angular-watch-translate.js

// watch - translate - assign other member of scope
// check out how it works at http://jsfiddle.net/SWe4r/1/

var app = angular.module('watcherApp', []);
app.controller('watcherController', function ($scope) {
    $scope.source = 'aaaaaa';
    $scope.foo = {
        bar: 'aaa'
    };
    $scope.bar = {
        foo: ''
    }
   
    $scope.translate = function (src) {
        return src.replace(/a/g, 'b');
    }
});
app.directive('watcher', function ($parse) {
    return {
        scope: {
            source: '@',
            target: '@',
            translator: '@'
        },
        transclue: true,
        restrict: 'E',
        link: function (scope, element, attr) {
            var translate = $parse(scope.translator)(scope.$parent);
            var setter = $parse(scope.target).assign;
            var getter = $parse(scope.source);
            scope.$parent.$watch(scope.source, function () {
                setter(scope.$parent, translate(getter(scope.$parent)))
            });
        }
    };
})