ViewPager2 Individual Options Menu Support for FragmentStateAdapter

FragmentStateAdapter.java

/**
 * Similar in behavior to {@link FragmentStatePagerAdapter}
 * <p>
 * Lifecycle within {@link RecyclerView}:
 * <ul>
 * <li>{@link RecyclerView.ViewHolder} initially an empty {@link FrameLayout}, serves as a
 * re-usable container for a {@link Fragment} in later stages.
 * <li>{@link RecyclerView.Adapter#onBindViewHolder} we ask for a {@link Fragment} for the
 * position. If we already have the fragment, or have previously saved its state, we use those.
 * <li>{@link RecyclerView.Adapter#onAttachedToWindow} we attach the {@link Fragment} to a
 * container.
 * <li>{@link RecyclerView.Adapter#onViewRecycled} and
 * {@link RecyclerView.Adapter#onFailedToRecycleView} we remove, save state, destroy the
 * {@link Fragment}.
 * </ul>
 */
public abstract class FragmentStateAdapter extends
        RecyclerView.Adapter<FragmentViewHolder> implements StatefulAdapter {
    // State saving config
    private static final String KEY_PREFIX_FRAGMENT = "f#";
    private static final String KEY_PREFIX_STATE = "s#";

    // Fragment GC config
    private static final long GRACE_WINDOW_TIME_MS = 10_000; // 10 seconds

    private final FragmentManager mFragmentManager;
    private final Lifecycle mLifecycle;

    // Fragment bookkeeping
    private final LongSparseArray<Fragment> mFragments = new LongSparseArray<>();
    private final LongSparseArray<Fragment.SavedState> mSavedStates = new LongSparseArray<>();
    private final LongSparseArray<Integer> mItemIdToViewHolder = new LongSparseArray<>();

    // Fragment GC
    @SuppressWarnings("WeakerAccess") // to avoid creation of a synthetic accessor
            boolean mIsInGracePeriod = false;
    private boolean mHasStaleFragments = false;

    /**
     * @param fragmentActivity if the {@link ViewPager2} lives directly in a
     *                         {@link FragmentActivity} subclass.
     * @see FragmentStateAdapter#FragmentStateAdapter(Fragment)
     * @see FragmentStateAdapter#FragmentStateAdapter(FragmentManager, Lifecycle)
     */
    public FragmentStateAdapter(@NonNull FragmentActivity fragmentActivity) {
        this(fragmentActivity.getSupportFragmentManager(), fragmentActivity.getLifecycle());
    }

    /**
     * @param fragment if the {@link ViewPager2} lives directly in a {@link Fragment} subclass.
     * @see FragmentStateAdapter#FragmentStateAdapter(FragmentActivity)
     * @see FragmentStateAdapter#FragmentStateAdapter(FragmentManager, Lifecycle)
     */
    public FragmentStateAdapter(@NonNull Fragment fragment) {
        this(fragment.getChildFragmentManager(), fragment.getLifecycle());
    }

    /**
     * @param fragmentManager of {@link ViewPager2}'s host
     * @param lifecycle       of {@link ViewPager2}'s host
     * @see FragmentStateAdapter#FragmentStateAdapter(FragmentActivity)
     * @see FragmentStateAdapter#FragmentStateAdapter(Fragment)
     */
    public FragmentStateAdapter(@NonNull FragmentManager fragmentManager,
                                @NonNull Lifecycle lifecycle) {
        mFragmentManager = fragmentManager;
        mLifecycle = lifecycle;
        super.setHasStableIds(true);
    }

    /**
     * Provide a Fragment associated with the specified position.
     */
    public abstract @NonNull
    Fragment getItem(int position);

    @NonNull
    @Override
    public final FragmentViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        return FragmentViewHolder.create(parent);
    }

