title: How to observe Instance member variables using traitlets library?
body: <p>I am trying to use the traitlets library provided by ipython in my code. Suppose if a trait is of instance of a particular class how do i observe change in value for a member of the class..</p>
  
  <p>example:</p>
  
  <pre><code>class A:
      def init(self,val):
      self.value = val
  
  class myApp(HasTraits):
      myA = Instance(A,kw={'val':2})
  </code></pre>
  
  <p>I want to have an observe method to be called if 'value' member variable of object myA is changed.. SOmething like below:</p>
  
  <pre><code>@observe('myA.value')
  def onValueChange(self,change):
      return
  </code></pre>
  
  <p>is it possible with the trailets implementation?</p>