Like a normal FragmentPagerAdapter but holds a list of instantiated fragments and the current primary fragment

SmartFragmentPagerAdapter.java

import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.util.SparseArray;
import android.view.ViewGroup;

import java.util.ArrayList;
import java.util.List;

/**
 * Like a normal {@link FragmentPagerAdapter} but holds a list of instantiated fragments
 * which can be queried using {@link #getInstantiatedFragments(int)}.
 * You can get the primary fragment using {@link #getPrimaryFragment()}.
 * <p>
 * When you want to know, when the ViewPager-page changes, subscribe as a {@link PrimaryFragmentListener}
 * instead of {@link ViewPager.OnPageChangeListener} because the latter will be called
 * <strong>before</strong> the page is actually instantiated.
 *
 * @see <a href="http://stackoverflow.com/a/15261142/2170109">http://stackoverflow.com/a/15261142/2170109</a>
 */
public abstract class SmartFragmentPagerAdapter extends FragmentPagerAdapter {

    private final SparseArray<Fragment> instantiatedFragments = new SparseArray<>();
    private List<PrimaryFragmentListener> primaryFragmentListeners;

    @Nullable private Fragment primaryFragment;

    protected SmartFragmentPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    // region PagerAdapter

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        Fragment fragment = (Fragment) super.instantiateItem(container, position);
        instantiatedFragments.put(position, fragment);

        return fragment;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        instantiatedFragments.remove(position);

        super.destroyItem(container, position, object);
    }

    @Override
    public void setPrimaryItem(ViewGroup container, int position, Object object) {
        super.setPrimaryItem(container, position, object);

        primaryFragment = (Fragment) object;
        dispatchSetPrimaryItem(position, object);
    }

    // endregion

    // region PrimaryFragmentListener

    public interface PrimaryFragmentListener {

        void onPrimaryFragmentChanged(int position, Fragment fragment);
    }

    @SuppressWarnings({"unused", "WeakerAccess"})
    public void addPrimaryFragmentListener(PrimaryFragmentListener listener) {
        if (primaryFragmentListeners == null) {
            primaryFragmentListeners = new ArrayList<>();
        }

        primaryFragmentListeners.add(listener);
    }

    @SuppressWarnings({"unused", "WeakerAccess"})
    public void removePrimaryFragmentListener(PrimaryFragmentListener listener) {
        if (primaryFragmentListeners != null) {
            primaryFragmentListeners.remove(listener);
        }
    }

    @SuppressWarnings({"unused", "WeakerAccess"})
    public void clearPrimaryFragmentListener() {
        if (primaryFragmentListeners != null) {
            primaryFragmentListeners.clear();
        }
    }

    // endregion

    private void dispatchSetPrimaryItem(int position, Object object) {
        if (primaryFragmentListeners != null) {
            for (int i = 0; i < primaryFragmentListeners.size(); i++) {

                PrimaryFragmentListener listener = primaryFragmentListeners.get(i);
                if (listener != null) {
                    listener.onPrimaryFragmentChanged(position, (Fragment) object);
                }
            }
        }
    }

    @SuppressWarnings({"unused", "WeakerAccess"})
    public Fragment getInstantiatedFragments(int position) {
        return instantiatedFragments.get(position);
    }

    @SuppressWarnings({"unused", "WeakerAccess"})
    @Nullable
    public Fragment getPrimaryFragment() {
        return primaryFragment;
    }
}