title: Angular 2/4 Displaying count of unread/ unseen messages
body: <p>I was trying to implement a message displaying app(just like gmail do) in Angular 2/4.
  I need to display the count of unread messages(say <code>inbox.component.ts</code>) on the sidebar(say <code>sidebar.component.ts</code>).</p>
  
  <p>In inbox component I have a variable(say <code>unSeenMsg</code>) that holds the value of unread messages(initialized on <code>ngOnInit()</code> ), on hitting one of unread messages I reduce the count by one <code>this.unSeenMsg = this.unSeenMsg-1</code>.</p>
  
  <p>Is there any method to inform the sidebar the value of <code>unSeenMsg</code> .</p>
  
  <p>Talking about the flow, I have a layout component(sidebar is a children of this one)
  inbox is another component that initialized in a later time</p>
  
  <p>Hope some one can help me </p>