An implementation of PagerAdapter for views

ViewPagerAdapter.java

/**
 * View Pager Adapter - An implementation of PagerAdapter for views
 * Simplified implementation which enables access to created view at a specific position
 * for controlling the view status and force pre creation of a view when necessary
 *
 * Created by tomergoldst on 15/11/2018.
 */

public abstract class ViewPagerAdapter<T> extends PagerAdapter {

    private SparseArray<T> mViews = new SparseArray<>();
    private final Object mLock = new Object();

    /**
     * Force create view at position
     *
     * @param position position
     * @return view created by createView(int position)
     */
    public final T forceViewCreation(int position) {
        return instantiateView(position);
    }

    /**
     * re-instantiate all cached views, this will trigger the abstract {@link #createView(int)} and
     * {@link #onViewCreated(Object, int)}
     */
    public final void reInstantiateCachedViews() {
        synchronized (mLock) {
            for (int i = 0, N = mViews.size(); i < N; i++) {
                int key = mViews.keyAt(i);
                instantiateView(key);
            }
        }
    }

    /**
     * Clear cache. Using this operation should follow a call to setAdapter which in turn will call
     * instantiateItem
     */
    public final void clearCache() {
        synchronized (mLock) {
            mViews.clear();
        }
    }

    /**
     * Create view at position
     *
     * @param position position
     * @return view created by createView(int position)
     */
    private T instantiateView(int position) {
        synchronized (mLock) {
            isValidPosition(position);
            T view = checkNotNull(createView(position));
            mViews.put(position, view);
            onViewCreated(view, position);
            return view;
        }
    }

    /**
     * Called after view was created
     *
     * @param view     created view
     * @param position position
     */
    public void onViewCreated(T view, int position) {
        // Empty implementation
    }

    /**
     * Check if position is valid
     *
     * @param position position
     */
    private void isValidPosition(int position) {
        if (position < 0 || position > getCount() - 1) {
            throw new RuntimeException("position " + position + " is not valid, " +
                    "getCount() == " + getCount());
        }
    }

    /**
     * Create view at position
     *
     * @param position viewpager position
     * @return DaysPickerView
     */
    public abstract T createView(int position);

    /**
     * get view at position. If view at position doesn't exist create the view
     *
     * @param position viewpager position
     * @return DaysPickerView
     */
    public final T getViewAtPosition(int position) {
        synchronized (mLock) {
            isValidPosition(position);
            T view = mViews.get(position);
            if (view == null) {
                view = instantiateView(position);
            }
            return view;
        }

    }

    /**
     * get cached view at position. If view at position doesn't exist return null
     *
     * @param position viewpager position
     * @return DaysPickerView
     */
    public final T getCachedViewAtPosition(int position) {
        synchronized (mLock) {
            isValidPosition(position);
            return mViews.get(position);
        }
    }

    /**
     * @param position requested view position
     * @return true if mViews contain view on position
     */
    protected final boolean isViewExist(int position) {
        synchronized (mLock) {
            isValidPosition(position);
            return mViews.get(position) != null;
        }
    }

    @Override
    abstract public int getCount();

    @NotNull
    @Override
    public final Object instantiateItem(ViewGroup container, int position) {
        View view = (View) getViewAtPosition(position);
        container.addView(view);
        return view;
    }

    @Override
    public final boolean isViewFromObject(@NotNull View view, @NotNull Object object) {
        return view == object;
    }

    @Override
    public void destroyItem(@NotNull ViewGroup container, int position, @NotNull Object object) {
        View view = (View) object;
        container.removeView(view);
        removeView(position);
    }

    /**
     * @param position of view to remove
     */
    private void removeView(int position) {
        synchronized (mLock) {
            isValidPosition(position);
            mViews.remove(position);
        }
    }

    private static <T> T checkNotNull(T reference) {
        if (reference == null) {
            throw new NullPointerException();
        } else {
            return reference;
        }
    }

}