public class WeakHolderTracker
{
    private final SparseArray<WeakReference<RecyclerView.ViewHolder>> mHoldersByPosition =
            new SparseArray<>();

    public void bindHolder(RecyclerView.ViewHolder holder, int position)
    {
        mHoldersByPosition.put(position, new WeakReference<>(holder));
    }

    @Nullable
    private RecyclerView.ViewHolder getHolder(int position)
    {
        WeakReference<RecyclerView.ViewHolder> holderRef = mHoldersByPosition.get(position);
        if (holderRef == null)
        {
            mHoldersByPosition.remove(position);
            return null;
        }

        RecyclerView.ViewHolder holder = holderRef.get();
        if (holder == null || (holder.getAdapterPosition() != position && holder.getAdapterPosition() != RecyclerView.NO_POSITION))
        {
            mHoldersByPosition.remove(position);
            return null;
        }

        return holder;
    }

    public List<RecyclerView.ViewHolder> getTrackedHolders()
    {
        List<RecyclerView.ViewHolder> holders = new ArrayList<>();

        for (int i = 0; i < mHoldersByPosition.size(); i++)
        {
            int key = mHoldersByPosition.keyAt(i);
            RecyclerView.ViewHolder holder = getHolder(key);

            if (holder != null)
            {
                holders.add(holder);
            }
        }

        return holders;
    }
}

public class CheckableAutofitHeightFrameLayout extends FrameLayout implements Checkable
{
    private static final int[] CHECKED_STATE_SET = {android.R.attr.state_checked};

    private boolean mIsChecked;
    private boolean mIsCheckable;

    public CheckableAutofitHeightFrameLayout(Context context)
    {
        super(context);
    }

    public CheckableAutofitHeightFrameLayout(Context context, AttributeSet attrs)
    {
        super(context, attrs);
    }

    public CheckableAutofitHeightFrameLayout(Context context, AttributeSet attrs, int defStyleAttr)
    {
        super(context, attrs, defStyleAttr);
    }

    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
    public CheckableAutofitHeightFrameLayout(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes)
    {
        super(context, attrs, defStyleAttr, defStyleRes);
    }

    @Override
    protected int[] onCreateDrawableState(int extraSpace)
    {
        final int[] drawableState = super.onCreateDrawableState(extraSpace + 1);
        if (isChecked())
        {
            mergeDrawableStates(drawableState, CHECKED_STATE_SET);
        }
        return drawableState;
    }

    @Override
    public boolean isCheckable()
    {
        return mIsCheckable;
    }

    @Override
    public void setCheckable(boolean isCheckable)
    {
        boolean wasCheckable = isCheckable();
        mIsCheckable = isCheckable;
        if (!isCheckable && isChecked())
        {
            setChecked(false);
        }
        else if (wasCheckable ^ mIsCheckable)
        {
            refreshDrawableState();
        }

    }

    @Override
    public void setChecked(boolean isChecked)
    {
        boolean wasChecked = isChecked();
        mIsChecked = isCheckable() && isChecked;

        if (wasChecked ^ mIsChecked)
        {
            refreshDrawableState();
        }
    }

    @Override
    public boolean isChecked()
    {
        return mIsChecked;
    }

    @Override
    public void toggle()
    {
        setChecked(!mIsChecked);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
    {
        //noinspection SuspiciousNameCombination
        super.onMeasure(widthMeasureSpec, widthMeasureSpec);
    }
}

public class GalleryActivity extends Activity
{
    private final ActionModeCallback mActionModeCallback = new ActionModeCallback();

    private SelectableRecyclerViewAdapter mAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        RecyclerView recyclerView = (RecyclerView) findViewById(R.id.gallery);
        int columnWidth = getResources().getDimensionPixelSize(R.dimen.column_width);
        int vSpacing = getResources().getDimensionPixelSize(R.dimen.grid_spacing_vertical);
        int hSpacing = getResources().getDimensionPixelSize(R.dimen.grid_spacing_horizontal);
        recyclerView.setLayoutManager(new GridAutofitLayoutManager(this, columnWidth));
        recyclerView.addItemDecoration(new GridSimpleSpacingDecoration(hSpacing, vSpacing));
        DataSet<Image> dataSet = new DummyImagesDataSet();
        mAdapter = new SelectableRecyclerViewAdapter(this, dataSet);
        recyclerView.setAdapter(mAdapter);
    }

    public void startActionMode()
    {
        startActionMode(mActionModeCallback);
    }

    private class ActionModeCallback implements ActionMode.Callback, SelectionObserver
    {
        private ActionMode mActionMode;

        @Override
        public boolean onPrepareActionMode(ActionMode actionMode, Menu menu)
        {
            return false;
        }

        @Override
        public void onDestroyActionMode(ActionMode actionMode)
        {
            SelectionHelper selectionHelper = mAdapter.getSelectionHelper();
            selectionHelper.unregisterSelectionObserver(this);
            mActionMode = null;
            selectionHelper.setSelectable(false);
        }

