var React = require('react');

var _listeners = [];

module.exports = {

  isInViewport: function() {

    var el = React.findDOMNode(this);

    if(!el) return false;

    var rect = el.getBoundingClientRect();

    var viewportHeight = global.window.innerHeight || global.document.documentElement.clientHeight;
    var viewportWidth = global.window.innerWidth || global.document.documentElement.clientWidth;

    return (
      rect.bottom >= 0 &&
      rect.right >= 0 &&
      rect.top <= viewportHeight &&
      rect.left <= viewportWidth
    );

  },

  componentDidMount: function() {

    if(typeof this.onInViewport === 'function') {

      _listeners.push(this);

      if( this.isInViewport() ) {
        this.onInViewport();
      }

    }

  },

  componentWillUnmount: function() {
    var index = _listeners.indexOf(this);
    if(index !== -1) return _listeners.splice(index, 1);
  }

};


var computeComponentsVisibilityDebounced = debounce(computeComponentsVisibility, 100);

// Start listening for changes
window.document.addEventListener('scroll', computeComponentsVisibilityDebounced, true);
window.addEventListener('resize', computeComponentsVisibilityDebounced);

function computeComponentsVisibility() {
  _listeners.forEach(function(listener) {
    if( listener.isInViewport() ) {
      listener.onInViewport();
    }
  });
}

function debounce(func, wait, immediate) {
  var timeout;
	return function() {
		var context = this, args = arguments;
		var later = function() {
			timeout = null;
			if (!immediate) func.apply(context, args);
		};
		var callNow = immediate && !timeout;
		clearTimeout(timeout);
		timeout = setTimeout(later, wait);
		if (callNow) func.apply(context, args);
	};
}