RecyclerView Scroll Tracking using a Sentinel View

gistfile1.txt

headerTrackScrollListener = new RecyclerView.OnScrollListener() {
    @Override
    public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
        super.onScrolled(recyclerView, dx, dy);
        float sentinelYPos = adapter.getSentinelViewYPos();
        if (sentinelYPos >= Utility.getScreenHeight(getContext()) || !areAuxiliaryTouchEventsEnabled) {
            return;
        }

        float topOfListScrollY = sentinelYPos - headerHeight;
        listBackgroundView.setY(topOfListScrollY);
        cameraPreviewView.setY(sentinelYPos - cameraPreviewHeight - headerHeight);

        if (topOfListScrollY <= 0 && headerView.getY() != 0 && prevTopOfListScrollY >= 0) {
            //WE ARE TRANSITIONING FROM PREVIEW BEING VISIBLE TO IT NOT BEING VISIBLE
            isCameraPreviewVisible = false;
            headerView.translateY(0 - (int) headerView.getY());
            cameraPresenter.onPause();
            cameraView.setVisibility(View.GONE);
        } else if (topOfListScrollY >= 0 && prevTopOfListScrollY <= 0) {
            //WE ARE TRANSITIONING FROM PREVIEW NOT BEING VISIBLE TO BEING VISIBLE
            isCameraPreviewVisible = true;
            headerView.cancelAnimators();
            headerView.setY(topOfListScrollY);
            if (!shouldTurnCameraOffUntilFullyVisible) {
                cameraView.setVisibility(View.VISIBLE);
            }
        } else if (topOfListScrollY > 0) {
            //PREVIEW IS VISIBLE AND HAS BEEN VISIBLE
            headerView.setY(topOfListScrollY);
            if (isVisible && !shouldTurnCameraOffUntilFullyVisible) {
                cameraPresenter.onResume(getContext());
                cameraView.setVisibility(View.VISIBLE);
            }
        }
        prevTopOfListScrollY = topOfListScrollY;
    }
};
recyclerView.addOnScrollListener(headerTrackScrollListener);

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cameraPreviewView.setOnTouchListener((v, event) -> {
    if (isCameraHolderAnimating) {
        return false;
    }
    switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN: {
            ((NonScrollableGridLayoutManager) recyclerView.getLayoutManager()).setScrollEnabled(false);
            headerView.setAreTouchEventsEnabled(false);
            ((LithiumActivity) getContext()).getTabHolder().setAreTouchEventsEnabled(false);
            if (velocityTracker == null) {
                velocityTracker = VelocityTracker.obtain();
            } else {
                velocityTracker.clear();
            }
            prevY = event.getRawY();
            velocityTracker.addMovement(event);
            break;
       }
       case MotionEvent.ACTION_MOVE: {
            velocityTracker.addMovement(event);
            float dy = event.getRawY() - prevY;
            prevY = event.getRawY();
            totalDy += Math.abs(dy);
            if (isDragging || Math.abs(dy) > touchSlop) {
                isDragging = true;
                float newY = studioHolder.getY() + dy;
                if (newY < 0) {
                    newY = 0;
                }
                studioHolder.setY(newY);
                updateCameraPreview(0f);
            }
            break;
        }
        case MotionEvent.ACTION_CANCEL:
        case MotionEvent.ACTION_UP: {
            isDragging = false;
            float yVel = 1;
            if (totalDy > ViewUtils.dpToPx(3, getContext())) {
                velocityTracker.computeCurrentVelocity(1000);
                yVel = velocityTracker.getYVelocity();
            }
            if (yVel > 0) {
                showCamera();
            } else {
                animateToDefault();
            }
            velocityTracker.recycle();
            velocityTracker = null;
            totalDy = 0;
            break;
        }
    }
    return true;
});