asyncres.js

define(['underscore'], function (_) {

    var AsyncRes = {};

    AsyncRes.extend = function (asyncRequest) {
        var asyncRes = Object.create(AsyncRes)

        // Resource value
        asyncRes.res        = null;

        // Queued callbacks waiting for the resource
        asyncRes.queue      = [];

        // Currenly loading the resource
        asyncRes.loading    = false;

        // Async load resource method
        asyncRes.asyncReq   = asyncRequest;

        return asyncRes;
    };


    AsyncRes._update = function () {
        // Is resource already being requested
        if (this.loading) return false;

        var inst = this,
            handled = false;

        this.res = null;
        this.loading = true;

        this.asyncReq(function (res) {
            if (handled) return;

            handled = true;
            inst.loading = false;
            inst._setRes(res);
        }, function (err) {
            if (handled) return;

            handled = true;
            inst.loading = false;
            inst._handleError(err);
        });
    };


    AsyncRes._setRes = function (res) {
        this.res = res;

        while (this.queue.length > 0) {
            this.queue.shift()(null, res);
        }
    };


    /**
     * Handles error occurred while loading the resource. 
     *
     * Responds to queued callbacks with the error
     * @param {Error} err Error object
     */
    AsyncRes._handleError = function (err) {
        this.error = err;

        // Move callbacks to another array
        var queue = this.queue.splice(0, this.queue.length);

        while (queue.length > 0) {
            queue.shift()(err);
        }
    };


    AsyncRes.set = function (res) {
        this.res = res;
    };


    AsyncRes.get = function (callback) {
        if (this.res !== null) {
            callback(null, this.res);
        } else {
            this.queue.push(callback);
            this._update();
        }
    };


    return AsyncRes;
});