FragmentStatePagerAdapter that caches each pages.

CacheFragmentStatePagerAdapter.java

/*
 * Copyright 2014 Soichiro Kashima
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.github.ksoichiro.android.observablescrollview;

import android.os.Bundle;
import android.os.Parcelable;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentStatePagerAdapter;
import android.util.SparseArray;
import android.view.ViewGroup;

/**
 * FragmentStatePagerAdapter that caches each pages.
 * FragmentStatePagerAdapter is also originally caches pages,
 * but its keys are not public nor documented, so depending
 * on how it create cache key is dangerous.
 * This adapter caches pages by itself and provide getter method to the cache.
 */
public abstract class CacheFragmentStatePagerAdapter extends FragmentStatePagerAdapter {

    private static final String STATE_SUPER_STATE = "superState";
    private static final String STATE_PAGES = "pages";
    private static final String STATE_PAGE_INDEX_PREFIX = "pageIndex:";
    private static final String STATE_PAGE_KEY_PREFIX = "page:";

    private FragmentManager mFm;
    private SparseArray<Fragment> mPages;

    public CacheFragmentStatePagerAdapter(FragmentManager fm) {
        super(fm);
        mPages = new SparseArray<Fragment>();
        mFm = fm;
    }

    @Override
    public Parcelable saveState() {
        Parcelable p = super.saveState();
        Bundle bundle = new Bundle();
        bundle.putParcelable(STATE_SUPER_STATE, p);

        bundle.putInt(STATE_PAGES, mPages.size());
        if (0 < mPages.size()) {
            for (int i = 0; i < mPages.size(); i++) {
                int position = mPages.keyAt(i);
                bundle.putInt(createCacheIndex(i), position);
                Fragment f = mPages.get(position);
                mFm.putFragment(bundle, createCacheKey(position), f);
            }
        }
        return bundle;
    }

    @Override
    public void restoreState(Parcelable state, ClassLoader loader) {
        Bundle bundle = (Bundle) state;
        int pages = bundle.getInt(STATE_PAGES);
        if (0 < pages) {
            for (int i = 0; i < pages; i++) {
                int position = bundle.getInt(createCacheIndex(i));
                Fragment f = mFm.getFragment(bundle, createCacheKey(position));
                mPages.put(position, f);
            }
        }

        Parcelable p = bundle.getParcelable(STATE_SUPER_STATE);
        super.restoreState(p, loader);
    }

    /**
     * Get a new Fragment instance.
     * Each fragments are automatically cached in this method,
     * so you don't have to do it by yourself.
     * If you want to implement instantiation of Fragments,
     * you should override {@link #createItem(int)} instead.
     *
     * {@inheritDoc}
     *
     * @param position position of the item in the adapter
     * @return fragment instance
     */
    @Override
    public Fragment getItem(int position) {
        Fragment f = createItem(position);
        // We should cache fragments manually to access to them later
        mPages.put(position, f);
        return f;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        if (0 <= mPages.indexOfKey(position)) {
            mPages.remove(position);
        }
        super.destroyItem(container, position, object);
    }

    /**
     * Get the item at the specified position in the adapter.
     *
     * @param position position of the item in the adapter
     * @return fragment instance
     */
    public Fragment getItemAt(int position) {
        return mPages.get(position);
    }

    /**
     * Create a new Fragment instance.
     * This is called inside {@link #getItem(int)}.
     *
     * @param position position of the item in the adapter
     * @return fragment instance
     */
    protected abstract Fragment createItem(int position);

    /**
     * Create an index string for caching Fragment pages.
     *
     * @param index index of the item in the adapter
     * @return key string for caching Fragment pages
     */
    protected String createCacheIndex(int index) {
        return STATE_PAGE_INDEX_PREFIX + index;
    }

    /**
     * Create a key string for caching Fragment pages.
     *
     * @param position position of the item in the adapter
     * @return key string for caching Fragment pages
     */
    protected String createCacheKey(int position) {
        return STATE_PAGE_KEY_PREFIX + position;
    }
}

this implementation is really good and useful - but slight modification in 72 on restoreinstance where the old mPage resides in memory seems and contains old value.clear the mPage(mPage.clear() ) and start setting the fragment by getFragment()
this solved issue when i changed orientation between portrait and landscape.

The error occurs:

java.lang.IllegalStateException: Fragment PlacesMapFragment{e264ad0 (36b68aeb-efd5-4d60-8dd1-65368a752a90)} is not currently in the FragmentManager
        at androidx.fragment.app.FragmentManagerImpl.putFragment(FragmentManagerImpl.java:350)
        at com.example.CacheFragmentStatePagerAdapter.saveState(CacheFragmentStatePagerAdapter.java:47)
        at androidx.viewpager.widget.ViewPager.onSaveInstanceState(ViewPager.java:1445)
        at android.view.View.dispatchSaveInstanceState(View.java:18792)
        at android.view.ViewGroup.dispatchSaveInstanceState(ViewGroup.java:3905)
        at android.view.ViewGroup.dispatchSaveInstanceState(ViewGroup.java:3911)
        at android.view.View.saveHierarchyState(View.java:18775)
        at androidx.fragment.app.FragmentManagerImpl.saveFragmentViewState(FragmentManagerImpl.java:2263)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:944)
        at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1228)
        at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:434)
        at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManagerImpl.java:2066)
        at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManagerImpl.java:1856)
        at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManagerImpl.java:1811)
        at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManagerImpl.java:1717)
        at androidx.fragment.app.FragmentManagerImpl$2.run(FragmentManagerImpl.java:150)
        at android.os.Handler.handleCallback(Handler.java:789)
        at android.os.Handler.dispatchMessage(Handler.java:98)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6944)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)