import React from 'react'

let firstRoute = true;

export default (getComponent) => class AsyncComponent extends React.Component {

  static Component = null;
  mounted = false;

  // Remembers scroll positions based on location->key
  static scrollPositions = {};

  state = {
    Component: AsyncComponent.Component
  };

  componentDidMount() {
    this.mounted = true;

    if ( AsyncComponent.Component === null ) {
      getComponent().then((m) => m.default).then((Component) => {
        if ( firstRoute ) {
          // We want to handle scroll restoration on our own from now on
          if ( 'scrollRestoration' in window.history ) {
            window.history.scrollRestoration = 'manual';
          }
          firstRoute = false;
        }
        AsyncComponent.Component = Component;
        if ( this.mounted ) {
          this.setState({Component});
        }
      })
    } else {
      const {action, location:{key="root"}} = this.props;
      // POP means user is going forward or backward in history, restore previous scroll position
      if ( action === 'POP' ) {
        const pos = AsyncComponent.scrollPositions[key];
        if ( pos ) {
          scroll(pos[0], pos[1]);
          return;
        }
      }
    }

    // Scroll to top of viewport
    scroll(0,0);
  }

  componentWillUnmount() {
    // Remember scroll position so we can restore if we return to this view via browser history
    const {location:{key="root"}} = this.props;
    AsyncComponent.scrollPositions[key] = [window.pageXOffset, window.pageYOffset];

    this.mounted = false;
  }

  render() {
    const {Component} = this.state;
    return Component === null
      ? <div>Loading...</div>
      : <Component {...this.props} />;
  }
};