    @Override
    public final void onBindViewHolder(final @NonNull FragmentViewHolder holder, int position) {
        final long itemId = holder.getItemId();
        final int viewHolderId = holder.getContainer().getId();
        final Long boundItemId = itemForViewHolder(viewHolderId); // item currently bound to the VH
        if (boundItemId != null && boundItemId != itemId) {
            removeFragment(boundItemId);
            mItemIdToViewHolder.remove(boundItemId);
        }

        mItemIdToViewHolder.put(itemId, viewHolderId); // this might overwrite an existing entry
        ensureFragment(position);

        /** Special case when {@link RecyclerView} decides to keep the {@link container}
         * attached to the window, but not to the view hierarchy (i.e. parent is null) */
        final FrameLayout container = holder.getContainer();
        if (ViewCompat.isAttachedToWindow(container)) {
            if (container.getParent() != null) {
                throw new IllegalStateException("Design assumption violated.");
            }
            container.addOnLayoutChangeListener(new View.OnLayoutChangeListener() {
                @Override
                public void onLayoutChange(View v, int left, int top, int right, int bottom,
                                           int oldLeft, int oldTop, int oldRight, int oldBottom) {
                    if (container.getParent() != null) {
                        container.removeOnLayoutChangeListener(this);
                        placeFragmentInViewHolder(holder);
                    }
                }
            });
        }

        gcFragments();
    }

    @SuppressWarnings("WeakerAccess")
        // to avoid creation of a synthetic accessor
    void gcFragments() {
        if (!mHasStaleFragments || shouldDelayFragmentTransactions()) {
            return;
        }

        // Remove Fragments for items that are no longer part of the data-set
        Set<Long> toRemove = new ArraySet<>();
        for (int ix = 0; ix < mFragments.size(); ix++) {
            long itemId = mFragments.keyAt(ix);
            if (!containsItem(itemId)) {
                toRemove.add(itemId);
                mItemIdToViewHolder.remove(itemId); // in case they're still bound
            }
        }

        // Remove Fragments that are not bound anywhere -- pending a grace period
        if (!mIsInGracePeriod) {
            mHasStaleFragments = false; // we've executed all GC checks

            for (int ix = 0; ix < mFragments.size(); ix++) {
                long itemId = mFragments.keyAt(ix);
                if (!mItemIdToViewHolder.containsKey(itemId)) {
                    toRemove.add(itemId);
                }
            }
        }

        for (Long itemId : toRemove) {
            removeFragment(itemId);
        }
    }

    private Long itemForViewHolder(int viewHolderId) {
        Long boundItemId = null;
        for (int ix = 0; ix < mItemIdToViewHolder.size(); ix++) {
            if (mItemIdToViewHolder.valueAt(ix) == viewHolderId) {
                if (boundItemId != null) {
                    throw new IllegalStateException("Design assumption violated: "
                            + "a ViewHolder can only be bound to one item at a time.");
                }
                boundItemId = mItemIdToViewHolder.keyAt(ix);
            }
        }
        return boundItemId;
    }

    private void ensureFragment(int position) {
        long itemId = getItemId(position);
        if (!mFragments.containsKey(itemId)) {
            Fragment newFragment = getItem(position);
            newFragment.setInitialSavedState(mSavedStates.get(itemId));
            mFragments.put(itemId, newFragment);
        }
    }

    @Override
    public final void onViewAttachedToWindow(@NonNull final FragmentViewHolder holder) {
        placeFragmentInViewHolder(holder);
        gcFragments();
    }

    /**
     * @param holder that has been bound to a Fragment in the {@link #onBindViewHolder} stage.
     */
    @SuppressWarnings("WeakerAccess")
    // to avoid creation of a synthetic accessor
    void placeFragmentInViewHolder(@NonNull final FragmentViewHolder holder) {
        Fragment fragment = mFragments.get(holder.getItemId());
        if (fragment == null) {
            throw new IllegalStateException("Design assumption violated.");
        }
        FrameLayout container = holder.getContainer();
        View view = fragment.getView();

