如何为'Required If'场景编写Angular指令？

I'm working on understanding how to use Angular Directives to implement front-end validations. While I'm familiar with the way that directives generally work, what I'm having a hard time finding in any tutorial, blogpost, or even the actual Angular docs, is how to meaningfully implement a useful validation. (By that, I mean one that isn't already there, as in I'm working on understanding how to use Angular