Angular Directive Isolated Scope binding cheatsheet

1. "@"   (  Text binding / one-way binding )
2. "="   ( Direct model binding / two-way binding )
3. "&"   ( Behaviour binding / Method binding  )
4. "=?"  ( Optional model binding, outer scope does not have to have the corresponding scope variable )
5. "@", then add observe/watch in the directive  ( Text / one-way binding, but will update inner scope whenever the value changes, as opposed to passing it into directive like a parameter once )

http://www.undefinednull.com/2014/02/11/mastering-the-scope-of-a-directive-in-angularjs/