Question

I’m trying to use NetUtil.asyncCopy to append data from one file to the end of another file from a Firefox extension. I have based this code upon a number of examples at https://developer.mozilla.org/en-US/docs/Code_snippets/File_I_O, particularly the ‘Copy a stream to a file’ example. Given what it says on that page, my code below:

1. Creates nsIFile objects for the file to copy from and file to append to and initialises these objects with the correct paths.

2. Creates an output stream to the output file.

3. Runs the NetUtil.asyncCopy function to copy between the file (which, I believe, behaves as an nsIInputStream) and the output stream.

I run this code as append_text_from_file("~/CopyFrom.txt", "~/AppendTo.txt");, but nothing gets copied across. The Appending Text and After ostream dumps appear on the console, but not the Done or Error dumps.

Does anyone have any idea what I’m doing wrong here? I’m fairly new to both Firefox extensions and javascript (although I am a fairly experienced programmer) – so I may be doing something really silly. If my entire approach is wrong then please let me know – I would have thought that this approach would allow me to append a file easily, and asynchronously, but it may not be possible for some reason that I don’t know about.

function append_text_from_file(from_filename, to_filename) {
        var from_file = Components.classes["@mozilla.org/file/local;1"].createInstance(Components.interfaces.nsILocalFile);
        from_file.initWithPath(from_filename);

        var to_file = Components.classes["@mozilla.org/file/local;1"].createInstance(Components.interfaces.nsILocalFile);
        to_file.initWithPath(to_filename);

        dump("Appending text\n");

        var ostream = FileUtils.openFileOutputStream(to_file, FileUtils.MODE_WRONLY | FileUtils.MODE_APPEND)
        dump("After ostream\n");
        NetUtil.asyncCopy(from_file, ostream, function(aResult) {
            dump("Done\n");
            if (!Components.isSuccessCode(aResult)) {
                // an error occurred!
                dump(aResult);
                dump("Error!\n")
            }
        });
    }

Answer 1

asyncCopy() requires an input stream not a file.
you can do this:

    var fstream = Cc["@mozilla.org/network/file-input-stream;1"].createInstance(Ci.nsIFileInputStream);
    fstream.init(from_file, 0x01, 4, null);

    NetUtil.asyncCopy(fstream, ostream, function(aResult)....