How Can I Reliably Detect File Download Completion in a Browser without Consistent 'load' Events?

Detecting File Download Completion in Browsers

Dynamic file generation and download can be a challenge when it comes to providing feedback to users. This article examines a scenario where a server generates a file in response to a request, but the browser doesn't provide a clear indication of when the download is complete.

The Problem: Event Detection Limitations

The author had difficulty detecting the download completion in their scenario where the file was generated dynamically and saved in a hidden iframe. The "load" event on the iframe didn't trigger consistently across browsers, leading to an unreliable indicator.

Alternative Solution: Client-Side Polling

One solution involves using a client-side polling mechanism in JavaScript. The algorithm works as follows:

• A unique token is generated and included in the download request.
• The browser shows a "waiting" indicator.
• A timer is started that checks for a cookie with the name "fileDownloadToken" periodically.
• If the cookie is present and matches the token, the "waiting" indicator is hidden.

Server-Side Response

The server-side code responds by setting a cookie with the name "fileDownloadToken" when the request contains a non-empty token value. This cookie serves as a signal to the client-side code that the download is complete.

Code Implementation

Example client-side JavaScript:

function blockResubmit() {
  // ...
  downloadTimer = window.setInterval(function() {
    var token = getCookie("fileDownloadToken");

    if (token == downloadToken || attempts == 0) {
      unblockSubmit();
    }

    attempts--;
  }, 1000);
}

Example server-side PHP:

$TOKEN = "downloadToken";

// Sets a cookie so that when the download begins the browser can
// unblock the submit button.
$this->setCookieToken($TOKEN, $_GET[$TOKEN], false);

$result = $this->sendFile();

Benefits of this Approach

• It doesn't require temporary file creation on the server.
• It's compatible with multiple browsers.
• It provides a reliable way to detect download completion in the specified scenario.

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