Question

When making an async WinJS call that returns a promise, how frequently does the promise’s progress callback function get called? Consider the following code:

WinJS.xhr({ url: "http://localhost:51299/api/values" }).done(

    function complete() {
        console.log("complete: " + new Date().getTime());
    },

    function error() {
    },

    function progress() {
        console.log("progress: " + new Date().getTime());
    }

);

When I run this code, I see the progress: message multiple times per millisecond, and overall I might see the progress: message 100-200 times during the 1-2 seconds it takes to get all the data back from the xhr call.

1. Does the progress function get called on a set interval, and if so, what is that interval?
2. Is it a performance issue to let it run this frequently?
3. Is it better practice to limit it to run less often using some sort of timer function, or is the impact insignificant?

Answer 1

Progress callbacks are determined by the source of the promise – in this case, the XHR system. You don’t get control over when they come out. In this case, XmlHttpRequest is updating progress. There’s no overall control, it’s all up to the async operation when, and if, progress updates happen.

You should consider throttling your responses to progress updates if you find a performance impact, but it’s hard to say in general what to do, it depends on your app.