问题
I have an application where I need to know when a user's Rails/Paperclip file download is complete. My app is set up to interact with Amazon S3 and I need to run a javascript function when the user has received the completed file.
How can I do this?
回答1:
Tracking weather or not the download completes is hard, especially in Javascript. There are a few blurred lines in your question which makes me think its not possible.
First, send_file passes a special header to tell the webserver telling it what to send. See the send_file docs. Rails doesn't actually send the file at all, it sets this header which tells the webserver to send the file but then returns immediately, and moves on to serve another request. To be able to track if the download completes you'll have to occupy your Rails application process sending the file and block until the user downloads it, instead of leaving that to the webserver (which is what its designed to do). This is super inefficient.
Next, how can you still be on a page to execute a javascript function if you are downloading a file? Your user clicks the file download link and is taken to wherever the file is, weather that be a send_file from Rails or a redirect to S3 or whatever, they are no longer on the page they came from. If you are thinking about the way Chrome or Firefox works where the download goes into a download manager and the user stays on the page, theres no more interaction with the server on the old page! If you want that page to be notified of download completion, then you'd need a periodic check or long poll to the server to see if the download is done.
I think you'd be better served by redirecting to the S3 file and setting a session variable to redirect the user to where you want them to go after the download is complete so that the next time they visit any page they are back in your planned flow.
Hope this helps!
来源：https://stackoverflow.com/questions/4787625/can-we-find-out-when-a-paperclip-download-is-complete