// inifinite scrolling of content without extra wrappers 
const { render, findDOMNode  } = ReactDOMFiber

class App extends React.Component {
  render() {
  // wrap the root element with an Intersection Observer, exposing .observe for children
    return <Intersection>
      <div style={{ height: 200, overflow: 'auto' }}>
        <Page offset={0} count={10} />
      </div>
    </Intersection>
  }
}


class Page extends React.Component {
  static contextTypes = {
    observe: React.PropTypes.func
  }
  state = {
    next: false
  }
  componentDidMount() {
    // load more content when this element comes into view
    this.context.observe(this.loadMore, () => this.setState({ next: true }))
  }
  render() {
    let { count, offset } = this.props
    return [
      times(count, i => 
        <div> 
          article {this.props.offset + i} 
        </div>), 
      this.state.next ? 
        <Page offset={offset + count} count={count}/> :
        <div ref={x => this.loadMore = x || this.loadMore }> load more ...</div>        
      ]
  }
}


class Intersection extends React.Component {
  static childContextTypes = {
    observe: React.PropTypes.func
  }
  getChildContext(){
    return {
      observe: this.observe
    }
  }
  elements = new Map()
  elementBuffer = []

  observe = (element, callback) => {
    this.elements.set(element, callback)
    // this funny bit to handle react's lifecycle order 
    if(!this.observer){      
      this.elementBuffer.push(element)
    }
    else {
      this.observer.observe(element)  
    }
  }
  onIntersect = (entries, observer) => {
    entries.forEach(entry => this.elements.get(entry.target)(entry))
  }
  
  componentDidMount(){    
    this.observer = new IntersectionObserver(this.onIntersect, { 
      root: findDOMNode(this), 
      rootMargin: '0px',
      threshold: 0.25 
    })
    this.elementBuffer.forEach(element => this.observer.observe(element))
    this.elementBuffer = []
  }
  render() {
    return this.props.children
  }
}

function times(n, fn){
  let arr = []
  for(let i=0; i< n; i++){
    arr.push(fn(i))
  }
  return arr
}


render(<App/>, window.app)

// homework - 
// reclaim memory by removing dom nodes from the top without jitter 
// go both ways; start from the middle and scroll up
// cleanup element handlers after loading