vue源碼解讀－component機制

身爲原來的jquery，angular使用者。後面接觸了react和vue。漸漸的喜歡上了vue。抱着學習的態度呀。看看源碼。果真菜要付出代價。一步步單步調試。頭好疼。看到哪裏記到哪裏。來一點點心得。錯誤的地方請幫我指出來。謝謝。最近看的是vue component部分。

先上一段最簡單的代碼，來剖析component機制：

<div id="app">
  <div>{{a}}</div>
  <input v-model="heihei">
  <button v-on:click="click1">
  click1
  </button>
  <my-component>
  <div slot='dudu'>111</div>
  <Child>{{a}}</Child>
  </my-component>
  <button @click.stop="click2">
  click2
  </button>
</div>
</body>
<script src="vue.js"></script>
<script type="text/javascript">
var Child = {
  template: '<div>A custom component!</div>'
} 
Vue.component('my-component', {
  name: 'my-component',
  template: '<div>A custom component!<Child></Child><slot></slot></div>',
  components: {
    Child:Child
  },
  created(){
    console.log(this);
  },
  mounted(){
    console.log(this);
  }
})
    new Vue({
  el: '#app',
  data: function () {
    return {
      heihei:{name:3333},
      a:1
    }
  },
  components: {
    Child:Child
  },
  created(){
  
  },
  methods: {
    click1: function(){
      console.log(this);
    },
    click2: function(){
      alert('click2')
    }
  }
})
</script>

咱們按照瀏覽器的思惟逐行來。執行到腳本時。首先執行了

Vue.component('my-component', {
  name: 'my-component',
  template: '<div>A custom component!<Child></Child><slot></slot></div>',
  components: {
    Child:Child
  },
  created(){
    console.log(this);
  },
  mounted(){
    console.log(this);
  }
})

咱們來看看這個函數經歷了什麼：
vue.js初始化的時候。調用了initGlobalAPI(vue),爲vue掛上了工具函數vue.component
通過initGlobalAPI（vue)中的initAssetRegisters (Vue) 後。變爲

vue.component = function (
      id,
      definition
    ) {
      if (!definition) {
        return this.options[type + 's'][id]
      } else {
        /* istanbul ignore if */
        {
          if (type === 'component' && config.isReservedTag(id)) {
            warn(
              'Do not use built-in or reserved HTML elements as component ' +
              'id: ' + id
            );
          }
        }
        if (type === 'component' && isPlainObject(definition)) {
          definition.name = definition.name || id;
          definition = this.options._base.extend(definition);
        }
        if (type === 'directive' && typeof definition === 'function') {
          definition = { bind: definition, update: definition };
        }
        this.options[type + 's'][id] = definition;／／全局的組件或者指令和過濾器。統一掛在vue.options上。等待init的時候利用策略合併侵入實例。供實例使用
        return definition
      }
    };

this.options._base在initGlobalAPI（vue)中爲Vue.options._base = Vue;
so vue.component調用了vue.extend。找到了源頭。咱們來好好看看這個vue.extend這個function。代碼以下：

Vue.extend = function (extendOptions) {
    extendOptions = extendOptions || {};
    var Super = this;
    var SuperId = Super.cid;
    var cachedCtors = extendOptions._Ctor || (extendOptions._Ctor = {});
    if (cachedCtors[SuperId]) {
      return cachedCtors[SuperId]／／若是組件已經被緩存在extendOptions上則直接取出
    }

    var name = extendOptions.name || Super.options.name;
    {
      if (!/^[a-zA-Z][\w-]*$/.test(name)) {
        warn(
          'Invalid component name: "' + name + '". Component names ' +
          'can only contain alphanumeric characters and the hyphen, ' +
          'and must start with a letter.'／／校驗組件名
        );
      }
    }

    var Sub = function VueComponent (options) {
      this._init(options);
    };
    Sub.prototype = Object.create(Super.prototype);
    Sub.prototype.constructor = Sub;／／將vue上原型的方法掛在Sub.prototype中，Sub的實例同時也繼承了vue.prototype上的全部屬性和方法。
    Sub.cid = cid++;
    Sub.options = mergeOptions(
      Super.options,
      extendOptions／／經過vue的合併策略合併添加項到新的構造器上
    );
    Sub['super'] = Super;緩存父構造器

