.open（）），但 beforeSend 方法返回一个XHR实例已经打开。我怎么会覆盖到适当加听众？
I need to addEventListener to listen for progress event before opening the XMLHttp connection (i.e. xhr.open()), but the beforeSend method returns an xhr instance already opened. How would I override it to add the listener properly?
（注意：您需要调用open（前添加事件监听器）的要求，否则进度的事件将不会触发。）
推荐答案
您可以覆盖 XHR 中的，甚至在每个单独的 $。阿贾克斯调用。这是记录在 $。阿贾克斯 文档的（感谢尼克！）的。
You can override the xhr function in ajaxSetup or even on each individual $.ajax call. This is documented in the $.ajax docs (thanks Nick!).
您code可能是这样的（未经测试）如果你想这样做的所有的时间：
Your code might look like this (untested) if you want to do this all the time:
(function() {
var originalXhr = jQuery.ajaxSettings.xhr;
jQuery.ajaxSetup({
xhr: function() {
var req = originalXhr();
if (req) {
// Add your progress handler
}
return req;
}
});
})();
...或者是这样的（活生生的例子）只是特定请求：
$.ajax({
url: "path/to/resource",
xhr: function() {
var req = $.ajaxSettings.xhr();
if (req) {
// Add your handler here
}
return req;
}
});
不幸的是，虽然覆盖 XHR 被记录，当前配置一（位置<$ C C $> jQuery.ajaxSettings.xhr ）似乎没有要，所以理论上讲你仍然通过在code。使用 jQuery.ajaxSettings.xhr 依托无证功能。你只需要仔细检查这仍然存在于每一个点的释放，它可能不会走动太多（和 jQuery.ajaxSettings 至少提的在文档，这里）。
Unfortunately, although overriding xhr is documented, the location of the currently-configured one (jQuery.ajaxSettings.xhr) doesn't seem to be, so technically you're still relying on an undocumented feature by using jQuery.ajaxSettings.xhr in your code. You just need to double-check that that's still there on each dot release, it's probably not going to move around too much (and jQuery.ajaxSettings is at least mentioned in the docs, here).
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