[Framework] Add extension sorter

Add priority ordering to loaded extensions in each category;
this allows control over the resulting order of extensions
acquired and used within the application. WTD-590

platform/framework/src/Constants.js

@@ -16,5 +16,13 @@ define({
         "tests": "test",
         "configuration": {},
         "extensions": {}
-    }
+    },
+    PRIORITY_LEVELS: {
+        "fallback": Number.NEGATIVE_INFINITY,
+        "default": 100,
+        "optional": 200,
+        "preferred": 400,
+        "mandatory": Number.POSITIVE_INFINITY
+    },
+    DEFAULT_PRIORITY: 0
 });

platform/framework/src/Main.js

@@ -26,9 +26,11 @@ define(
         './resolve/RequireConfigurator',
         './register/CustomRegistrars',
         './register/ExtensionRegistrar',
+        './register/ExtensionSorter',
         './bootstrap/ApplicationBootstrapper'
     ],
-    function (require,
+    function (
+        require,
         es6promise,
         angular,
         angularRoute,
@@ -41,6 +43,7 @@ define(
         RequireConfigurator,
         CustomRegistrars,
         ExtensionRegistrar,
+        ExtensionSorter,
         ApplicationBootstrapper
     ) {
         "use strict";
@@ -68,6 +71,7 @@ define(
                 registrar = new ExtensionRegistrar(
                     app,
                     new CustomRegistrars(app, $log),
+                    new ExtensionSorter($log),
                     $log
                 ),
                 bootstrapper = new ApplicationBootstrapper(

platform/framework/src/register/ExtensionRegistrar.js

@@ -17,9 +17,11 @@ define(
          * @param {Object.<string,function>} customRegistrars an object
          *        containing custom registration functions, primarily for
          *        Angular built-ins.
+         * @param {ExtensionSorter} sorter the sorter which will impose
+         *        priority ordering upon extensions
          * @param {*} $log Angular's logging service
          */
-        function ExtensionRegistrar(app, customRegistrars, $log) {
+        function ExtensionRegistrar(app, customRegistrars, sorter, $log) {
             // Track which extension categories have already been registered.
             // Exceptions will be thrown if the same extension category is
             // registered twice.
@@ -163,7 +165,7 @@ define(
                 Object.keys(extensionGroup).forEach(function (category) {
                     registerExtensionsForCategory(
                         category,
-                        extensionGroup[category]
+                        sorter.sort(extensionGroup[category])
                     );
                 });
 

platform/framework/src/register/ExtensionSorter.js

@@ -0,0 +1,97 @@
+/*global define*/
+
+define(
+    ["../Constants"],
+    function (Constants) {
+        "use strict";
+
+        /**
+         * Responsible for applying priority order to extensions in a
+         * given category. This will sort in reverse order of the numeric
+         * priority given for extensions in the `priority` priority (such
+         * that large values are registered first.) Extensions may also
+         * specify symbolic properties as strings (instead of numbers),
+         * which will be looked up from the table `Constants.PRIORITY_LEVELS`.
+         * @param $log Angular's logging service
+         * @constructor
+         */
+        function ExtensionSorter($log) {
+
+            // Handle unknown or malformed priorities specified by extensions
+            function unrecognizedPriority(extension) {
+                // Issue a warning
+                $log.warn([
+                    "Unrecognized priority '",
+                    (extension || {}).priority,
+                    "' specified for extension from ",
+                    ((extension || {}).bundle || {}).path,
+                    "; defaulting to ",
+                    Constants.DEFAULT_PRIORITY
+                ].join(''));
+
+                // Provide a return value (default priority) to make this
+                // useful in an expression.
+                return Constants.DEFAULT_PRIORITY;
+            }
+
+            function getPriority(extension) {
+                var priority =
+                    (extension || {}).priority || Constants.DEFAULT_PRIORITY;
+
+                // If it's a symbolic priority, look it up
+                if (typeof priority === 'string') {
+                    priority = Constants.PRIORITY_LEVELS[priority];
+                }
+
+                // Should be a number; otherwise, issue a warning and
+                // fall back to default priority level.
+                return (typeof priority === 'number') ?
+                        priority : unrecognizedPriority(extension);
+            }
+
+            // Attach a numeric priority to an extension; this is done in
+            // one pass outside of the comparator, mainly because getPriority
+            // may log warnings, and we only want this to happen once
+            // (instead of the many times that might occur during a sort.)
+            function prioritize(extension, index) {
+                return {
+                    // The extension itself, for later unwrapping
+                    extension: extension,
+                    // The index, to provide a stable sort (see compare)
+                    index: index,
+                    // The numeric priority of the extension
+                    priority: getPriority(extension)
+                };
+            }
+
+            // Unwrap the original extension
+            // (for use after ordering has been applied)
+            function deprioritize(prioritized) {
+                return prioritized.extension;
+            }
+
+            // Compare two prioritized extensions
+            function compare(a, b) {
+                // Reverse order by numeric priority; or, original order.
+                return (b.priority - a.priority) || (a.index - b.index);
+            }
+
+            return {
+                /**
+                 * Sort extensions according to priority.
+                 *
+                 * @param {object[]} extensions array of resolved extensions
+                 * @returns {object[]} the same extensions, in priority order
+                 */
+                sort: function (extensions) {
+                    return (extensions || [])
+                        .map(prioritize)
+                        .sort(compare)
+                        .map(deprioritize);
+                }
+            };
+        }
+
+        return ExtensionSorter;
+    }
+);