making Jasmine's async specs beautiful, with promises and an AsyncSpec helper

foo.spec.js

describe('live tile', function () {

    describe('when updating the tile', function () {

        var async = new AsyncSpec();

        async.beforeEach(function (complete) {
            Shared.getImages()
                .then(Tiles.buildThumbails)
                .then(async.storage.store('fileNames'))
                .then(complete);
        });

        async.it('should write the thumbnails to the tile thumbnails folder', function (storage) {
            storage['fileNames'].forEach(function (file) {

                var thumbnailExists = Shared.thumbnailFileExists(file);
                async.await(thumbnailExists).then(function (fileExists) {
                    expect(fileExists).toBe(true);
                });

            });
        });

    });

});

x-async-spec-helper.js

// Built on WinJS/WinRT/Win8 but easily adapted to any JS runtime
// --------------------------------------------------------------

(function (global) {

    var Storage = WinJS.Class.define(null, {
        store: function (fieldName, cb) {
            var that = this;
            return function (data) {
                if (cb) { data = cb(data); }
                that[fieldName] = data;
                return new WinJS.Promise(function (complete) {
                    complete(data);
                });
            }
        }
    });

    var AsyncSpec = WinJS.Class.define(function () {
        this.storage = new Storage();
    }, {
        it: function (description, block) {
            var that = this;
            var storage = this.storage;

            global.it(description, function () {

                waitsFor(function () {
                    return that.done;
                });

                runs(function () {
                    block(storage);
                });
            });
        },

        beforeEach: function (block) {
            this.done = false;
            var that = this;
            global.beforeEach(function () {
                block(that.complete.bind(that));
            });
        },

        complete: function () {
            this.done = true;
        },

        await: function (waitingPromise) {
            var done = false;
            var count = 0;

            var promise = new WinJS.Promise(function (complete) {

                global.runs(function () {
                    waitingPromise.then(function () {
                        complete.apply(null, arguments);
                        done = true;
                    });
                });

                global.waitsFor(function () {
                    return done;
                });

            });

            return promise;
        }
    });

    global.AsyncSpec = AsyncSpec;
})(this);