        /*
        possible states:
        - fragment: { added, notAdded }
        - view: { created, notCreated }
        - view: { attached, notAttached }

        combinations:
        - { f:added, v:created, v:attached } -> check if attached to the right container
        - { f:added, v:created, v:notAttached} -> attach view to container
        - { f:added, v:notCreated, v:attached } -> impossible
        - { f:added, v:notCreated, v:notAttached} -> schedule callback for when created
        - { f:notAdded, v:created, v:attached } -> illegal state
        - { f:notAdded, v:created, v:notAttached } -> illegal state
        - { f:notAdded, v:notCreated, v:attached } -> impossible
        - { f:notAdded, v:notCreated, v:notAttached } -> add, create, attach
         */

        // { f:notAdded, v:created, v:attached } -> illegal state
        // { f:notAdded, v:created, v:notAttached } -> illegal state
        if (!fragment.isAdded() && view != null) {
            throw new IllegalStateException("Design assumption violated.");
        }

        // { f:added, v:notCreated, v:notAttached} -> schedule callback for when created
        if (fragment.isAdded() && view == null) {
            scheduleViewAttach(fragment, container);
            return;
        }

        // { f:added, v:created, v:attached } -> check if attached to the right container
        if (fragment.isAdded() && view.getParent() != null) {
            if (view.getParent() != container) {
                addViewToContainer(view, container);
            }

            //Set menu visibility of All Fragments to false
            setMenuVisibilityOfAllFragments(false);
            //Set menu visibility of primary item
            fragment.setMenuVisibility(true);
            return;
        }

        // { f:added, v:created, v:notAttached} -> attach view to container
        if (fragment.isAdded()) {
            addViewToContainer(view, container);
            return;
        }

        // { f:notAdded, v:notCreated, v:notAttached } -> add, create, attach
        if (!shouldDelayFragmentTransactions()) {
            scheduleViewAttach(fragment, container);

            //Set menu visibility of All Fragments to false
            setMenuVisibilityOfAllFragments(false);
            //Set menu visibility of primary item
            fragment.setMenuVisibility(true);
            mFragmentManager.beginTransaction().add(fragment, "f" + holder.getItemId()).commitNow();
        } else {
            if (mFragmentManager.isDestroyed()) {
                return; // nothing we can do
            }
            mLifecycle.addObserver(new LifecycleEventObserver() {
                @Override
                public void onStateChanged(@NonNull LifecycleOwner source,
                                           @NonNull Lifecycle.Event event) {
                    if (shouldDelayFragmentTransactions()) {
                        return;
                    }
                    source.getLifecycle().removeObserver(this);
                    if (ViewCompat.isAttachedToWindow(holder.getContainer())) {
                        placeFragmentInViewHolder(holder);
                    }
                }
            });
        }
    }

    /**
     * Sets visibility of all existing fragments with visibility param
     * @param visibility
     */
    private void setMenuVisibilityOfAllFragments(Boolean visibility) {
        for (int i = 0; i < mFragments.size(); i++) {
            long key = mFragments.keyAt(i);
            // get the object by the key.
            Fragment obj = mFragments.get(key);
            if (obj != null) {
                obj.setMenuVisibility(visibility);
            }
        }
    }

    private void scheduleViewAttach(final Fragment fragment, final FrameLayout container) {
        // After a config change, Fragments that were in FragmentManager will be recreated. Since
        // ViewHolder container ids are dynamically generated, we opted to manually handle
        // attaching Fragment views to containers. For consistency, we use the same mechanism for
        // all Fragment views.
        mFragmentManager.registerFragmentLifecycleCallbacks(
                new FragmentManager.FragmentLifecycleCallbacks() {
                    @Override
                    public void onFragmentViewCreated(@NonNull FragmentManager fm,
                                                      @NonNull Fragment f, @NonNull View v,
                                                      @Nullable Bundle savedInstanceState) {
                        if (f == fragment) {
                            fm.unregisterFragmentLifecycleCallbacks(this);
                            addViewToContainer(v, container);
                        }
                    }
                }, false);
    }

