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Re: using BlobBuilder append method with generated binary data

From: Charles Pritchard <chuck@visc.us>
Date: Mon, 06 Dec 2010 13:03:06 -0800
Message-ID: <4CFD4F8A.4030704@visc.us>
To: Paul Kinlan <paulkinlan@google.com>
CC: Toni Ruottu <toni.ruottu@iki.fi>, Eric Uhrhane <ericu@google.com>, "public-webapps@w3.org" <public-webapps@w3.org>
Keep in mind the recent discussion from Moz: that IFRAME needs to be 
within the DOM tree to work
on FF4 and below. I'm going to track down some more discussion there, so 
warning, that statement isn't fact checked yet.

Afaik, WebKit (or just Chrome) does not have that restriction, and 
iframes can be moved across DOMs
without re-initializing the underlying content window.


On 12/6/2010 12:41 PM, Paul Kinlan wrote:
> That's what I thought when I first looked at it, however you can 
> pre-set the array length and then iterate over every byte and set it. 
> It is not ideal, but works.  I have it running on 
> http://appmator.appspot.com/ but it basically boils down to the 
> following:
>
> var ui8a = new Uint8Array(output.length);
> for(var i = 0; i< output.length; i++) {
>   ui8a[i] = output.charCodeAt(i);
> }
> bb.append(ui8a.buffer);
> var blob = bb.getBlob("application/octet-stream");
>
> var saveas = document.createElement("iframe");
> saveas.style.display = "none";
> saveas.src = window.createObjectURL(blob);
>
> P
>
> On Mon, Dec 6, 2010 at 8:34 PM, Toni Ruottu <toni.ruottu@iki.fi 
> <mailto:toni.ruottu@iki.fi>> wrote:
>
>     I do not think UInt8Array has any data. It is just a view for
>     accessing the buffer.
>
>      --Toni
>
>     On Sat, Dec 4, 2010 at 1:12 AM, Paul Kinlan <paulkinlan@google.com
>     <mailto:paulkinlan@google.com>> wrote:
>     > 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
>     > ArrayBuffer?
>     >
>     > I use it to generate zip files client side and attach them to an
>     > iframe so they are downloaded.
>     >
>     > P
>     > On Thursday, December 2, 2010, Toni Ruottu <toni.ruottu@iki.fi
>     <mailto: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 <mailto: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 <mailto: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 <mailto: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
>     > tw: @Paul_Kinlan <http://twitter.com/paul_kinlan>
>     > LinkedIn: http://uk.linkedin.com/in/paulkinlan
>     > Blog: http://paul.kinlan.me
>     > Skype: paul.kinlan
>     >
>
>
>
>
> -- 
> Paul Kinlan
> Developer Advocate @ Google for Chrome and HTML5
> t: +447730517944
> tw: @Paul_Kinlan
> LinkedIn: http://uk.linkedin.com/in/paulkinlan
> Blog: http://paul.kinlan.me
> Skype: paul.kinlan
Received on Tuesday, 7 December 2010 10:17:14 GMT

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