title: Ipywidgets observe method on interactive instead of widget
body: <p>Both ipython widgets and interactive objects have observe() methods. (See the results of the print statements.)
  With the following example, I can confirm the actions of the observe() method on a slider widget but not on the interactive (ie) object.</p>
  <p><strong>Q:</strong> Is there any way to use the interactive's observe method or I have to call separate observe() method on it's all widget components ? If so, why ?</p>
  <p><strong>Expected behavior:</strong> Printing 'ie change observed' after changing inp1,</p>
  <pre><code>from IPython.display import display
  import ipywidgets as widgets
  
  int_range0_slider = widgets.IntSlider()
  int_range1_slider = widgets.IntSlider()
  output = widgets.Output()
  
  def interactive_function(inp0,inp1):
      with output:
          print('ie changed. int_range0_slider: '+str(inp0)+' int_range1_slider: '+str(inp1))
      return
  
  def report_int_range0_change(change):
      with output:
          print('int_range0 change observed'+str(change))
      return
  
  def report_ie_change(change):
      with output:
          print('ie change observed'+str(change))
      return
  
  ie = widgets.interactive(interactive_function, inp0=int_range0_slider,inp1=int_range1_slider)
  
  print(int_range0_slider.observe)
  print(ie.observe)
  int_range0_slider.observe(report_int_range0_change, names='value')
  ie.observe(report_ie_change)
  
  display(int_range0_slider,int_range1_slider,output)
  </code></pre>
  <p>I'm a newbie, any help on the correct usage would be appreciated.</p>