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Re: XMLHttpRequest.responseBlob

From: Darin Fisher <darin@chromium.org>
Date: Wed, 28 Apr 2010 12:45:24 -0700
Message-ID: <q2zbd8f24d21004281245x58e38a93n615b5124da8d4385@mail.gmail.com>
To: Michael Nordman <michaeln@google.com>
Cc: Jonas Sicking <jonas@sicking.cc>, Web Applications Working Group WG <public-webapps@w3.org>
On Wed, Apr 28, 2010 at 11:57 AM, Michael Nordman <michaeln@google.com>wrote:

>
>
> On Wed, Apr 28, 2010 at 11:21 AM, Jonas Sicking <jonas@sicking.cc> wrote:
>
>> Ugh, sent this originally to just Darin. Resending to the list.
>>
>> On Wed, Apr 28, 2010 at 10:11 AM, Darin Fisher <darin@chromium.org>
>> wrote:
>> > On Tue, Apr 27, 2010 at 2:04 PM, Jonas Sicking <jonas@sicking.cc>
>> wrote:
>> >>
>> >> On Tue, Apr 27, 2010 at 1:59 PM, Darin Fisher <darin@chromium.org>
>> wrote:
>> >> > On Tue, Apr 27, 2010 at 1:33 PM, Jonas Sicking <jonas@sicking.cc>
>> wrote:
>> >> >>
>> >> >> On Tue, Apr 27, 2010 at 1:26 PM, Darin Fisher <darin@chromium.org>
>> >> >> wrote:
>> >> >> >> It would be nice to be able to allow streaming such that every
>> time
>> >> >> >> a
>> >> >> >> progress event is fired only the newly downloaded data is
>> available.
>> >> >> >> The UA is then free to throw away that data once the event is
>> done
>> >> >> >> firing. This would be useful in the cases when the page is able
>> to
>> >> >> >> do
>> >> >> >> incremental parsing of the resulting document.
>> >> >> >>
>> >> >> >> If we add a 'load mode' flag on XMLHttpRequest, which can't be
>> >> >> >> modified after send() is called, then streaming to a Blob could
>> >> >> >> simply
>> >> >> >> be another enum value for such a flag.
>> >> >> >>
>> >> >> >> There is still the problem of how the actual blob works. I.e.
>> does
>> >> >> >> .responseBlob return a new blob every time more data is returned?
>> Or
>> >> >> >> should the same Blob be constantly modifying? If modifying, what
>> >> >> >> happens to any in-progress reads when the file is modified? Or do
>> >> >> >> you
>> >> >> >> just make the Blob available once the whole resource has been
>> >> >> >> downloaded?
>> >> >> >>
>> >> >> >
>> >> >> >
>> >> >> > This is why I suggested using FileWriter.  FileWriter already has
>> to
>> >> >> > deal with
>> >> >> > most of the problems you mentioned above,
>> >> >>
>> >> >> Actually, as far as I can tell FileWriter is write-only so it
>> doesn't
>> >> >> deal with any of the problems above.
>> >> >
>> >> > When you use createWriter, you are creating a FileWriter to an
>> existing
>> >> > File.
>> >> > The user could attempt to create a FileReader to the very same File
>> >> > while
>> >> > a FileWriter is open to it.
>> >> > It is true that for <input type=saveas> there is no way to get at the
>> >> > underlying
>> >> > File object.  That is perhaps a good thing for the use case of
>> >> > downloading
>> >> > to
>> >> > a location specified by the user.
>> >>
>> >> Ah. But as far as I can tell (and remember), it's still fairly
>> >> undefined what happens when the OS file under a File/Blob object is
>> >> mutated.
>> >>
>> >> / Jonas
>> >
>> > Agreed.  I don't see it as a big problem.  Do you?  The application
>> > developer is
>> > in control.  They get to specify the output file (via FileWriter) that
>> XHR
>> > sends its
>> > output to, and they get to know when XHR is done writing.  So, the
>> > application
>> > developer can avoid reading from the file until XHR is done writing.
>>
>> Well, it seems like a bigger deal here since the file is being
>> constantly modified as we're downloading data into it, no? So for
>> example if you grab a File object after the first progress event, what
>> does that File object contain after the second? Does it contain the
>> whole file, including the newly downloaded data? Or does it contain
>> only the data after the first progress event? Or is the File object
>> now invalid and can't be used?
>>
>
> What gears did about that was to provide a 'snapshot' of the
> downloaded data each time responseBlob was called, with
> the 'snapshot' being consistent with the progress events
> having been seen by the caller. The 'snapshot' would remain
> valid until discarded by the caller. Each snapshot just provided
> a view onto the same data which maybe was in memory or
> maybe had spilled over to disk unbeknownst to the caller.
>
>
>>
>> I'm also still unsure that a FileWriter is what you want generally. If
>> you're just downloading temporary data, but data that happens to be so
>> large that you don't want to keep it in memory, you don't want to
>> bother the user asking for a location for that temporary file. Nor do
>> you want that file to be around once the user leaves the page.
>>
>
> I think the point about not requiring the caller to manage the 'file' are
> important.
>
>
>> Sure, if the use case is actually downloading and saving a file for
>> the user to use, rather than for the page to use, then a FileWriter
>> seems like it would work. I.e. if you want something like
>> "Content-Disposition: attachment", but where you can specify request
>> headers. Is that the use case?
>>
>
> Mods to xhr to access the response more opaquely is a fairly general
> feature request. One specific use case is to download a resource via xhr
> and then save the results in a sandboxed file system. So "for the page to
> use".
>

^^^ That is the use case I'm primarily interested in.

I think it would be beneficial if downloading to disk was not rate-limited
by routing chunks through JS.

I don't care as much about downloading to a user specified location, but I
think that's an interesting use case as well.  XHR gives the app more
flexibility (custom headers, cross-origin, etc.), and FileWriter allows the
app to "save as" URLs that do not have a C-D header that forces a download.



>
> The notion of having a streaming interface on xhr is interesting. That with
> a
> BlobBuilder capability could work. If a streaming xhr mode provided new
> data in the form of 'blobs' where each blob was just the newly received
> data,
> the caller could use a BlobBuilder instance to concatenate the set of
> received
> data blobs. And then take blobBuilder.getBlob() and do what they will with
> it.   xhr.ondatareceived = function (data) { builder.appendBlob(data); }
>
>
^^^ I like that proposal for streaming.

-Darin
Received on Wednesday, 28 April 2010 19:45:55 GMT

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