$(selector).on(event,childSelector,data,function,map)

$("#btnParent").on("click","#btn",function(){
  console.log('binded!');
 });

on: function( types, selector, data, fn, /*INTERNAL*/ one ) {
  var type, origFn;

  // 类型可以使类型映射或者句柄映射
  if ( typeof types === "object" ) {
    // ( types-Object, selector, data )
    if ( typeof selector !== "string" ) {
      // ( types-Object, data )
      data = data || selector;
      selector = undefined;
    }
    // 遍历types对象，针对每一个属性绑定on()方法
    // 将types[type]作为fn传入
    for ( type in types ) {
      this.on( type, selector, data, types[ type ], one );
    }
    return this;
  }

  // 参数修正
  // jQuery这种参数修正的方法很好
  // 可以兼容多种参数形式
  // 可见在灵活调用的背后做了很多处理
  if ( data == null && fn == null ) {
    // ( types, fn )
    fn = selector;
    data = selector = undefined;
  } else if ( fn == null ) {
    if ( typeof selector === "string" ) {
      // ( types, selector, fn )
      fn = data;
      data = undefined;
    } else {
      // ( types, data, fn )
      fn = data;
      data = selector;
      selector = undefined;
    }
  }
  if ( fn === false ) {
    // fn传入false时，阻止该事件的默认行为
    // function returnFalse() {return false;}
    fn = returnFalse;
  } else if ( !fn ) {
    return this;
  }

  // one()调用on()
  if ( one === 1 ) {
    origFn = fn;
    fn = function( event ) {
      // Can use an empty set, since event contains the info
      // 用一个空jQuery对象，这样可以使用.off方法，
      // 并且event带有remove事件需要的信息
      jQuery().off( event );
      return origFn.apply( this, arguments );
    };
    // Use same guid so caller can remove using origFn
    // 事件删除依赖于guid
    fn.guid = origFn.guid || ( origFn.guid = jQuery.guid++ );
  }

  // 这里调用jQuery的each方法遍历调用on()方法的jQuery对象
  // 如$('li').on(...)则遍历每一个li传入add()
  // 推荐使用$(document).on()或者集合元素的父元素
  return this.each( function() {
    jQuery.event.add( this, types, fn, data, selector );
  });
},

jQuery.fn.extend( {

  bind: function( types, data, fn ) {
    return this.on( types, null, data, fn );
    //注意this.on()
  },
  unbind: function( types, fn ) {
    return this.off( types, null, fn );
    //注意this.off()
  },

  delegate: function( selector, types, data, fn ) {
    return this.on( types, selector, data, fn );
  },
  undelegate: function( selector, types, fn ) {
    return arguments.length === 1 ?
      this.off( selector, "**" ) :
      this.off( types, selector || "**", fn );
  }
} );