A few helpful things to remember:
Goal: Write out binary data stored in a ArrayBuffer (html5 typed array) to a file
What I learned:
If you open a file with FileUtils.openSafeFileOutputStream, then the only way to properly close it is with FileUtils.closeSafeFileOutputStream. This is something the docs forget to tell you but very important.
Typed Arrays have bounds checking on instantiation. If byteLength%BYTES_IN_TYPE!=0 then an exception is thrown.
String are written to memory sequentially. Array indexes are not. TypedArray index locations can't be accessed directly (reading them actually reads their wrapper object, or to be more precise, it seems if you run "var a = buffer[i], b = buffer[i+1];" you would find that &a is not related to &b).
Writing out in an OutputStream will convert to a string whatever object you pass into the buffer parameter!!! The result of such a call (toString) will be what is set as the location in memory of the buffer, assuming typeof buffer.toString() == "string". If you could spoof that, that would be wearrryy wuseful.
Ways to write to a file
1) NetUtil.asyncCopy
2) nsIFileOutputStream
3) nsIBinaryOutputStream <--- <3 some love
I tried and tried and tried some more, to find some Firefox service or component which could just write the binary out WITHOUT conversion to a string first. Couldn't find one. They ALL took in a nsIInputStream only. Which sucks because my data isn't an input stream, AND I can't just implement nsIInputStream in javascript due to "Native Only" method calls.
So, any way to have a ArrayBuffer backed InputStream?? That would be nice. But no soup for me, it seemed.
I was just trying to avoid implementing my own buffered writer. Anyways, given that FileOutputStream only accepts string input, and NetUtil.asyncCopy only takes a legit input stream, I've no choice but to convert to a string manually **(or so I thought).
So, given the items I learned above, this is what I had to do:
But but, right after I finished with this, it occurred to me that for every InputStream, there must be an OutputStream. Since there was a BinaryInputStream, I did a search and indeed, I found BinaryOutputStream. I'd no particular reason to believe that it would behave differently than FileOutputStream, but just to make sure, I tested using a Typed Array as the output. Low and behold, it worked!! Seems BOS runs through the index values of an object. I would wager that it actually creates an Iterator and runs through that, but I didn't test that.
So once I knew about BOS, my code simplified to using just service calls, which is nice.
