[webkit-dev] XHR responseArrayBuffer attribute: possible implementation

Geoffrey Garen ggaren at apple.com
Mon Oct 25 12:17:48 PDT 2010

Hi Chris.

I like the efficiency of this approach. And I agree with your premise that a developer will probably only want one type of data (raw, text, or XML) per request, and not more than one.

My biggest concern with this idea is that there's nothing obvious about the API pattern of three properties -- .responseText, .responseXML, and .responseArrayBuffer -- that makes clear that accessing one should prohibit access to the others. I wonder if there's a good way to make this clearer.

Maybe the API should require the programmer to specify in advance what type of data he/she will ask for. For example, an extra responeType parameter passed to open. The default behavior would be the values currently supported, but you could opt into something specific for extra safety/performance, and new types of data:

request.open("GET", "data.xml", true, "Text");
request.open("GET", "data.xml", true, "XML");
request.open("GET", "data.xml", true, "Bytes");


On Oct 22, 2010, at 4:47 PM, Chris Rogers wrote:

> A few weeks ago I brought up the idea of implementing the responseArrayBuffer attribute for XHR:
> http://dev.w3.org/2006/webapi/XMLHttpRequest-2/#the-responsearraybuffer-attribute
> One of the concerns was that it might require double the memory usage since the raw bytes would have to be accumulated along with the decoded text as it's being built up.  One possible solution which I've been discussing with James Robinson and Ken Russell is to defer decoding the text, and instead buffer the raw data as it comes in.  If there's any access to responseText (or responseXML), then the buffered data can be decoded into text at that time, and the buffered raw data discarded.  If that case happens, then from that point on no raw data buffering would happen and the text would be accumulated as it is right now.  Otherwise, if responseText is never accessed then the raw data continues to buffer until it's completely loaded.  Then an access to responseArrayBuffer can easily convert the raw bytes to an ArrayBuffer.
> The idea is that once responseText or responseXML is accessed, then it would no longer be possible to access responseArrayBuffer (an exception would be thrown).
> Conversely, once responseArrayBuffer is accessed, then it would no longer be possible to use responseText or responseXML (an exception would be thrown).
> This approach does seem a little strange because of the mutually exclusive nature of the access.  However, it seems that it would be hard to come up for a reasonable use case where both the raw bytes *and* the text would be needed for the same XHR.
> How does this sound as an approach?
> Chris
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> webkit-dev at lists.webkit.org
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