function isElementInViewport(el) {
  var rect = el.getBoundingClientRect();

  var width_st = rect.width / 2,
    height_st = rect.height / 2;

  var innerHeight = window.innerHeight,
    innerWidth = window.innerWidth;


  if (  rect.top <=0 && rect.height > innerHeight 
    || rect.left <= 0 && rect.width > innerWidth
  ) {
    return rect.left * rect.right <= 0
      || rect.top * rect.bottom <= 0
  }

  return (
      rect.height > 0 
    && rect.width > 0 
    && ( ( rect.top >= 0 && rect.top <= innerHeight - height_st )
      || ( rect.bottom >= height_st && rect.bottom <= innerHeight ) )
    && ( ( rect.left >= 0 && rect.left <= innerWidth - width_st )
      || ( rect.right >= width_st && rect.right <= innerWidth ) )
  );
}

var nodes = document.querySelectorAll(".item")
function report(node) {
  // 上报的逻辑
}
window.onscroll = function() {
  nodes.forEach(node => {
    if( isElementInViewport(node) ) {
      report(node)
    }
  })
  
}

function report(node) {
  // 上报的逻辑
}
var intersectionObserver = new IntersectionObserver(entries => {
  entries.forEach(entry => {
    if( entry.intersectionRatio > 0 ) {
      report(entry.target)
    }
  })
})
var nodes = document.querySelectorAll(".item")
nodes.forEach(node => {
  intersectionObserver.observe(node)
})

require('intersection-observer'); // polyfill

class Exposure {
  constructor(callback) {
    if (!callback || typeof callback !== 'function') {
      throw new Error("need callback or selector param")
      return
    }
    this.intersectionObserver = new IntersectionObserver((entries) => {
      entries.forEach(item => {
        if (item.intersectionRatio > 0) {
          if (item.target) {
            callback(item.target, item)
            this.intersectionObserver.unobserve(item.target)
          }
        }
      })
    });
  }

  observe(selector, ignoreExposured) {
    if (!this.intersectionObserver || !selector) {
      return
    }
    let nodes = []
    if( this.isDOM(selector) ) { // dom节点
      nodes = [selector]
    }else { // 选择器
      nodes = document.querySelectorAll(selector)
    }
    if (!nodes.length) {
      return
    }
    nodes.forEach(node => {
      if (!ignoreExposured && node.__wg__tracker__exposured__) {
        return
      }
      node.__wg__tracker__exposured__ = true
      // 开始观察
      this.intersectionObserver.observe(
        node
      );
    })
  }

  disconnect() {
    if (!this.intersectionObserver) {
      return
    }
    this.intersectionObserver.disconnect()
  }

  isDOM(obj) {
    if( !obj ) {
      return false
    }
    if( typeof HTMLElement === 'object' ) {
      return obj instanceof HTMLElement
    }
    if( typeof obj === 'object' && obj.nodeType === 1 && typeof obj.nodeName === 'string' ) {
      return true
    }
    return false
  }
}

export default Exposure

function report() {}
var exposurer = new Exposure((node) => {
  report(node)
})
exposurer.observe(".item)