无需编写自定义指令.Angular 的文档很好，但并不完整.其实，有一个指令叫做ngRequired，这需要一个 Angular 表达式.

There's no need to write a custom directive. Angular's documentation is good but not complete. In fact, there is a directive called ngRequired, that takes an Angular expression.

<input type='email'
       name='email'
       ng-model='contact.email' 
       placeholder='your@email.com'
       ng-required='!contact.phone' />

<input type='text'
       ng-model='contact.phone'             
       placeholder='(xxx) xxx-xxxx'
       ng-required='!contact.email' />  

这里有一个更完整的例子:http://jsfiddle.net/uptnx/1/

Here's a more complete example: http://jsfiddle.net/uptnx/1/