var InteractionManager = function(options) {
  _.bindAll(this, "reposition", "_reposition");

  this.options = _.defaults(options || {}, {
    fromTop: 50,
    margin: 20
  });

  this.items = [];
};

InteractionManager.prototype = {
  add: function(scope, selector) {
    if(!scope.bind) { throw "Item must respond to '.bind'" }

    if(_.any(this.items, function(other) { return scope.cid == other.scope.cid })) { return; }

    this.items.push({scope: scope, selector: selector});
    scope.bind("element:reposition", this.reposition);
  },

  reposition: function() {
    var allItems = _.extend([], this.items);

    this._reposition(allItems);
  },

  _reposition: function(list){
    var object = list.pop();

    if(!object) { return this.options.fromTop; }
    var offset = this._reposition(list);

    var el      = object.scope.$(object.selector)
      , manager = this;

    if(!el.is(':visible')) { return offset; }

    el.clearQueue().animate({'top': offset});
    offset = offset + el.outerHeight(true) + manager.options.margin;

    return offset;
  }
};

_.extend(InteractionManager.prototype, Backbone.Events);