How do I use OS.File.open?

Question

function write_text(filename, text) {
    let pfh = yield OS.File.open("/tmp/foo", {append: true});
    yield pfh.write(text);
    yield pfh.flush();
    yield pfh.close(); 
}

I tried without the yield which is the more natural form but that broke: In python i'd do yielded_object.next()

error: scribus-web-slurp: An exception occurred.
TypeError: pfh.write is not a function
resource://jid1-orxy9dnn8jbfeq-at-jetpack/scribus-web-slurp/lib/main.js 28
Traceback (most recent call last):

I know Javascript but it's the Firefox-extensions that are causing problems - are there any tutorials that can walk me through the process or bring me up to scratch?? The MDN documentation is too exhaustive and I don;t know where to start.

I'm, assuming 'yield' gives me a generator object - but I don't understand why Open should give me such an object?? Open should just give me a file-handle? — , Oct 30 '13 at 04:22

score 3 · Accepted Answer · edited May 23 '17 at 12:09

The async OS.File API returns Promises. It is best used with Task.jsm

function write_text(filename, text) {
    var encoder = new TextEncoder();
    var data = encoder.encode(text);
    Task.spawn(function() {
        let pfh = yield OS.File.open("/tmp/foo", {write: true});
        yield pfh.write(data);
        yield pfh.close(); 
    });
}

The documentation has a some examples.

Also, don't flush() if you don't have to (and flush() in the async API is only available in Firefox 27 anyway)

Edit: Ah, you're using the SDK, I gather when re-reading the actual error of your question.

You need to import TextEncoder explicitly from some other module, as SDK modules lack the class.
append: is only supported in Firefox 27+
You write: true to write to a file.

Here is a full, working example I tested in Firefox 25 (main.js)

const {Cu} = require("chrome");
const {TextEncoder, OS} = Cu.import("resource://gre/modules/osfile.jsm", {});
const {Task} = Cu.import("resource://gre/modules/Task.jsm", {});

function write_text(filename, text) {
    var encoder = new TextEncoder();
    var data = encoder.encode(text);
    filename = OS.Path.join(OS.Constants.Path.tmpDir, filename);
    Task.spawn(function() {
       let file = yield OS.File.open(filename, {write: true});
       yield file.write(data);
       yield file.close(); 
       console.log("written to", filename);
    }).then(null, function(e) console.error(e));
}

write_text("foo", "some text");

See also your other question for more commentary on using this stuff in the SDK.

How do I use OS.File.open?

1 Answers1