Use cookies to help execute custom JavaScript after a file starts to download

RELATED TO:

AUTHOR: Peter Milchev

DATE POSTED: October 15, 2018

HOW TO

Detect when the file is ready to start downloading.

DESCRIPTION

Usually, when a file is being downloaded, the server clears the response and responds with the file to download. 

SOLUTION

To workaround this, we can add a cookie to the response on the server and look for it in the client-side. To achieve that, before sending the request to the server, we can start checking for this cookie on a given interval of time until the cookie is present. Once the cookie is present, we will know that the server responded with the file and we can do our custom logic and clear the cookie.

<telerik:RadButton runat="server" ID="RadButton1" OnClientClicking="OnClientClicking" OnClick="RadButton1_Click" Text="Download File" AutoPostBack="true" />
function OnClientClicking(sender, args) {
    startPolling();
}

var cookieAPI = (function (undefined) {
    function clearCookie(cookieName) {
        setCookie(cookieName, "", new Date());
    }
 
    function getCookie(cname) {
        var name = cname + "=";
        var decodedCookie = decodeURIComponent(document.cookie);
        var ca = decodedCookie.split(';');
        for (var i = 0; i < ca.length; i++) {
            var c = ca[i];
            while (c.charAt(0) == ' ') {
                c = c.substring(1);
            }
            if (c.indexOf(name) == 0) {
                return c.substring(name.length, c.length);
            }
        }
        return "";
    }
    function setCookie(cname, cvalue, exdays) {
        var d = new Date();
        d.setTime(d.getTime() + (exdays * 24 * 60 * 60 * 1000));
        var expires = "expires=" + d.toUTCString();
        document.cookie = cname + "=" + cvalue + ";" + expires + ";path=/";
    }
 
    return {
        getCookie: getCookie,
        setCookie: setCookie,
        clearCookie: clearCookie
    }
})();

 

function startPolling() {
    var cookieName = "file-download";
    cookieAPI.clearCookie(cookieName);
 
    var interval = setInterval(function () {
        var cookieMessage = cookieAPI.getCookie(cookieName);
 
        if (cookieMessage) {
            clearInterval(interval);
 
            // use message form server
            console.log(cookieMessage);
 
            // clear cookie
            cookieAPI.clearCookie(cookieName)
        }
    }, 500);
}

 

protected void RadButton1_Click(object sender, EventArgs e)
{
    Response.Clear();
 
    //Indicate the type of data being sent
    Response.ContentType = "application/octet-stream";
 
    //Name the file
    Response.AddHeader("Content-Disposition", "attachment; filename=\"someFile.txt\"");
 
    // simulate long server processing
    Thread.Sleep(4000);
    Response.BinaryWrite(Encoding.UTF8.GetBytes("This is a dummy file"));
 
    // add cookie to know the file begins downloading
    HttpCookie myCookie = new HttpCookie("file-download");
    myCookie.Value = "The file started downloading!";
    HttpContext.Current.Response.Cookies.Add(myCookie);
 
    Response.End();
}

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