feat($rootScope): Add custom compare and copy functions to $watch

Closes #10069

Author: Toilal

Diff for: src/ng/rootScope.js

@@ -353,11 +353,13 @@ function $RootScopeProvider() {
        *    - `newVal` contains the current value of the `watchExpression`
        *    - `oldVal` contains the previous value of the `watchExpression`
        *    - `scope` refers to the current scope
-       * @param {boolean=} objectEquality Compare for object equality using {@link angular.equals} instead of
-       *     comparing for reference equality.
+       * @param {boolean=|function()} objectEquality Compare for object equality using {@link angular.equals} instead of
+       *     comparing for reference equality. If a function is passed, it will be used as a replacement for {@link angular.equals}.
+       * @param {function()} copier Replacement function for {@link angular.copy} to save value into oldVal for next
+            change event.
        * @returns {function()} Returns a deregistration function for this listener.
        */
-      $watch: function(watchExp, listener, objectEquality) {
+      $watch: function(watchExp, listener, objectEquality, copier) {
         var get = $parse(watchExp);
 
         if (get.$$watchDelegate) {
@@ -375,6 +377,14 @@ function $RootScopeProvider() {
 
         lastDirtyWatch = null;
 
+        if (isFunction(objectEquality)) {
+          watcher.comparator = objectEquality;
+        }
+
+        if (isFunction(copier)) {
+          watcher.copier = copier;
+        }
+
         if (!isFunction(listener)) {
           watcher.fn = noop;
         }
@@ -763,12 +773,12 @@ function $RootScopeProvider() {
                   if (watch) {
                     if ((value = watch.get(current)) !== (last = watch.last) &&
                         !(watch.eq
-                            ? equals(value, last)
+                            ? (watch.comparator ? watch.comparator(value, last) : equals(value, last))
                             : (typeof value === 'number' && typeof last === 'number'
                                && isNaN(value) && isNaN(last)))) {
                       dirty = true;
                       lastDirtyWatch = watch;
-                      watch.last = watch.eq ? copy(value, null) : value;
+                      watch.last = watch.eq ? (watch.copier ? watch.copier(value, null) : copy(value, null)) : value;
                       watch.fn(value, ((last === initWatchVal) ? value : last), current);
                       if (ttl < 5) {
                         logIdx = 4 - ttl;

Diff for: test/ng/rootScopeSpec.js

@@ -458,6 +458,67 @@ describe('Scope', function() {
       expect(log).toEqual([]);
     }));
 
+    it('should use custom equality and copier',
+      inject(function($rootScope) {
+        var log = [];
+        var oldLog = [];
+
+        function logger(newVal, oldVal) {
+          var val = (newVal === oldVal || (newVal !== oldVal && oldVal !== newVal)) ? newVal : 'xxx';
+          log.push(val);
+        }
+
+        function oldLogger(newVal, oldVal) {
+          var val = (newVal === oldVal || (newVal !== oldVal && oldVal !== newVal)) ? oldVal : 'xxx';
+          oldLog.push(val);
+        }
+
+        var watched = "ABC";
+        var watchedFn = function() {
+          return watched;
+        };
+
+        // Comparator returns true if string starts with same character
+        var comparator = function(newVal, oldVal) {
+          if (oldVal === newVal) return true;
+          if (oldVal === undefined || newVal === undefined) return false;
+          if (newVal.length === oldVal.length === 0) return true;
+          if (newVal.length > 0 && oldVal.length > 0 && oldVal[0] === newVal[0]) {
+            return true;
+          }
+          return false;
+        };
+
+        // Copier add " copy" to value
+        var copier = function(value) {
+          if (value !== undefined) {
+            return value + " copy";
+          }
+          return undefined;
+        };
+
+        $rootScope.$watch(watchedFn, logger, comparator, copier);
+        $rootScope.$watch(watchedFn, oldLogger, comparator, copier);
+
+        $rootScope.$digest();
+        expect(log).toEqual(['ABC']);
+        expect(oldLog).toEqual(['ABC']);
+        log = [];
+        oldLog = [];
+
+        watched = "DEF";
+        $rootScope.$digest();
+        expect(log).toEqual(['DEF']);
+        expect(oldLog).toEqual(['ABC copy']);
+        log = [];
+        oldLog = [];
+
+        watched = "DZZ";
+        $rootScope.$digest();
+        expect(log).toEqual([]);
+        expect(oldLog).toEqual([]);
+      }));
+
 
     describe('$watch deregistration', function() {