title: How can I know the fragment transaction is finished?
body: <p>I want to make two fragment transactions in a row.</p>
  
  <pre><code>Fragment1 fragment1 = new Fragment1();
  FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
  transaction.replace(R.id.fragment_container, fragment1);
  transaction.addToBackStack(null);
  transaction.commit();
  
  Fragment2 fragment2 = new Fragment2();
  FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
  transaction.replace(R.id.fragment_container, fragment2);
  transaction.addToBackStack(null);
  transaction.commit();
  </code></pre>
  
  <p>However, if I do so, there will be some concurrence problem.
  Is there a way for me to implement a callback function so that I can start the second transaction right after the first one is finished?</p>