Detect when browser receives file download

Question

I have a page that allows the user to download a dynamically-generated file. It takes a long time to generate, so I\'d like to show a \"waiting\" indicator. The problem is,

Answer 1

This Java/Spring example detects the end of a Download, at which point it hides the "Loading..." indicator.

Approach: On the JS side, set a Cookie with a Max Expiration Age of 2 min, and poll every second for cookie expiration. Then the server-side overrides this cookie with an earlier expiration age -- the completion of the server process. As soon as the cookie expiration is detected in the JS polling, "Loading..." is hidden.

JS Side

function buttonClick() { // Suppose this is the handler for the button that starts
    $("#loadingProgressOverlay").show();  // show loading animation
    startDownloadChecker("loadingProgressOverlay", 120);
    // Here you launch the download URL...
    window.location.href = "myapp.com/myapp/download";
}

// This JS function detects the end of a download.
// It does timed polling for a non-expired Cookie, initially set on the 
// client-side with a default max age of 2 min., 
// but then overridden on the server-side with an *earlier* expiration age 
// (the completion of the server operation) and sent in the response. 
// Either the JS timer detects the expired cookie earlier than 2 min. 
// (coming from the server), or the initial JS-created cookie expires after 2 min. 
function startDownloadChecker(imageId, timeout) {

    var cookieName = "ServerProcessCompleteChecker";  // Name of the cookie which is set and later overridden on the server
    var downloadTimer = 0;  // reference to timer object    

    // The cookie is initially set on the client-side with a specified default timeout age (2 min. in our application)
    // It will be overridden on the server side with a new (earlier) expiration age (the completion of the server operation), 
    // or auto-expire after 2 min.
    setCookie(cookieName, 0, timeout);

    // set timer to check for cookie every second
    downloadTimer = window.setInterval(function () {

        var cookie = getCookie(cookieName);

        // If cookie expired (NOTE: this is equivalent to cookie "doesn't exist"), then clear "Loading..." and stop polling
        if ((typeof cookie === 'undefined')) {
            $("#" + imageId).hide();
            window.clearInterval(downloadTimer);
        }

    }, 1000); // Every second
}

// These are helper JS functions for setting and retrieving a Cookie
function setCookie(name, value, expiresInSeconds) {
    var exdate = new Date();
    exdate.setTime(exdate.getTime() + expiresInSeconds * 1000);
    var c_value = escape(value) + ((expiresInSeconds == null) ? "" : "; expires=" + exdate.toUTCString());
    document.cookie = name + "=" + c_value + '; path=/';
}

function getCookie(name) {
    var parts = document.cookie.split(name + "=");
    if (parts.length == 2 ) {
        return parts.pop().split(";").shift();
    }
}

Java/Spring Server Side

    @RequestMapping("/download")
    public String download(HttpServletRequest request, HttpServletResponse response) throws Exception {
        //... Some logic for downloading, returning a result ...

        // Create a Cookie that will override the JS-created Max-Age-2min Cookie 
        // with an earlier expiration (same name)
        Cookie myCookie = new Cookie("ServerProcessCompleteChecker", "-1");
        myCookie.setMaxAge(0); // this is immediate expiration, 
                               // but can also add +3 sec. for any flushing concerns
        myCookie.setPath("/");
        response.addCookie(myCookie);
        //... -- presumably the download is writing to the Output Stream...
        return null;
}