AngularJS directive with default options

Question

I'm just starting with angularjs, and am working on converting a few old JQuery plugins to Angular directives. I'd like to define a set of default options for my (element) directive, which can be overridden by specifying the option value in an attribute.

I've had a look around for the way others have done this, and in the angular-ui library the ui.bootstrap.pagination seems to do something similar.

First all default options are defined in a constant object:

.constant('paginationConfig', {
  itemsPerPage: 10,
  boundaryLinks: false,
  ...
})

Then a getAttributeValue utility function is attached to the directive controller:

this.getAttributeValue = function(attribute, defaultValue, interpolate) {
    return (angular.isDefined(attribute) ?
            (interpolate ? $interpolate(attribute)($scope.$parent) :
                           $scope.$parent.$eval(attribute)) : defaultValue);
};

Finally, this is used in the linking function to read in attributes as

.directive('pagination', ['$parse', 'paginationConfig', function($parse, config) {
    ...
    controller: 'PaginationController',
    link: function(scope, element, attrs, paginationCtrl) {
        var boundaryLinks = paginationCtrl.getAttributeValue(attrs.boundaryLinks,  config.boundaryLinks);
        var firstText = paginationCtrl.getAttributeValue(attrs.firstText, config.firstText, true);
        ...
    }
});

This seems like a rather complicated setup for something as standard as wanting to replace a set of default values. Are there any other ways to do this that are common? Or is it normal to always define a utility function such as getAttributeValue and parse options in this way? I'm interested to find out what different strategies people have for this common task.

Also, as a bonus, I'm not clear why the interpolate parameter is required.

Answer 1

You can use compile function - read attributes if they are not set - fill them with default values.

.directive('pagination', ['$parse', 'paginationConfig', function($parse, config) {
    ...
    controller: 'PaginationController',
    compile: function(element, attrs){
       if (!attrs.attrOne) { attrs.attrOne = 'default value'; }
       if (!attrs.attrTwo) { attrs.attrTwo = 42; }
    },
        ...
  }
});

Answer 2

Use the =? flag for the property in the scope block of the directive.

angular.module('myApp',[])
  .directive('myDirective', function(){
    return {
      template: 'hello {{name}}',
      scope: {
        // use the =? to denote the property as optional
        name: '=?'
      },
      controller: function($scope){
        // check if it was defined.  If not - set a default
        $scope.name = angular.isDefined($scope.name) ? $scope.name : 'default name';
      }
    }
  });

Answer 3

I'm using AngularJS v1.5.10 and found the preLink compile function to work rather well for setting default attribute values.

Just a reminder:

• attrs holds the raw DOM attribute values which are always either undefined or strings.
• scope holds (among other things) the DOM attribute values parsed according to the provided isolate scope specification (= / < / @ / etc.).

Abridged snippet:

.directive('myCustomToggle', function () {
  return {
    restrict: 'E',
    replace: true,
    require: 'ngModel',
    transclude: true,
    scope: {
      ngModel: '=',
      ngModelOptions: '<?',
      ngTrueValue: '<?',
      ngFalseValue: '<?',
    },
    link: {
      pre: function preLink(scope, element, attrs, ctrl) {
        // defaults for optional attributes
        scope.ngTrueValue = attrs.ngTrueValue !== undefined
          ? scope.ngTrueValue
          : true;
        scope.ngFalseValue = attrs.ngFalseValue !== undefined
          ? scope.ngFalseValue
          : false;
        scope.ngModelOptions = attrs.ngModelOptions !== undefined
          ? scope.ngModelOptions
          : {};
      },
      post: function postLink(scope, element, attrs, ctrl) {
        ...
        function updateModel(disable) {
          // flip model value
          var newValue = disable
            ? scope.ngFalseValue
            : scope.ngTrueValue;
          // assign it to the view
          ctrl.$setViewValue(newValue);
          ctrl.$render();
        }
        ...
    },
    template: ...
  }
});