node callback and event emitter utilities

[zeroware]

While using the lib I found that at some point people will have to use node callback and event emitter based lib with channels.

Here is two utilities functions I ended up with :

/**
 * Create a channel from an async node function
 * Passed function must accept a node callback as last args
 *
 * @param {function} f
 * @return {Channel}
 */
function fromNodeCallback(f) {

    var args = 2 <= arguments.length ? Array.prototype.slice.call(arguments, 1) : [];
    var chan = csp.chan();

    args.push(function(err, value) {
        csp.putAsync(chan, err ? err : value, function() {
            chan.close();
        });
    });

    f.apply(this, args);

    return chan;
}


/**
 * Create a channel from an event emitter object
 *
 * @param {function} eventEmitter
 * @param {string} eventName
 * @param {Channel} [chan]
 *
 * @return {Channel}
 */
function fromEvent(eventEmitter, eventName, chan) {

    chan = chan || csp.chan();

    eventEmitter.on(eventName, function(data) {
        csp.putAsync(chan, data);
    });

    return chan;
}

[ubolonton]

fromEvent can be useful.

We have been holding off things like fromNodeCallback to wait for a better error handling story (#14).

[nmn]

I have pull request #37 for the same.

I think the error handling story is important to solve. But also, we need to decide on behaviour related to closed channels.

I think a closed channel should be a signal to remove all bindings. In the fromEvent function for example:

function fromEvent(eventEmitter, eventName, chan) {

    chan = chan || csp.chan();
    var handler = function(data){
        if(chan === csp.CLOSED){ // I need the documentation for this :(
            eventEmitter.off(eventName, handler);
        } else {
            csp.putAsync(chan, data);
        }
    }
    eventEmitter.on(eventName, handler);
    return chan;
}