title: After called popbackstack(),all the added fragments shows up, why?
body: <p>I have 3 Fragment A, B, C , in MainActivity.</p>
  
  <p>A ,B,C  extents the BaseFragment:</p>
  
  <pre><code>class BaseFragment extents Fragment{
    public void onHiddenChanged(boolean hidden) {
      super.onHiddenChanged(hidden);
      Log.d("onHiddenChanged:",this.getClass().getName()+" hidden:"+hidden);    
      }
      ...
  }
  </code></pre>
  
  <p>and In the MainActiviy like that:</p>
  
  <pre><code>class MainActiviy extents Activity{
     onCreate(){
       ...
       fragmentTransaction.add(android.R.id.content, fragmentA, fragmentA.getClass().getName());
       fragmentTransaction.addToBackStack(fragmentA.getClass().getName());
       fragmentTransaction.commit();
  
       ...
  
    }
  }
  </code></pre>
  
  <p>When Click fragmentA,just enter to fragmentB （just Add B and Hide A, and add B to BackStack）:</p>
  
  <pre><code>fragmentTransaction.add(android.R.id.content, fragmentB, fragmentB.getClass().getName());
  fragmentTransaction.addToBackStack(fragmentB.getClass().getName());
  fragmentTransaction.hide(fragmentA).commit();
  </code></pre>
  
  <p>When Click fragmentB,just enter to fragmentC,just like above. </p>
  
  <p>So, I enter A ,enter B ,enter C by step,  and then in the FragmentC, I press the back button,
  I notice that, all the added fragment show's up:</p>
  
  <pre><code>onHiddenChanged:FragmentB
  onHiddenChanged:FragmentA
  </code></pre>
  
  <p>that's not my expects. When I press BackButton in FragnmentC, I just want to show FragmentB, and the FragmentA keep hidden. </p>