title: Detect parent class variable or property is changed by other child class in objective-c
body: <p>I have parent UIViewController A which has some properties. And I have child UIViewController B,C,D which are inheriting from A.</p>
  
  <p>My question is:
  In the B, I change Parent properties value. How can C,D know Parent value is changed from B?</p>
  
  <p>Basically I want to share properties between children.</p>
  
  <p>Thanks in advance.</p>
  
  <p>Code example;</p>
  
  <pre><code>@interface A : UIViewController{
  NSString *string1; //This is just example
  }
  @property (nonatomic, strong) NSString *string2;
  @end
  </code></pre>
  
  <p>Child B</p>
  
  <pre><code>@interface B : A
  @end
  
  @implementation B
  //here I change string1 and string2;
  @end
  </code></pre>
  
  <p>Child C</p>
  
  <pre><code>@interface C : A
  @end
  
  @implementation C
  //here I want to get changed string1 and string2 by Child A;
  @end
  </code></pre>
  
  <p>Child D</p>
  
  <pre><code>@interface D : A
  @end
  
  @implementation D
  //here I want to get changed string1 and string2 by Child A;
  @end
  </code></pre>
  
  <p>At the moment, I am using <strong>Singleton</strong> to store all the value and It is working well, but I think <strong>there should be better way</strong> in such situation?</p>