Part 3 - Creates onScroll method and related state to prepare for the drag feature

Post.js

class Post extends PureComponent {
  state = {
    dragProgress: 0
  }

  /* ... */

  onScroll = () => {
    const scrollTop = windowScroll.get('top');
    if (scrollTop < 0 && !this.isTransitioningFromDrag) {
      if (scrollTop > -DRAG_THRESHOLD) {
        const dragProgress = Math.min(1, -scrollTop / DRAG_LIMIT);
        this.setState({ dragProgress });
      } else {
        this.isTransitioningFromDrag = true;
        this.props.navigateTo('home');
      }
      return;
    }

    this.setState({ dragProgress: 0 });
  };

  /* ... */
}