本文共4块
Bootstrap是基于jQuery实现的UI库，没有jQuery就无法运行。

结构图

如图共4个部分，着重看看第四部分(结构体都类似),以Alert为例。

结构图 Bootstrap源码1.png
Bootstrap源码2.png

• $.fn[NAME] = Alert._jQueryInterface;定义了jQuery的扩展，$.fn是指jquery的命名空间,加上fn上的方法及属性,会对jquery实例每一个有效：$.fn[NAME] === jQuery.prototype[NAME]，它对每个$实例生效，这样就可以调用了 ：
  $("#alert1"). alert();

• 我们再来看看另外一个api:$('#identifier').alert('close');会调用Alert定义中的Alert.prototype.close,并将$('#identifier')作为参数传入；

• Alert.prototype.close方法中会查找到class="alert"的父元素, 再调用_triggerCloseEvent将父元素直接关闭掉，其实低层会调用jQuery中的$(element).trigger($.Event(Event.CLOSE));，
  最后，如果没有阻止事情，会_removeElement，其实就是将alert隐藏起来，并不是删除dom。

• 2个钩子方法,会在相应事件调用时被钩子住：
  $('#myalert').bind('close.bs.alert', function () { // 当调用 close 实例方法时立即触发该事件。 })
$('#myalert').bind('closed.bs.alert', function () { // 当警告框被关闭时触发该事件（将等待 CSS 过渡效果完成）... })

  /**
   * --------------------------------------------------------------------------
  * Bootstrap (v4.0.0-alpha.6): alert.js
  * Licensed under MIT (https://github.com/twbs/bootstrap/blob/master/LICENSE)
  * --------------------------------------------------------------------------
  */
  
  var Alert = function ($) {
  
  /**
   * ------------------------------------------------------------------------
   * Constants
   * ------------------------------------------------------------------------
   */
  
  var NAME = 'alert';
  var VERSION = '4.0.0-alpha.6';
  var DATA_KEY = 'bs.alert';
  var EVENT_KEY = '.' + DATA_KEY;
  var DATA_API_KEY = '.data-api';
  var JQUERY_NO_CONFLICT = $.fn[NAME];
  var TRANSITION_DURATION = 150;
  
  var Selector = {
    DISMISS: '[data-dismiss="alert"]'
  };
  
  var Event = {
    CLOSE: 'close' + EVENT_KEY,
    CLOSED: 'closed' + EVENT_KEY,
    CLICK_DATA_API: 'click' + EVENT_KEY + DATA_API_KEY
  };
  
  var ClassName = {
    ALERT: 'alert',
    FADE: 'fade',
    SHOW: 'show'
  };
  
  /**
   * ------------------------------------------------------------------------
   * Class Definition
   * ------------------------------------------------------------------------
   */
  
  var Alert = function () {
    function Alert(element) {
      _classCallCheck(this, Alert);  // 第27行定义的：function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }
  
      this._element = element;
    }
  
    // getters
  
    // public
  
    Alert.prototype.close = function close(element) {
      element = element || this._element;
  
      var rootElement = this._getRootElement(element);
      var customEvent = this._triggerCloseEvent(rootElement);
  
      if (customEvent.isDefaultPrevented()) {
        return;
      }
  
      this._removeElement(rootElement);
    };
  
    Alert.prototype.dispose = function dispose() {
      $.removeData(this._element, DATA_KEY);
      this._element = null;
    };
  
    // private
  
    Alert.prototype._getRootElement = function _getRootElement(element) {
      var selector = Util.getSelectorFromElement(element);
      var parent = false;
  
      if (selector) {
        parent = $(selector)[0];
      }
  
      if (!parent) {
        parent = $(element).closest('.' + ClassName.ALERT)[0];
      }
  
      return parent;
    };
  
    Alert.prototype._triggerCloseEvent = function _triggerCloseEvent(element) {
      var closeEvent = $.Event(Event.CLOSE);
  
      $(element).trigger(closeEvent);
      return closeEvent;
    };
  
    Alert.prototype._removeElement = function _removeElement(element) {
      var _this2 = this;
  
      $(element).removeClass(ClassName.SHOW);
  
      if (!Util.supportsTransitionEnd() || !$(element).hasClass(ClassName.FADE)) {
        this._destroyElement(element);
        return;
      }
  
      $(element).one(Util.TRANSITION_END, function (event) {
        return _this2._destroyElement(element, event);
      }).emulateTransitionEnd(TRANSITION_DURATION);
    };
  
    Alert.prototype._destroyElement = function _destroyElement(element) {
      $(element).detach().trigger(Event.CLOSED).remove();
    };
  
    // static
  
    Alert._jQueryInterface = function _jQueryInterface(config) {
      return this.each(function () {
        var $element = $(this);
        var data = $element.data(DATA_KEY);
  
        if (!data) {
          data = new Alert(this);
          $element.data(DATA_KEY, data);
        }
  
        if (config === 'close') {
          data[config](this);
        }
      });
    };
  
    Alert._handleDismiss = function _handleDismiss(alertInstance) {
      return function (event) {
        if (event) {
          event.preventDefault();
        }
  
        alertInstance.close(this);
      };
    };
  
    _createClass(Alert, null, [{
      key: 'VERSION',
      get: function get() {
        return VERSION;
      }
    }]);
  
    return Alert;
  }();
  
  /**
   * ------------------------------------------------------------------------
   * Data Api implementation
   * ------------------------------------------------------------------------
   */
  
  $(document).on(Event.CLICK_DATA_API, Selector.DISMISS, Alert._handleDismiss(new Alert()));
  
  /**
   * ------------------------------------------------------------------------
   * jQuery
   * ------------------------------------------------------------------------
   */
  
  $.fn[NAME] = Alert._jQueryInterface;
  $.fn[NAME].Constructor = Alert;
  $.fn[NAME].noConflict = function () {
    $.fn[NAME] = JQUERY_NO_CONFLICT;
    return Alert._jQueryInterface;
  };
  
  return Alert;
  }(jQuery);