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Re: using BlobBuilder append method with generated binary data

From: Paul Kinlan <paulkinlan@google.com>
Date: Fri, 3 Dec 2010 23:12:45 +0000
Message-ID: <AANLkTi=9M0DnOKYFV1OThQuUeLaN-349Vu_Prvjon+8s@mail.gmail.com>
To: Toni Ruottu <toni.ruottu@iki.fi>
Cc: Eric Uhrhane <ericu@google.com>, "public-webapps@w3.org" <public-webapps@w3.org>
I might have missed something but for appending client-side generated
data to a blob can't you just append a UInt8Array as that is based off

I use it to generate zip files client side and attach them to an
iframe so they are downloaded.

On Thursday, December 2, 2010, Toni Ruottu <toni.ruottu@iki.fi> wrote:
> My code had a bug in it. The APIs seem to be working ok for my purposes.
> Using the APIs, I was able to write Firefox's sendAsBinary method for chrome.
> It is available from http://javascript0.org/wiki/Portable_sendAsBinary
>   --Toni
> On Tue, Nov 16, 2010 at 5:25 PM, Toni Ruottu <toni.ruottu@iki.fi> wrote:
>> Sorry for slow answer. Took me a while to get on top of this. I think
>> they just might answer my problem. However, I can not really tell
>> before I see a working implementation. I wrote a test. See
>> http://www.cs.helsinki.fi/u/twruottu/testi/bpost3.html It does not
>> currently work with the latest development version of Chrome. I
>> wonder, if I am doing something wrong.
>>  --Toni
>> On Wed, Oct 20, 2010 at 6:17 PM, Eric Uhrhane <ericu@google.com> wrote:
>>> Toni:
>>>    BlobBuilder now has an append() method that takes an ArrayBuffer,
>>> and FileReader has readAsArrayBuffer.  Do these together satisfy your
>>> needs?
>>>        Eric
>>> On Wed, Oct 20, 2010 at 3:39 AM, Toni Ruottu <toni.ruottu@iki.fi> wrote:
>>>> I have discussed the topic before on some Chromium bug threads. I
>>>> searched the archives of this mailing list for blobs and generated
>>>> content, but was not too successful. There may be multiple different
>>>> reasons why one would need to build blobs from generated data. The one
>>>> that concerns me, is being able to do binary POSTs with XmlHTTPRequest
>>>> in a standard manner. To send binary data over XHR one is required to
>>>> send out a blob, but first the blob needs to be constructed with
>>>> BlobBuilder. Let me fill you in on how I see the current situation.
>>>> Maybe someone can spot an error in my line of thought.
>>>> A new (empty) BlobBuilder is created by stating...
>>>> var bb = new BlobBuilder();
>>>> After creating a BlobBuilder, the user appends some data into the blob
>>>> being generated by using the append function. The append function is
>>>> overloaded and can be used to append multiple different types of data
>>>> into the blob. Currently only existing blobs, and utf-8 text are
>>>> supported.
>>>> One can add an existing blob into the new one by doing...
>>>> bb.append(existingBlob);
>>>> One can add a text string in utf-8 format by doing...
>>>> bb.append(aTextString);
>>>> Once the data is in there, it is possible to construct the new blob with...
>>>> var blob = bb.getBlob()
>>>> A blob has a slice method which makes it possible to turn a large blob
>>>> into smaller ones. It is thus possible to cut out single bytes from a
>>>> blob and use them construct new blobs by appending copies of these
>>>> sample bytes together with the BlobBuilder.
>>>> As sample bytes can only be generated by writing text into the blob as
>>>> utf-8, it is not possible to generate all 8-bit patterns. More
>>>> specifically, the bit patterns that are illegal in any given utf-8
>>>> string, are not possible to generate using the current API. This leads
>>>> developers into using various hacks to overcome the issue.
>>>> For example one could ask the user to provide a file with sample
>>>> bytes. Methods for reading a file into a blob are available, so there
>>>> is no need to generate the sample bytes. There is however no way to
>>>> make sure that the user provided a file with correct bit-patterns as
>>>> the blob interface is to a large degree a read-only interface. A
>>>> demonstration of using a file to overcome the issue is available at
>>>> http://www.cs.helsinki.fi/u/twruottu/testi/bpost.html
>>>> To solve the problem a third append function which accepts a list of
>>>> integers, and adds the corresponding bytes into the blob is needed.
>>>> Once such method has been introduced it should be possible to write
>>>> the 7-bit ascii string "foo" into a BlobBuilder by doing...
>>>> bb.append([102,111,111]);
>>>> It should also be possible to add bytes 0x00 a

Paul Kinlan
Developer Advocate @ Google for Chrome and HTML5
t: +447730517944
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Received on Friday, 3 December 2010 23:13:17 UTC

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