How Can I Reliably Detect File Download Completion in a Browser?

Handle File Download Completion Detection

Problem:

Detecting when a browser fully receives a dynamically generated file download becomes crucial when displaying waiting indicators. Currently, using an iframe's "load" event fails to address this issue due to browser inconsistencies.

Solution 1: Browser-Based Token Verification

A client-side JavaScript solution involves:

• Generating a unique token for the download request.
• Submitting the request with the token.
• Displaying a "waiting" indicator.
• Setting a timer to periodically check for a corresponding cookie with the token value.
• Hiding the indicator upon finding the cookie.

On the server-side, the algorithm:

• Checks for the token in the request.
• Sets a cookie with the token value if found.

Code Snippet (JavaScript):

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

Code Snippet (PHP):

// Set a cookie to unblock submit button when download begins.
$TOKEN = "downloadToken";
$this->setCookieToken($TOKEN, $_GET[$TOKEN]);

Solution 2: Server Polling

Alternatively, a server-side approach involves:

• Initiating file creation.
• Polling the server until completion.
• Downloading the ready file.

This method avoids creating temporary server files. However, it requires client-server communication during the waiting period.

Pros and Cons:

Using a browser token allows for dynamic detection, but may have browser compatibility issues. Server polling is a more reliable but slower option.

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