title: When do we need to use @Injectable on our services in Angular2?
body: <p>All:</p>
  
  <p>Im pretty new to Angular2, when I reach dependency injection section, that <code>@Injectable()</code> notation make me a lil confused. Just want to make sure my understanding about @Injectable() is correct:</p>
  
  <ol>
  <li><code>@Injectable()</code> indicates the class follows it can be injected as service.</li>
  <li><code>@Injectable()</code> indicates the class follows it has other <code>@Injectable()</code> defined service injected into.</li>
  </ol>
  
  <p>Is this pretty much what <code>@Injectable()</code> means? Anything else needs to pay special attention about this notation?</p>
  
  <p>And We can not decide which case happens(or both happen) until we look into the class definition, is it correct?</p>
  
  <p>Thanks</p>