[whatwg] File API - does Blob 'track' the changes in the underlying file on the disk?

Dmitry Titov dimich at google.com
Fri Jan 8 14:19:39 PST 2010

I think I should have used public-webapps maillist. File API is discussed
there. Sorry for spam.

On Fri, Jan 8, 2010 at 1:56 PM, Dmitry Titov <dimich at google.com> wrote:

> Hi,
> Does the Blob, which is obtained as File (so it refers to an actual file on
> disk) track the changes in the underlying file and 'mutates', or does it
> represent the 'snapshot' of the file, or does it become 'invalid'?
> Today, if a user selects a file using <input type=file>, and then the file
> on the disk changes before the 'submit' is clicked, the form will submit the
> latest version of the file.
> This may be a surprisingly popular use case, when user submits a file via
> form and wants to do 'last moment' changes in the file, after partial
> pre-populating the form. It works 'intuitively' today.
> Now, if the page decides to use XHR to upload the file, I think
> var file = myInputElement.files[0];
> var xhr = ...
> xhr.send(file);
> should also send the version of the file that exists at the moment of
> xhr.send(file), not when user picked the file (for consistency with form
> action).
> Assuming this is desired behavior, what should the following do:
> var file = myInputElement.files[0];
> var blob = file.slice(0, file.size);
> // ... now file on the disk changes ...
> xhr.send(blob);
> Will it:
> - send the new version of the whole file (and update blob.size?)
> - send captured number of bytes from the new version of the file (perhaps
> truncated since file may be shorter now)
> - send original bytes from the previous version of the file that existed
> when Blob was created (sort of 'copy on write')
> - throw exception
> ?
> Thanks,
> Dmitry
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