剥皮猫的方法很多。
[PLUNKER](http://plnkr.co/edit/YHsxmmLzayOt5DLo2LCG?p=preview)
app.directive('lowerThan', [ function() { var link = function($scope, $element, $attrs, ctrl) { var validate = function(viewValue) { var comparisonModel = $attrs.lowerThan; if(!viewValue || !comparisonModel){ // It's valid because we have nothing to compare against ctrl.$setValidity('lowerThan', true); } // It's valid if model is lower than the model we're comparing against ctrl.$setValidity('lowerThan', parseInt(viewValue, 10) < parseInt(comparisonModel, 10) ); return viewValue; }; ctrl.$parsers.unshift(validate); ctrl.$formatters.push(validate); $attrs.$observe('lowerThan', function(comparisonModel){ // Whenever the comparison model changes we'll re-validate return validate(ctrl.$viewValue); }); }; return { require: 'ngModel', link: link }; }]);用法：
<input name="min" type="number" ng-model="field.min" lower-than="{{field.max}}" /><span ng-show="form.min.$error.lowerThan"> Min cannot exceed max.</span>
