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Re: FormData questions

From: Jonas Sicking <jonas@sicking.cc>
Date: Sun, 14 Feb 2010 19:32:57 -0800
Message-ID: <63df84f1002141932k5dc69d33s5f192a865c0ea11e@mail.gmail.com>
To: Dmitry Titov <dimich@chromium.org>
Cc: Webapps WG <public-webapps@w3.org>
On Sun, Feb 14, 2010 at 6:04 PM, Dmitry Titov <dimich@chromium.org> wrote:
>
>
> On Sat, Feb 13, 2010 at 6:44 PM, Jonas Sicking <jonas@sicking.cc> wrote:
>>
>> On Sat, Feb 13, 2010 at 6:02 PM, Dmitry Titov <dimich@chromium.org> wrote:
>> > On Fri, Feb 12, 2010 at 5:32 PM, Jonas Sicking <jonas@sicking.cc> wrote:
>> >>
>> >> Hi WebApps fans!
>> >>
>> >> Working on implementing FormData and ran into a couple of questions.
>> >>
>> >> First of all, I assume that it is intended that a FromData object can
>> >> be submitted several times. I.e. that the following code is ok:
>> >>
>> >> fd = new FormData;
>> >> fd.append("name1", "foo");
>> >> xhr1 = new XMLHttpRequest;
>> >> xhr1.open(...);
>> >> xhr1.send(fd);
>> >> fd.append("name2", "bar");
>> >> xhr2 = new XMLHttpRequest;
>> >> xhr2.open(...);
>> >> xhr2.send(fd);
>> >>
>> >> where the first XHR will send 1 name/value pair, and the second XHR
>> >> will send 2. I do think this should be allowed, but I wanted to make
>> >> sure others agreed.
>> >
>> > What can be a reason to disallow this? FormData is just a collection of
>> > data
>> > objects. So assuming those XHR objects are sync, the code should work as
>> > you
>> > described.
>>
>> It complicates implementation a tiny amount, but IMHO not enough to
>> disallow it.
>>
>> > Interesting question though - what happens if XHR is async and the
>> > content
>> > of FormData changes while async operation is in progress. By analogy
>> > with
>> > scenario when underlying file of Blob object changes while async reading
>> > operation is in progress, would it be reasonable to fail the send and
>> > return
>> > 'error' ProgressEvent?
>>
>> I don't think raising an 'error' event should be correct, no. In my
>> example above I think the two requests should succeed successfully,
>> and the first one should submit one name/value pairs, and the second
>> should submit two.
>
> Does it mean that implementation should basically produce actual form data
> synchronously (do a deep copy) at the moment of xhr.send() even if the xhr
> is asynchronous? In case FormData includes Blobs backed by files on the disk
> it may be prohibitive.

I believe the text (non-Blob) data should be synchronously copied,
yes. The Blob data needs no special handling beyond what you already
have to do.

The way we implement this in Firefox is that we create a multiplex
data stream which consists of textual data mixed with Blob-backed
data. So we create a stream implementation that will alternately read
from in memory textual data, alternately from one or more Blobs.

The object we create synchronously upon the call to send() just
contains the text data and pointers to various Blobs. While this
object is created synchronously, no data is read from the Blob. Once
the network code reads from the stream, data is asynchronously read
from the Blob.

Don't know what setup you have for normal form submissions, so I'm not
sure if you can duplicate this behavior exactly.

Another way to look at it is that for the following code:

fd = new FormData;
fd.append("foo", "bar");
fd.append("name", myFile);

This synchronously creates a FormData object that contains enough
information to submit both textual data and a Blob. And since you can
create that information synchronously, you should for the following
code

xhr.send(fd);

be able to synchronously create a clone of the fd that similarly
contains enough information to submit both textual and Blob data, and
then set up to transmit that clone.

/ Jonas
Received on Monday, 15 February 2010 03:33:50 GMT

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