[Android, Java] HeightWrappingViewPager - a custom ViewPager that can be used as a child view in ScrollView. With custom swipe listener

HeightWrappingViewPager.java

/**
 * A custom scrollable ViewPager
 * NOTE: it uses in a production app.
 * <p>
 * Created by Dmitriy V. Chernysh on 2/7/19.
 * <p>
 * https://instagr.am/mobiledevpro
 * #MobileDevPro
 */

public class HeightWrappingViewPager extends ViewPager {
    public static final int CUSTOM_EVENT_SWIPE_LEFT = 1;
    public static final int CUSTOM_EVENT_SWIPE_RIGHT = 2;
    public static final float CUSTOM_SWIPE_MIN_DISTANCE = 50F;

    private boolean isSwipable = true;
    private boolean isHeightMatchParent;
    private CustomSwipePagerListener customSwipePagerListener;

    private float motionEventStartX = 0F;
    private boolean isCustomSwipeDone;

    public interface CustomSwipePagerListener {
        void onNext();

        void onPrev();
    }

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

    public HeightWrappingViewPager(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
  
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int mode = MeasureSpec.getMode(heightMeasureSpec);
        // Unspecified means that the ViewPager is in a ScrollView WRAP_CONTENT.
        // At Most means that the ViewPager is not in a ScrollView WRAP_CONTENT.
        if (mode == MeasureSpec.UNSPECIFIED || mode == MeasureSpec.AT_MOST) {
            super.onMeasure(widthMeasureSpec, heightMeasureSpec);

            if (isHeightMatchParent) return;

            int childCount = getChildCount();
            int height = 0;

            for (int i = 0; i < childCount; i++) {
                final View child = getChildAt(i);

                //don't set height if RecyclerView is a first child.
                //in some cases RecyclerView should be scrollable. Example, fragment_exam_questions_breakdown.xml
                if (child instanceof RecyclerView) {
                    return;
                }
                final ViewPager.LayoutParams lp = (ViewPager.LayoutParams) child.getLayoutParams();
                int position = 0;
                //find position of visible page
                try {
                    Field f = lp.getClass().getDeclaredField("position");
                    f.setAccessible(true);
                    position = f.getInt(lp); //IllegalAccessException
                } catch (NoSuchFieldException | IllegalAccessException ex) {
                    ex.printStackTrace();
                }

                //if page found position and current item position are equals
                if (position == getCurrentItem()) {
                    //change height of visible page
                    child.measure(widthMeasureSpec, MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED));
                    int h = child.getMeasuredHeight();
                    if (h > height) height = h;
                    break;
                }
            }
            heightMeasureSpec = MeasureSpec.makeMeasureSpec(height, MeasureSpec.EXACTLY);

        }
        // super has to be called again so the new specs are treated as exact measurements
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        if (!isSwipable) {

            //handle custom events
            if (customSwipePagerListener != null) {
                int swipeDirection = swipeDirection(ev);

                if (swipeDirection == CUSTOM_EVENT_SWIPE_LEFT) {
                    isCustomSwipeDone = true;
                    customSwipePagerListener.onPrev();
                } else if (swipeDirection == CUSTOM_EVENT_SWIPE_RIGHT) {
                    isCustomSwipeDone = true;
                    customSwipePagerListener.onNext();
                }
            }

            return false;
        } else {
            return super.onInterceptTouchEvent(ev);
        }
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        if (!isSwipable) {
            return false;
        } else {
            return super.onTouchEvent(ev);
        }
    }

    public void onChangePagerIndex() {
        //it lets to optimize a height of viewpager
        requestLayout();
    }

    public void setSwipable(boolean isSwipable) {
        this.isSwipable = isSwipable;
        requestLayout();
    }

    public void setHeightMatchParent(boolean b) {
        this.isHeightMatchParent = b;
        requestLayout();
    }

    public void setCustomSwipeListener(CustomSwipePagerListener listener) {
        customSwipePagerListener = listener;
    }

    private int swipeDirection(MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                isCustomSwipeDone = false;
                motionEventStartX = event.getX();
                return 0;

            case MotionEvent.ACTION_MOVE:
            case MotionEvent.ACTION_UP:
                //prevent callbacks doubling
                if (isCustomSwipeDone) return 0;

                float deltaX = motionEventStartX - event.getX();

                if (deltaX > 0 && deltaX > CUSTOM_SWIPE_MIN_DISTANCE) {
                    return CUSTOM_EVENT_SWIPE_RIGHT;
                } else if (deltaX < 0 && -deltaX > CUSTOM_SWIPE_MIN_DISTANCE) {
                    return CUSTOM_EVENT_SWIPE_LEFT;
                } else
                    return 0;

            default:
                return 0;
        }
    }
}

howToUse.java

     viewPager.setAdapter(adapter);
     // fit to parent view height
     viewPager.setHeightMatchParent(true); 
     viewPager.measure(LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.MATCH_PARENT);

     viewPager.setSwipable(false);
     viewPager.setCustomSwipeListener(new HeightWrappingViewPager.CustomSwipePagerListener() {
            @Override
            public void onNext() {
                //do something
            }

            @Override
            public void onPrev() {
                   //do something
            }
        });