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);
	}
}

Cool have been looking for this

thank you very mutch... work like a charm

Nice!!!!!

I would make a little change and use generics so we don't need to cast when getting the fragment:

public abstract class SmartFragmentStatePagerAdapter<T extends Fragment> extends FragmentStatePagerAdapter {

    private SparseArray<T> registeredFragments = new SparseArray<T>();

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

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

Now, when you extends this class, specify your fragment type:

public class MyPageAdapter extends SmartFragmentStatePagerAdapter<MyFragment> {
....

Nice, it help me a lot for understanding a cache for fragment

Can you please give me a small example how I can manage my Fragments (with AsyncTasks) of ViewPager inside a Fragment by using this adapter?

This will lose its state on configuration change. The FragmentStatePagerAdapter will restore its cached fragment instances, and this object will have no registered instances.

@hashbrown yep!

It helped so much ! 👍 👍

Is there anyway to refresh current fragment?

@gustavogianordoli

Hi thank you very much,
How can i use getRegisteredFragment method on SmartFragmentStatePagerAdapter deployed viewPager ? is it for check caching fragment?

firstly thanks you! hoping it is useful to me,

For those running into orientation change issues on rotation, I wanted to post a quick fix until we have a better workaround. In the SmartFragmentStatePagerAdapter class, extend with:

    // Add to `SmartFragmentStatePagerAdapter.java` as a new method
    // Returns the fragment for the position (if instantiated)
    public Fragment fetchFragmentByPosition(ViewGroup pagerInstance, int position) {
        Fragment existingInstance = registeredFragments.get(position);
        if (existingInstance != null) {
            return existingInstance;
         } else {
  	    return instantiateItem(pagerInstance, position);
         }
    }

In the parent activity or fragment where the adapter instance is available:

// In the Activity or wherever you have access to the view pager instance
adapter.fetchFragmentByPosition(mPager, 0);

This should allow you to get access to the instance even when the registeredFragments becomes null on orientation change. Can anyone else report back on if this code a) is working on orientation change b) isn't working on orientation change or c) proposed workaround

@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?