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File API to separate reading from files

From: Nikunj R. Mehta <nikunj.mehta@oracle.com>
Date: Wed, 19 Aug 2009 11:47:57 -0700
Message-Id: <5D0F3662-D26C-40A9-AD85-C3AB4214256F@oracle.com>
To: Web Applications Working Group WG <public-webapps@w3.org>
Here's an alternative, more easily extensible, proposal for reading  
files. It provides applications a way to read small amounts of data at  
a time. It also allows applications to concurrently read the same file.

Firstly, there is a simple interface to access file metadata. This  
metadata is always accessed synchronously. A file object could be  
passed to XHR, in which case it can upload the file during the send()  
process.

interface File {
   readonly attribute DOMString name;
   readonly attribute DOMString mediaType;
   readonly atribute DOMString url;
   readonly attribute unsigned long long size;
}

Secondly, a list of files can be obtained using some UI.

typedef sequence<File> FileList;

Thirdly, an abstract interface is an input stream that is not limited  
to files. It works at the level of bytes that files are made of. The  
read() operation can specify the extent that is required. If an  
application wishes to read small increments, it can thus specify those  
increments. Of course, the File interface identifies its size, so the  
application can suitably choose increments. Processing of blocks read  
from the file occurs in callbacks. XHR could also consider taking an  
InputStream parameter during the send() operation.

interface InputStream {
   read(in DataHandler, [optional in] long long offset, [optional in]  
long long length);	
   abort();
   attribute Function onerror;
}

Fourthly, reading a block of bytes is supported through an interface  
that accepts an array of integers. This is similar to the Gears Blob  
interface.

[CallBack=FunctionOnly]
interface DataHandler {
   handle(in sequence<int> data);
}

Fifthly, a file can be used for reading an input stream by specifying  
the name of a file when constructing the stream
[Constructor(in File toOpen)]
interface FileInputStream : InputStream {
}

Sixthly, one can create various kinds of derived readers such as text  
reader, binary string reader, and data URL reader. By inheriting from  
InputStream, the basic mechanisms such as abort and onerror are  
inherited. Moreover, the base read behavior is altered by the subclass  
although it behaves in a similar manner, except that the data seen  
outside is different.

[Constructor(in InputStream base)]
interface BinaryStringInputStream : InputStream {
   read(in StringDataHandler, [optional in] long long offset,  
[optional in] long long length);	
}

The callback is provided a DOMString. The String's length is expected  
to match the increment requested.

[CallBack=FunctionOnly]
interface StringDataHandler {
	handle(in DOMString data);
}

For text reading, encoding is optionally specified.

[Constructor(in InputStream base, [optional in] DOMString encoding)]
interface TextInputStream : InputStream {
   read(in StringDataHandler, [optional in] long long offset,  
[optional in] long long length);	
}


A file can be alternatively read as a dataURL using a similar kind of  
handler as above.

[Constructor(in InputStream base)]
interface FileDataURL: InputStream {
   read(in StringDataHandler, [optional in] long long offset,  
[optional in] long long length);	
}

This API has the advantage that it can cleanly be extended to deal  
with both writing use cases and binary data. Furthermore, it can also  
support extensions that perform cryptographic, compression, or coding  
on top of the basic interfaces.

To compare with the editor's draft, here's a typical programming case  
in JavaScript:

var fileList = ...
// There is a mistake in the example provided in Section 3 where it  
does fileList.files[0]
var myFile = fileList[0];

// *According to editor's draft*
myFile.getAsText(handleDataAsText)
function handleDataAsText(fileContent, error) {
   if (error) {

   }
}

// *According to my proposal*
var stream = new TextInputStream(new FileInputStream(myFile), "UTF-16");
stream.read(handleDataAsText);
stream.onerror = errorHandler;
function handleDataAsText(fileContent) {

}

function errorHandler(error) {

}

Note the two differences:
1. Error handling is separated from file reading
2. Two extra objects are needed to read text data out of the file.  
However, the composability of input streams enables a far richer  
library to operate.

This API matches more closely the Java API for IO. It also benefits  
from the extensibility model used in Java, while retaining the  
asynchronous processing nature that is preferred in ECMAScript  
environments. It is also not too different from the editor's draft in  
that it does not introduce a completely different kind of data  
processing - we are still looking at callbacks. However, the  
improvement is in the composability of streams as well as supporting  
multiple concurrent file readers and processing blocks of data at a  
time.

Progress events can be built on top but I welcome suggestions to build  
them in to this proposal.

Nikunj
http://o-micron.blogspot.com
Received on Wednesday, 19 August 2009 18:50:30 GMT

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