    @SuppressWarnings("WeakerAccess")
        // to avoid creation of a synthetic accessor
    void addViewToContainer(@NonNull View v, FrameLayout container) {
        if (container.getChildCount() > 1) {
            throw new IllegalStateException("Design assumption violated.");
        }

        if (v.getParent() == container) {
            return;
        }

        if (container.getChildCount() > 0) {
            container.removeAllViews();
        }

        if (v.getParent() != null) {
            ((ViewGroup) v.getParent()).removeView(v);
        }

        container.addView(v);
    }

    @Override
    public final void onViewRecycled(@NonNull FragmentViewHolder holder) {
        final int viewHolderId = holder.getContainer().getId();
        final Long boundItemId = itemForViewHolder(viewHolderId); // item currently bound to the VH
        if (boundItemId != null) {
            removeFragment(boundItemId);
            mItemIdToViewHolder.remove(boundItemId);
        }
    }

    @Override
    public final boolean onFailedToRecycleView(@NonNull FragmentViewHolder holder) {
        // This happens when a ViewHolder is in a transient state (e.g. during custom
        // animation). We don't have sufficient information on how to clear up what lead to
        // the transient state, so we are throwing away the ViewHolder to stay on the
        // conservative side.
        onViewRecycled(holder); // the same clean-up steps as when recycling a ViewHolder
        return false; // don't recycle the view
    }

    private void removeFragment(long itemId) {
        Fragment fragment = mFragments.get(itemId);

        if (fragment == null) {
            return;
        }

        if (fragment.getView() != null) {
            ViewParent viewParent = fragment.getView().getParent();
            if (viewParent != null) {
                ((FrameLayout) viewParent).removeAllViews();
            }
        }

        if (!containsItem(itemId)) {
            mSavedStates.remove(itemId);
        }

        if (!fragment.isAdded()) {
            mFragments.remove(itemId);
            return;
        }

        if (shouldDelayFragmentTransactions()) {
            mHasStaleFragments = true;
            return;
        }

        if (fragment.isAdded() && containsItem(itemId)) {
            mSavedStates.put(itemId, mFragmentManager.saveFragmentInstanceState(fragment));
        }
        mFragmentManager.beginTransaction().remove(fragment).commitNow();
        mFragments.remove(itemId);
    }

    @SuppressWarnings("WeakerAccess")
        // to avoid creation of a synthetic accessor
    boolean shouldDelayFragmentTransactions() {
        return mFragmentManager.isStateSaved();
    }

    /**
     * Default implementation works for collections that don't add, move, remove items.
     * <p>
     * TODO(b/122670460): add lint rule
     * When overriding, also override {@link #containsItem(long)}.
     * <p>
     * If the item is not a part of the collection, return {@link RecyclerView#NO_ID}.
     *
     * @param position Adapter position
     * @return stable item id {@link RecyclerView.Adapter#hasStableIds()}
     */
    @Override
    public long getItemId(int position) {
        return position;
    }

    /**
     * Default implementation works for collections that don't add, move, remove items.
     * <p>
     * TODO(b/122670460): add lint rule
     * When overriding, also override {@link #getItemId(int)}
     */
    public boolean containsItem(long itemId) {
        return itemId >= 0 && itemId < getItemCount();
    }

    @Override
    public final void setHasStableIds(boolean hasStableIds) {
        throw new UnsupportedOperationException(
                "Stable Ids are required for the adapter to function properly, and the adapter "
                        + "takes care of setting the flag.");
    }

    @Override
    public @NonNull
    Parcelable saveState() {
        /** TODO(b/122670461): use custom {@link Parcelable} instead of Bundle to save space */
        Bundle savedState = new Bundle(mFragments.size() + mSavedStates.size());

        /** save references to active fragments */
        for (int ix = 0; ix < mFragments.size(); ix++) {
            long itemId = mFragments.keyAt(ix);
            Fragment fragment = mFragments.get(itemId);
            if (fragment != null && fragment.isAdded()) {
                String key = createKey(KEY_PREFIX_FRAGMENT, itemId);
                mFragmentManager.putFragment(savedState, key, fragment);
            }
        }

