Android SmartFragmentStatePagerAdapter that intelligently caches the Fragments in the ViewPager

SmartFragmentStatePagerAdapter.java

/* 
   Extension of FragmentStatePagerAdapter which intelligently caches 
   all active fragments and manages the fragment lifecycles. 
   Usage involves extending from SmartFragmentStatePagerAdapter as you would any other PagerAdapter.
*/
public abstract class SmartFragmentStatePagerAdapter extends FragmentStatePagerAdapter {
	// Sparse array to keep track of registered fragments in memory
	private SparseArray<Fragment> registeredFragments = new SparseArray<Fragment>();

	public SmartFragmentStatePagerAdapter(FragmentManager fragmentManager) {
		super(fragmentManager);
	}

	// Register the fragment when the item is instantiated
	@Override
	public Object instantiateItem(ViewGroup container, int position) {
		Fragment fragment = (Fragment) super.instantiateItem(container, position);
		registeredFragments.put(position, fragment);
		return fragment;
	}

	// Unregister when the item is inactive
	@Override
	public void destroyItem(ViewGroup container, int position, Object object) {
		registeredFragments.remove(position);
		super.destroyItem(container, position, object);
	}

	// Returns the fragment for the position (if instantiated)
	public Fragment getRegisteredFragment(int position) {
		return registeredFragments.get(position);
	}
}

@nesquena
Above code works great, just checked. One small correction to the above code, a type cast is required for the return statement -

return (Fragment)instantiateItem(pagerInstance, position); 

To simplify the class, I think we can have only one method instead of two doing the same work of getting the Registered Fragment -

// Returns the fragment for the position (if instantiated)
public Fragment getRegisteredFragment(int position) {
	Fragment existingInstance = registeredFragments.get(position);
	if (existingInstance != null) {
		return existingInstance;
	} else {
		return (Fragment) instantiateItem(pagerInstance, position);
	}
}

Wish I had this code earlier. I came across such a situation, and did not know what to do. Spent a lot of time, and finally made a hack from reading the internal code of FragmentStatePagerAdapter. Below is what I came up with, but your solution is very simple.

//Sparse Array to keep track of the registered fragments in memory
private SparseArray<Fragment> mRegisteredFragments = new SparseArray<>();

//Stores a reference to the FragmentManager used
private FragmentManager mFragmentManager;

public SmartFragmentStatePagerAdapter(FragmentManager fragmentManager) {
    super(fragmentManager);
    mFragmentManager = fragmentManager;
}

/**
 * Overriding to restore the state of Registered Fragments array
 *
 * @param state  is the Parcelable state
 * @param loader is the ClassLoader required for restoring the state
 */
@Override
public void restoreState(Parcelable state, ClassLoader loader) {
	super.restoreState(state, loader);
	if (state != null) {
		//When the state is present
		Bundle bundle = (Bundle) state;
		//Setting the ClassLoader passed, onto the Bundle
		bundle.setClassLoader(loader);

		//Retrieving the keys used in Bundle
		Set<String> keyStringSet = bundle.keySet();
		//Iterating over the Keys to find the Fragments
		for (String keyString : keyStringSet) {
			if (keyString.startsWith("f")) {
				//Fragment keys starts with 'f' followed by its position index
				int position = Integer.parseInt(keyString.substring(1));
				//Getting the Fragment from the Bundle using the Key through the FragmentManager
				Fragment fragment = mFragmentManager.getFragment(bundle, keyString);
				if (fragment != null) {
					//If Fragment is valid, then update the Sparse Array of Registered Fragments
					mRegisteredFragments.put(position, fragment);
				}
			}
		}
	}
}

I'm getting FragmentStatePagerAdapter cannot be applied to (android.app.FragmentManager)
on the line:

public SmartFragmentStatePagerAdapter(FragmentManager fragmentManager) {
        super(fragmentManager);
    }

Do you know what might be causing this?

EDIT:

I had to change:

import android.app.Fragment;
import android.app.FragmentManager;

to:

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;

I'm still having trouble getting this to work after rotation. Is there an example project?