[Identity] Add identity aggregator

Add an aggregator to handle exposing the user's identity,
nasa/openmctweb#92

platform/identity/src/IdentityAggregator.js

@@ -0,0 +1,65 @@
+/*global define*/
+
+define(
+    function () {
+        "use strict";
+
+        /**
+         * Provides information about the currently logged-in
+         * user, if available.
+         *
+         * @interface IdentityService
+         */
+
+        /**
+         * Get information about the current user. This returns a promise
+         * which will resolve to metadata about the user, or undefined if
+         * no information about the user is available.
+         *
+         * @method IdentityService#getUser
+         * @returns {Promise.<UserMetadata>} metadata about the current user
+         */
+
+        /**
+         * Metadata about a user.
+         *
+         * @typedef UserMetadata
+         * @property {string} name the user's human-readable name
+         * @property {string} key the user's machine-readable name
+         */
+
+        /**
+         * Aggregator for multiple identity services. Exposes the first
+         * defined identity provided by any provider, according to
+         * priority order.
+         *
+         * @implements {IdentityService}
+         * @memberof platform/identity
+         */
+        function IdentityAggregator($q, providers) {
+            this.providers = providers;
+            this.$q = $q;
+        }
+
+        function delegateGetUser(provider) {
+            return provider.getUser();
+        }
+
+        function identity(value) {
+            return value;
+        }
+
+        function giveFirst(results) {
+            return results.filter(identity)[0];
+        }
+
+        IdentityAggregator.prototype.getUser = function () {
+            var $q = this.$q,
+                promises = this.providers.map(delegateGetUser);
+
+            return $q.all(promises).then(giveFirst);
+        };
+
+        return IdentityAggregator;
+    }
+);