title: How to get one of observable field's validation result in the computed field using Knockout Validation framework
body: <p>i have a view model defined following:</p>
  
  <pre><code>var ViewModel = function() {
      var self = this;
      self.name = ko.observable().extend({ required: true });
      self.identityCode = ko.observable().extend({ required: true, maxLength: 18, minLength: 15 });
      self.gender = ko.computed(function() {
          // get gender information from the identiy code here
      });
      self.birthdate = ko.computed(function() {
         // get birthdate information from the identity code here
      });
      self.form_onsubmit = function (form) {
          if (!self.isValid()) {
              self.errors.showAllMessages();
              return false;
          } else {
              return true;
          }
      };
  };
  </code></pre>
  
  <p>as you can see the code above, the gender field and brithdate field are computed field which be got from the identity code. i just want to know how to get the validation result of identity code before doing it. thanks!</p>