ajax - 异步 AJAX 调用返回什么？

Question

我正在尝试创建一个测试用例来监控多个并行异步服务器任务的进度。我的代码可以正常工作，但有几处我不明白。首先，下面的 $.ajax 调用返回了什么？理论上，它应该返回 undefined，但事实并非如此。

function doParallel() {    
    var promiseA, promiseB, handleSuccess, handleFailure;
    var dataA = JSON.stringify({ size: a });
    var dataB = JSON.stringify({ size: b });

    promiseA = $.ajax({
        url: testGlobal.urlA, 
        data: dataA,
        type: "POST",
        async: true,
        contentType: 'application/json; charset=utf-8',
        dataType: "json",
        success: function (rtnData) {
            //  Get the result
            result = (rtnData === undefined) ? null : $.parseJSON(rtnData.d);
        },
        error: function (xhr, textStatus, errorThrown) {
            //  Whoops! didn't work
            reportAjaxError(xhr, textStatus, url, data);
        },
        complete: function (xhr, textStatus) {
            //  Errors have already been handled, so only 
            //  deal with success cases
        }
    });  <--- WHAT GETS RETURNED TO PROMISE HERE?

... (same code for promiseB, etc.
    var notifyingPromiseA = intervalPromise(2000, 'a');
    var notifyingPromiseB = intervalPromise(2000, 'b');

...
    promiseA.done(function() {
        log("A done");
    }
    promiseB.done(function() {
        log("B done");
    }
    $.when(promiseA, promiseB).done(function() { log ("All done") });
}

function intervalPromise(millis, source) {
    var deferred = $.Deferred();
    //checkProgress();
    log("Checking progress on " + source);
    var id = setInterval(function () {
        deferred.notify();
        if (testGlobal.aDone && testGlobal.bDone) {
            clearInterval(id);
            deferred.resolve();
        }
    }, millis);
    return deferred.promise();
}

...

Answer 1

$.ajax() 返回 XMLHttpRequest 对象。从 jQuery v1.5 开始，$.ajax() 还实现并返回一个 Promise/Deferred 接口(interface)。

http://api.jquery.com/jQuery.ajax/

http://api.jquery.com/category/deferred-object/

使用 Promise，您可以根据原始 ajax 调用的结果链接其他回调。

// setup interval / timer to update UI not finished / still working logic

$.ajax().done(function() {
    // clear UI not fninshed / still working logic
});