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Re: Transferable and structured clones, was: Re: [FileAPI] Deterministic release of Blob proposal

From: Greg Billock <gbillock@google.com>
Date: Tue, 6 Mar 2012 12:04:10 -0800
Message-ID: <CAAxVY9coG976xP5-QJs49sSy7AW5pygevokmAgsdwqX=0CszjA@mail.gmail.com>
To: "public-webapps@w3.org" <public-webapps@w3.org>
On Mon, Mar 5, 2012 at 6:46 PM, Charles Pritchard <chuck@jumis.com> wrote:
> On 3/5/2012 5:56 PM, Glenn Maynard wrote:
> On Mon, Mar 5, 2012 at 7:04 PM, Charles Pritchard <chuck@jumis.com> wrote:
>> Do you see old behavior working something like the following?
>> var blob = new Blob("my new big blob");
>> var keepBlob = blob.slice(); destination.postMessage(blob, '*', [blob]);
>> // is try/catch needed here?
> You don't need to do that.  If you don't want postMessage to transfer the
> blob, then simply don't include it in the transfer parameter, and it'll
> perform a normal structured clone.  postMessage behaves this way in part for
> backwards-compatibility: so exactly in cases like this, we can make Blob
> implement Transferable without breaking existing code.
> See http://dev.w3.org/html5/postmsg/#posting-messages and similar
> postMessage APIs.
> Web Intents won't have a transfer map argument.
> http://dvcs.w3.org/hg/web-intents/raw-file/tip/spec/Overview.html#widl-Intent-data
> For the Web Intents structured cloning algorithm, Web Intents would be
> inserting into step 3:
>     If input is a Transferable object, add it to the transfer map.
> http://www.whatwg.org/specs/web-apps/current-work/multipage/common-dom-interfaces.html#internal-structured-cloning-algorithm
> Then Web Intents would move the first section of the structured cloning
> algorithm to follow the internal cloning algorithm section, swapping their
> order.
> http://www.whatwg.org/specs/web-apps/current-work/multipage/common-dom-interfaces.html#safe-passing-of-structured-data
> That's my understanding.

We've been discussing the merits of this approach vs using a transfer
array argument. There's a lot to like about this alternative -- it
conserves arguments and looks simpler than the transfer map, as well
as not having the headaches of whether you can do (null, [port]) or
(port, [port]) and concerns like that.

The advantage of using the transfer map param is that it is more
contiguous with existing practice. We'd kind of hoped that this
particular debate was finalized before we got to the point of needing
to make a decision, so we bluffed and left it out of the web intents
spec draft. :-) At this point, I'm leaning toward needing to add a
transfer map parameter, and then dealing with that alongside other
uses, given the state of thinking on Transferables support and the
need to make this pretty consistent across structure clone

I do think that complexity might be better solved by the type system
(i.e. a "new Transferable(ArrayBuffer)"), which would require a
different developer mechanic to set up clone vs transfer, but would
relieve complexity in the invocation of structured clone itself:
transferables could just always transfer transparently. I don't know
if, given current practice with MessagePort, that kind of solution is

> Something like this may be necessary if Blob were a Transferable:
> var keepBlob = blob.slice();
> var intent = new Intent("-x-my-intent", blob);
> navigator.startActivity(intent, callback);
>> And we might have an error on postMessage stashing it in the transfer
>> array if it's not a Transferable on an older browser.
> Example of how easy the neutered concept applies to Transferrable:
> var blob = new Blob("my big blob");
> blob.close();
> I like the idea of having Blob implement Transferrable and adding close to
> the Transferrable interface.
> File.close could have a better relationship with the cache and/or locks on
> data.
> Some history on Transferrable and structured clones:
> Note: MessagePort does have a close method and is currently the only
> Transferrable mentioned in WHATWG:
> http://www.whatwg.org/specs/web-apps/current-work/multipage/common-dom-interfaces.html#transferable-objects
> ArrayBuffer is widely implemented. It was the second item to implement
> Transferrable:
> http://www.khronos.org/registry/typedarray/specs/latest/#9
> Subsequently, ImageData adopted Uint8ClampedArray for one of its properties,
> adopting TypedArrays:
> http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#imagedata
> This has lead to some instability in the structured clone algorithm for
> ImageData as the typed array object for ImageData is read-only.
> https://www.w3.org/Bugs/Public/show_bug.cgi?id=13800
> ArrayBuffer is still in a strawman state.
> -Charles
Received on Tuesday, 6 March 2012 20:04:43 UTC

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