// 方式一
var MyComponent = Vue.extend({
  name: 'my-component',
  template: '<div>A custom component!</div>'
});
Vue.component('my-component', MyComponent);

// 方式二
Vue.component('my-component', {
  name: 'my-component',
  template: '<div>A custom component!</div>'
});

// 使用组件
<div id="example">
  <my-component></my-component>
</div>

Vue.extend = function (extendOptions) {
 extendOptions = extendOptions || {};
 var Super = this;
 var isFirstExtend = Super.cid === 0;
 if (isFirstExtend && extendOptions._Ctor) {
  return extendOptions._Ctor;
 }
 var name = extendOptions.name || Super.options.name;
 // 如果有name属性，即组件名称，检测name拼写是否合法
 if ('development' !== 'production') {
  if (!/^[a-zA-Z][\w-]*$/.test(name)) {
   warn('Invalid component name: "' + name + '". Component names ' + 'can only contain alphanumeric characaters and the hyphen.');
   name = null;
  }
 }
 // 创建一个VueComponent 构造函数，函数名为‘VueComponent'或者name
 var Sub = createClass(name || 'VueComponent');
 // 构造函数原型继承Vue.prototype
 Sub.prototype = Object.create(Super.prototype);
 Sub.prototype.constructor = Sub;
 Sub.cid = cid++;
 // 合并Vue.options和extendOptions，作为新构造函数的静态属性options  
 Sub.options = mergeOptions(Super.options, extendOptions);
 //'super'静态属性指向Vue函数
 Sub['super'] = Super;
 // start-----------------拷贝Vue静态方法  
 // allow further extension
 Sub.extend = Super.extend;
 // create asset registers, so extended classes
 // can have their private assets too.
 config._assetTypes.forEach(function (type) {
  Sub[type] = Super[type];
 });
 // end-----------------拷贝Vue静态方法  
 // enable recursive self-lookup
 if (name) {
  Sub.options.components[name] = Sub;
 }
 // cache constructor：缓存该构造函数
 if (isFirstExtend) {
  extendOptions._Ctor = Sub;
 }
 return Sub;
};

// _assetTypes: ['component', 'directive', 'elementDirective', 'filter', 'transition', 'partial']
config._assetTypes.forEach(function (type) {
 // 静态方法Vue.component
 Vue[type] = function (id, definition) {
  if (!definition) {
   return this.options[type + 's'][id];
  } else {
   /* istanbul ignore if */
   if ('development' !== 'production') {
    if (type === 'component' && (commonTagRE.test(id) || reservedTagRE.test(id))) {
     warn('Do not use built-in or reserved HTML elements as component ' + 'id: ' + id);
    }
   }
   // 如果第二个参数是简单对象，则需要通过Vue.extend创建组件构造函数
   if (type === 'component' && isPlainObject(definition)) {
    if (!definition.name) {
     definition.name = id;
    }
    definition = Vue.extend(definition);
   }
   // 将组件函数加入Vue静态属性options.components中，也就是，全局注入该组件
   this.options[type + 's'][id] = definition;
   return definition;
  }
 };
});

var MyComponent = Vue.extend({
  template: '<div>A custom component!</div>'
});

new Vue({
  el: '#example',
  components: {
    'my-component': MyComponent,
    'other-component': {
      template: '<div>A custom component!</div>'
    }
  }
});

Vue.prototype._init = function (options) {
  options = options || {};
  ....
  // merge options.
  options = this.$options = mergeOptions(this.constructor.options, options, this);
  ...
};

function mergeOptions(parent, child, vm) {
  //解析components属性
  guardComponents(child);
  guardProps(child);
  ...
}

function guardComponents(options) {
  if (options.components) {
    // 将对象转为数组
    var components = options.components = guardArrayAssets(options.components);
    //ids数组包含组件名
    var ids = Object.keys(components);
    var def;
    if ('development' !== 'production') {
      var map = options._componentNameMap = {};
    }
    // 遍历组件数组
    for (var i = 0, l = ids.length; i < l; i++) {
      var key = ids[i];
      if (commonTagRE.test(key) || reservedTagRE.test(key)) {
        'development' !== 'production' && warn('Do not use built-in or reserved HTML elements as component ' + 'id: ' + key);
        continue;
      }
      // record a all lowercase <-> kebab-case mapping for
      // possible custom element case error warning
      if ('development' !== 'production') {
        map[key.replace(/-/g, '').toLowerCase()] = hyphenate(key);
      }
      def = components[key];
      // 如果是组件定义是简单对象-对象字面量，那么需要根据该对象创建组件函数
      if (isPlainObject(def)) {
        components[key] = Vue.extend(def);
      }
    }
  }
}