Detect when browser receives file download

04/16/2020 21:00:01

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, I can't figure out how to detect when the browser has received the file, so I can hide the indicator.

I'm making the request in a hidden form, which POSTs to the server, and targets a hidden iframe for its results. This is so I don't replace the entire browser window with the result. I listen for a "load" event on the iframe, in the hope that it will fire when the download is complete.

I return a "Content-Disposition: attachment" header with the file, which causes the browser to show the "Save" dialog. But the browser doesn't fire a "load" event in the iframe.

One approach I tried is using a multi-part response. So it would send an empty HTML file, as well as the attached downloadable file. For example:

Content-type: multipart/x-mixed-replace;boundary="abcde"

Content-type: text/html

Content-type: application/vnd.fdf
Content-Disposition: attachment; filename=foo.fdf


This works in Firefox; it receives the empty HTML file, fires the "load" event, then shows the "Save" dialog for the downloadable file. But it fails on IE and Safari; IE fires the "load" event but doesn't download the file, and Safari downloads the file (with the wrong name and content-type), and doesn't fire the "load" event.

A different approach might be to make a call to start the file creation, then poll the server until it's ready, then download the already-created file. But I'd rather avoid creating temporary files on the server.

Does anyone have a better idea?

Verified Answer (445 Votes)

11/13/2010 01:54:14

One possible solution uses JavaScript on the client.

The client algorithm:

  1. Generate a random unique token.
  2. Submit the download request, and include the token in a GET/POST field.
  3. Show the "waiting" indicator.
  4. Start a timer, and every second or so, look for a cookie named "fileDownloadToken" (or whatever you decide).
  5. If the cookie exists, and its value matches the token, hide the "waiting" indicator.

The server algorithm:

  1. Look for the GET/POST field in the request.
  2. If it has a non-empty value, drop a cookie (e.g. "fileDownloadToken"), and set its value to the token's value.

Client source code (JavaScript):

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

function expireCookie( cName ) {
    document.cookie = 
        encodeURIComponent(cName) + "=deleted; expires=" + new Date( 0 ).toUTCString();

function setCursor( docStyle, buttonStyle ) {
    document.getElementById( "doc" ).style.cursor = docStyle;
    document.getElementById( "button-id" ).style.cursor = buttonStyle;

function setFormToken() {
    var downloadToken = new Date().getTime();
    document.getElementById( "downloadToken" ).value = downloadToken;
    return downloadToken;

var downloadTimer;
var attempts = 30;

// Prevents double-submits by waiting for a cookie from the server.
function blockResubmit() {
    var downloadToken = setFormToken();
    setCursor( "wait", "wait" );

    downloadTimer = window.setInterval( function() {
        var token = getCookie( "downloadToken" );

        if( (token == downloadToken) || (attempts == 0) ) {

    }, 1000 );

function unblockSubmit() {
  setCursor( "auto", "pointer" );
  window.clearInterval( downloadTimer );
  expireCookie( "downloadToken" );
  attempts = 30;

Example server code (PHP):

$TOKEN = "downloadToken";

// Sets a cookie so that when the download begins the browser can
// unblock the submit button (thus helping to prevent multiple clicks).
// The false parameter allows the cookie to be exposed to JavaScript.
$this->setCookieToken( $TOKEN, $_GET[ $TOKEN ], false );

$result = $this->sendFile();


public function setCookieToken(
    $cookieName, $cookieValue, $httpOnly = true, $secure = false ) {

    // See:
    // See:
    // See:
        2147483647,            // expires January 1, 2038
        "/",                   // your path
        $_SERVER["HTTP_HOST"], // your domain
        $secure,               // Use true over HTTPS
        $httpOnly              // Set true for $AUTH_COOKIE_NAME

Answer #2 (27 Votes)

06/04/2011 22:25:51

A very simple (and lame) one line solution is to use the window.onblur() event to close the loading dialog. Of course, if it takes too long and the user decides to do something else (like reading emails) the loading dialog will close.


Answer #3 (14 Votes)

09/11/2010 03:16:21

old thread, i know...

but those, that are lead here by google might be interested in my solution. it is very simple, yet reliable. and it makes it possible to display real progress messages (and can be easily plugged in to existing processes):

the script that processes (my problem was: retrieving files via http and deliver them as zip) writes the status to the session.

the status is polled and displayed every second. thats all (ok, its not. you have to take care of a lot of details [eg concurrent downloads], but its a good place to start ;-)).

the downloadpage:

    <a href="download.php?id=1" class="download">DOWNLOAD 1</a>
    <a href="download.php?id=2" class="download">DOWNLOAD 2</a>
    <div id="wait">
    Please wait...
    <div id="statusmessage"></div>
//this is jquery
               $('#statusmessage').html('prepare loading...');
               setTimeout('getstatus()', 1000);
    function getstatus(){
          url: "/getstatus.php",
          type: "POST",
          dataType: 'json',
          success: function(data) {
              setTimeout('getstatus()', 1000);


echo json_encode($_SESSION['downloadstatus']);


//and spit the generated file to the browser
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