function testIsValidRequiredField()
 {
     $rule_string = new MockRule_String();
     $rule_string->expectCallCount('isValid', 3);
     $f = new Tracker_FormElement_Field_TextTestVersion();
     $f->setReturnValue('isRequired', true);
     $f->setReturnReference('getRuleString', $rule_string);
     $a = new MockTracker_Artifact();
     $f->isValid($a, 'This is a text');
     $f->isValid($a, '2009-08-45');
     $f->isValid($a, 25);
     $this->assertFalse($f->isValidRegardingRequiredProperty($a, ''));
     $this->assertFalse($f->isValidRegardingRequiredProperty($a, null));
 }