        /** Write {@link mSavedStates) into a {@link Parcelable} */
        for (int ix = 0; ix < mSavedStates.size(); ix++) {
            long itemId = mSavedStates.keyAt(ix);
            if (containsItem(itemId)) {
                String key = createKey(KEY_PREFIX_STATE, itemId);
                savedState.putParcelable(key, mSavedStates.get(itemId));
            }
        }

        return savedState;
    }

    @Override
    public void restoreState(@NonNull Parcelable savedState) {
        if (!mSavedStates.isEmpty() || !mFragments.isEmpty()) {
            throw new IllegalStateException(
                    "Expected the adapter to be 'fresh' while restoring state.");
        }

        Bundle bundle = (Bundle) savedState;

        for (String key : bundle.keySet()) {
            if (isValidKey(key, KEY_PREFIX_FRAGMENT)) {
                long itemId = parseIdFromKey(key, KEY_PREFIX_FRAGMENT);
                Fragment fragment = mFragmentManager.getFragment(bundle, key);
                mFragments.put(itemId, fragment);
                continue;
            }

            if (isValidKey(key, KEY_PREFIX_STATE)) {
                long itemId = parseIdFromKey(key, KEY_PREFIX_STATE);
                Fragment.SavedState state = bundle.getParcelable(key);
                if (containsItem(itemId)) {
                    mSavedStates.put(itemId, state);
                }
                continue;
            }

            throw new IllegalArgumentException("Unexpected key in savedState: " + key);
        }

        if (!mFragments.isEmpty()) {
            mHasStaleFragments = true;
            mIsInGracePeriod = true;
            gcFragments();
            scheduleGracePeriodEnd();
        }
    }

    private void scheduleGracePeriodEnd() {
        final Handler handler = new Handler(Looper.getMainLooper());
        final Runnable runnable = new Runnable() {
            @Override
            public void run() {
                mIsInGracePeriod = false;
                gcFragments(); // good opportunity to GC
            }
        };

        mLifecycle.addObserver(new LifecycleEventObserver() {
            @Override
            public void onStateChanged(@NonNull LifecycleOwner source,
                                       @NonNull Lifecycle.Event event) {
                if (event == Lifecycle.Event.ON_DESTROY) {
                    handler.removeCallbacks(runnable);
                    source.getLifecycle().removeObserver(this);
                }
            }
        });

        handler.postDelayed(runnable, GRACE_WINDOW_TIME_MS);
    }

    // Helper function for dealing with save / restore state
    private static @NonNull
    String createKey(@NonNull String prefix, long id) {
        return prefix + id;
    }

    // Helper function for dealing with save / restore state
    private static boolean isValidKey(@NonNull String key, @NonNull String prefix) {
        return key.startsWith(prefix) && key.length() > prefix.length();
    }

    // Helper function for dealing with save / restore state
    private static long parseIdFromKey(@NonNull String key, @NonNull String prefix) {
        return Long.parseLong(key.substring(prefix.length()));
    }
}

FragmentViewHolder.java

/**
 * {@link ViewHolder} implementation for handling {@link Fragment}s. Used in
 * {@link FragmentStateAdapter}.
 */
public final class FragmentViewHolder extends RecyclerView.ViewHolder {
    private FragmentViewHolder(FrameLayout container) {
        super(container);
    }

    static FragmentViewHolder create(ViewGroup parent) {
        FrameLayout container = new FrameLayout(parent.getContext());
        container.setLayoutParams(
                new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
                        ViewGroup.LayoutParams.MATCH_PARENT));
        container.setId(ViewCompat.generateViewId());
        container.setSaveEnabled(false);
        return new FragmentViewHolder(container);
    }

    FrameLayout getContainer() {
        return (FrameLayout) itemView;
    }
}

Usage.kt

inner class ViewPagerAdapter : FragmentStateAdapter(this) {
        override fun getItem(position: Int): Fragment {
            return YourFragment.newInstance()
        }

        override fun getItemCount(): Int {
            return yourList.size
        }
    }

Copy and Paste FragmentStateAdapter.java and FragmentViewHolder.java into the same package. Then use it as in Usage.kt.