Attach a new helper function "$glance" to scope. Enables complex conditional scope one-time observation.

glance.js

angular.module('scope.glance', [])

.factory('attach$glance', ['$q', function ($q) {
  function $glance(watchExpression, checkFunction, objectEquality) {
    var scope = this;

    var dfd = $q.defer(),
        clearWatch;

    checkFunction = checkFunction || function (newVal, oldVal, scope) {
      return !!newVal;
    };

    clearWatch = scope.$watch(watchExpression, function () {

      var args = Array.prototype.slice.call(arguments),
          res = checkFunction.apply(this, args);

      if (res) {
        dfd.resolve(args);
        clearWatch();
      }

    }, objectEquality);

    return dfd.promise;
  }

  return function (scope, name) {
    name = name || '$glance';
    scope[name] = $glance;
  };
}]);

Example

angular.module('myApp', ['scope.glance'])

.run(function (attach$glance, $rootScope) {
  attach$glance($rootScope);
})

.controller('MainCtrl', function MainCtrl($scope, $q) {

  // Retrieval time varies
  $scope.dataA = retrieveDataFrom('A');
  $scope.dataB = retreiveDataFrom('B');

  $q.all([
    $scope.glance('dataA.init'),
    $scope.glance('dataB.init', function (newVal, oldVal) {
      // Resolve condition
      return (newVal === 'foo');
    })
  ])
  .then(function () {
    // Initialization ready
  });

  function retreiveDataFrom(remote) {
    // Retrieve some data
  }
});