fragment-back-nightmare-post

## after-first-back.java
// the back is equivalent to (removed boilerplate)
fm.beginTransaction()
  .remove(bFragment)
  .add(aFragment)
  .commit();

## second-b.java
// After A -> B -> C (back [A,C] shown)

// user goes to B with this transaction
fm.beginTransaction()
  .replace(R.id.content, new B_Fragment(), "fragment-b")
  .addToBackStack("a")
  .commit();

// which is the same as
fm.beginTransaction()
  .remove(fm.findFragmentById(R.id.content))
  .add(R.id.content, new B_Fragment(), "fragment-b")
  .addToBackStack("a")
  .commit();

## solution.java
  public static void performNoBackStackTransaction(FragmentManager fragmentManager, String tag, Fragment fragment) {
    final int newBackStackLength = fragmentManager.getBackStackEntryCount() +1;

    fragmentManager.beginTransaction()
        .replace(R.id.content, fragment, tag)
        .addToBackStack(tag)
        .commit();

    fragmentManager.addOnBackStackChangedListener(new FragmentManager.OnBackStackChangedListener() {
      @Override
      public void onBackStackChanged() {
        int nowCount = fragmentManager.getBackStackEntryCount();
        if (newBackStackLength != nowCount) {
          // we don't really care if going back or forward. we already performed the logic here.
          fragmentManager.removeOnBackStackChangedListener(this);

          if ( newBackStackLength > nowCount ) { // user pressed back
            fragmentManager.popBackStackImmediate();
          }
        }
      }
    });
  }

## what-2.java
//fm is FragmentManager
// Fragment a is on the screen
Fragment a = new A_Fragment()
fm.beginTransaction()
  .remove(null /*no fragments in R.id.content*/)
  .add(R.id.content, aFragment, "fragment-a")
  .commit();


// user wants to go from A to B
Fragment bFragment = new B_Fragment();
fm.beginTransaction()
  .remove(fm.findFragmentById(R.id.content)) // resolves to A_Fragment instance
  .add(R.id.content, bFragment, "fragment-b")
 .addToBackStack("a")
 .commit();

// user wants to go from B to C
fm.beginTransaction()
  .remove(fm.findFragmentById(R.id.content)) // resolves to B_Fragment instance
  .add(R.id.content, new C_Fragment(), "fragment-c")
 .commit();


## what.java
//fm is FragmentManager
// Fragment a is on the screen
fm.beginTransaction()
  .replace(R.id.content, new A_Fragment(), "fragment-a")
  .commit();


// user wants to go from A to B
fm.beginTransaction()
 .replace(R.id.content, new B_Fragment(), "fragment-b")
 .addToBackStack("a")
 .commit();

// user wants to go from B to C
fm.beginTransaction()
 .replace(R.id.content, new C_Fragment(), "fragment-c")
 .commit();