add fragment to backstack that is already inside it

Question

What happen if a add to BackStack a fragment that is already inside it? For example:

Fragment one = new Fragment();
Fragment two = new Fragment();

getSupportFragmentManager().beginTransaction().add(R.id.viewGroup, one).addToBackStack(null).commit();
//...
getSupportFragmentManager().beginTransaction().replace(R.id.viewGroup, two).addToBackStack(null).commit();
//...
getSupportFragmentManager().beginTransaction().replace(R.id.viewGroup, one).addToBackStack(null).commit();

When i add again fragment "one", two references to it is inside stack or only lastest is mantained ?

Answer 1

Even through the instance of the fragment is same, the BackStack entry will be increased, when an already exist Fragment is added to BackStack again.

When you add First Fragment , the back stack entry will be one.

When you replace Second Fragment ,the back stack entry will be two

When you replace First Fragment ( even though the same First Fragment instance), The older (first) Fragment instance will be reused. but the BackStack entry will be three

Now the stack will look like this

`First Fragment` -> `Second Fragment` -> `First Fragment`

So when Back pressed Fragments will be popped out as Fist Fragment , then Second Fragment and then again First Fragment.

This is the addBackStackState method in FragmentManager

 void addBackStackState(BackStackRecord state) {
    if (mBackStack == null) {
        mBackStack = new ArrayList<BackStackRecord>();
    }
    mBackStack.add(state);
    reportBackStackChanged();
}

You can look at the BackStackRecord class to understand more how addBackStackState method is called.