<input ng-model="somefield">
<span ng-show="!somefield.length">Please enter something!</span>
<span ng-show="somefield.length">Good boy!</span>

你也可以使用ng-hide =“somefield.length”而不是ng-show =“！somefield.length”，如果这更自然地为你读。

一个更好的选择可能是真正利用form abilities of Angular：

<form name="myform">
  <input name="myfield" ng-model="somefield" ng-minlength="5" required>
  <span ng-show="myform.myfield.$error.required">Please enter something!</span>
  <span ng-show="!myform.myfield.$error.required">Good boy!</span>
</form>

Updated Plnkr here.