How Can We Reliably Detect Browser File Download Completion Without Browser Events?

Detecting Browser File Download Completion

Problem Statement

A page featuring a dynamic file download mechanism requires a visual indicator to signal when the download completes. However, browsers do not always trigger events to indicate file reception when a "Save" dialog appears.

Server Response Limitations

The provision of a "Content-Disposition: attachment" header instructs browsers to display the "Save" dialog, but it suppresses the loading of the iframe used for event monitoring.

Proposed Solution

Implement a client-based solution leveraging JavaScript and server-side cookies:

Client Algorithm

1. Generate a unique token.
2. Submit the download request with the token.
3. Display the "waiting" indicator.
4. Poll the server at regular intervals for a cookie named "fileDownloadToken" that matches the token.
5. If the cookie is found, remove the "waiting" indicator.

Server Algorithm

1. Extract the token from the request.
2. Create a cookie with the name "fileDownloadToken" and set its value to the received token.

Example Implementation

• Client-Side JavaScript (simplified)

• Server-Side PHP (simplified)

Benefits

• Does not require temporary file creation.
• Detects file download readiness without relying on event listeners.
• Can be customized to specific token names and cookie settings.

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