        @Override
        public boolean onCreateActionMode(ActionMode actionMode, Menu menu)
        {
            mActionMode = actionMode;
            mActionMode.getMenuInflater().inflate(R.menu.gallery_selection, menu);
            mAdapter.getSelectionHelper().registerSelectionObserver(this);
            return true;
        }

        @Override
        public boolean onActionItemClicked(ActionMode actionMode, MenuItem menuItem)
        {
            switch (menuItem.getItemId())
            {
                case R.id.menu_toast:
                    Toast.makeText(GalleryActivity.this,
                            R.string.text_simple_toast, Toast.LENGTH_SHORT).show();
                    break;
            }
            return true;
        }

        @Override
        public void onSelectedChanged(RecyclerView.ViewHolder holder, boolean isSelected)
        {
            if (mActionMode != null)
            {
                int checkedImagesCount = mAdapter.getSelectionHelper().getSelectedItemsCount();
                mActionMode.setTitle(String.valueOf(checkedImagesCount));
            }
        }

        @Override
        public void onSelectableChanged(boolean isSelectable)
        {
            if (!isSelectable)
            {
               mActionMode.finish();
            }
        }
    }
}

public class SelectableRecyclerViewAdapter extends RecyclerView.Adapter<RecyclerView.ViewHolder> implements SelectionObserver
{
    private final SelectionHelper mSelectionHelper;

    public SelectableRecyclerViewAdapter()
    {
        mSelectionHelper = new SelectionHelper();
        mSelectionHelper.registerSelectionObserver(this);
    }

    public SelectionHelper getSelectionHelper()
    {
        return mSelectionHelper;
    }

    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int position)
    {
        LayoutInflater inflater = LayoutInflater.from(viewGroup.getContext());
        ImageViewHolder holder = new ImageViewHolder(
                inflater.inflate(R.layout.gallery_item, viewGroup, false));
        return mSelectionHelper.wrapClickable(holder);
    }

    @Override
    public void onHolderClick(RecyclerView.ViewHolder holder)
    {
		// perform item click
    }

    @Override
    public boolean onHolderLongClick(RecyclerView.ViewHolder holder)
    {
	    // perform item long click
        return false;
    }

    //...
}

public class GridAutofitLayoutManager extends GridLayoutManager
{
    private int mColumnWidth;
    private boolean mColumnWidthChanged = true;

    public GridAutofitLayoutManager(Context context, int columnWidth)
    {
        /* Initially set spanCount to 1, will be changed automatically later. */
        super(context, 1);
        setColumnWidth(checkedColumnWidth(context, columnWidth));
    }

    public GridAutofitLayoutManager(Context context, int columnWidth, int orientation, boolean reverseLayout)
    {
        /* Initially set spanCount to 1, will be changed automatically later. */
        super(context, 1, orientation, reverseLayout);
        setColumnWidth(checkedColumnWidth(context, columnWidth));
    }

    private int checkedColumnWidth(Context context, int columnWidth)
    {
        if (columnWidth <= 0)
        {
            context.getResources().getDimensionPixelSize(R.dimen.rv_def_column_width);
        }
        return columnWidth;
    }

    public void setColumnWidth(int newColumnWidth)
    {
        if (newColumnWidth > 0 && newColumnWidth != mColumnWidth)
        {
            mColumnWidth = newColumnWidth;
            mColumnWidthChanged = true;
        }
    }

    /* I don't actually remember why I choose to set span count in onLayoutChildren, I wrote this
    class some time ago. But the point is we need to do so after view get measured,
    so we can get its height and width. */
    @Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state)
    {
        if (mColumnWidthChanged && mColumnWidth > 0)
        {
            int totalSpace;
            if (getOrientation() == VERTICAL)
            {
                totalSpace = getWidth() - getPaddingRight() - getPaddingLeft();
            }
            else
            {
                totalSpace = getHeight() - getPaddingTop() - getPaddingBottom();
            }
            int spanCount = Math.max(1, totalSpace / mColumnWidth);
            setSpanCount(spanCount);
            mColumnWidthChanged = false;
        }
        super.onLayoutChildren(recycler, state);
    }
}

public class GalleryActivity extends Activity
{
    private final ActionModeCallback mActionModeCallback = new ActionModeCallback();

    private SelectableRecyclerViewAdapter mAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        RecyclerView recyclerView = (RecyclerView) findViewById(R.id.gallery);
        int columnWidth = getResources().getDimensionPixelSize(R.dimen.column_width);
        int vSpacing = getResources().getDimensionPixelSize(R.dimen.grid_spacing_vertical);
        int hSpacing = getResources().getDimensionPixelSize(R.dimen.grid_spacing_horizontal);
        recyclerView.setLayoutManager(new GridAutofitLayoutManager(this, columnWidth));
        recyclerView.addItemDecoration(new GridSimpleSpacingDecoration(hSpacing, vSpacing));
        DataSet<Image> dataSet = new DummyImagesDataSet();
        mAdapter = new SelectableRecyclerViewAdapter(this, dataSet);
        recyclerView.setAdapter(mAdapter);
    }
	
	//...