    // For props and computed properties, we define the proxy getters on
    // the Vue instances at extension time, on the extended prototype. This
    // avoids Object.defineProperty calls for each instance created.
    if (Sub.options.props) {
      initProps$1(Sub);
    }
    if (Sub.options.computed) {   ／／處理props和computed相關響應式配置項
      initComputed$1(Sub);
    }

    // allow further extension/mixin/plugin usage
    Sub.extend = Super.extend;
    Sub.mixin = Super.mixin;
    Sub.use = Super.use;

    // create asset registers, so extended classes
    // can have their private assets too.              ／／在新的構造器上掛上vue的工具方法
    ASSET_TYPES.forEach(function (type) {
      Sub[type] = Super[type];
    });
    // enable recursive self-lookup
    if (name) {
      Sub.options.components[name] = Sub;
    }

    // keep a reference to the super options at extension time.
    // later at instantiation we can check if Super's options have
    // been updated.
    Sub.superOptions = Super.options;
    Sub.extendOptions = extendOptions;
    Sub.sealedOptions = extend({}, Sub.options);

    // cache constructor
    cachedCtors[SuperId] = Sub;//緩存組件構造器在extendOptions上
    return Sub
  };
}

function initProps$1 (Comp) {
  var props = Comp.options.props;
  for (var key in props) {
    proxy(Comp.prototype, "_props", key);
  }
}

function initComputed$1 (Comp) {
  var computed = Comp.options.computed;
  for (var key in computed) {
    defineComputed(Comp.prototype, key, computed[key]);
  }
}

總的來講vue.extend是返回了一個帶有附加配置相的新的vue的構造器。在函數中，構造器叫作Sub，等待render時候初始化。
通過vue.component的調用。vue增長了一個全局組件my-component;此時vue.options.component以下圖：

前三個是vue內置的三個組件，在initgloabalapi的時候初始化。
至此全局組件建立完成。全局組件放置在最底層。在之後的策略合併裏會在子組件中的component項的__proto__中。

---

經過組件的遞歸建立渲染來看vue總體的生命週期（理解vue如何巧妙構建應用）

上圖：

vue官方的生命週期圖，其實也就是vue組件的構成的生命週期。沿着new Vue()咱們來大概看看這些生命週期在什麼階段

Vue.prototype._init = function (options) {
    var vm = this;
    // a uid
    vm._uid = uid$1++;

    var startTag, endTag;
    /* istanbul ignore if */
    if ("development" !== 'production' && config.performance && mark) {
      startTag = "vue-perf-init:" + (vm._uid);
      endTag = "vue-perf-end:" + (vm._uid);
      mark(startTag);
    }

    // a flag to avoid this being observed
    vm._isVue = true;
    // merge options
    if (options && options._isComponent) {
      // optimize internal component instantiation
      // since dynamic options merging is pretty slow, and none of the
      // internal component options needs special treatment.
      initInternalComponent(vm, options);//內部組件調用此快捷方法
    } else {
      vm.$options = mergeOptions(
        resolveConstructorOptions(vm.constructor),//策略合併，每項屬性都有對應的合併規則
        options || {},
        vm
      );
    }
    /* istanbul ignore else */
    {
      initProxy(vm);／／屬性代理，即vm.xx = vm.data.xx
    }
    // expose real self
    vm._self = vm;
    initLifecycle(vm);//初始化生命週期狀態變量，創建子父關係初始值，如$children，$parent.
    initEvents(vm);// 初始化事件
    initRender(vm);//初始化render核心函數_$createElement和$slots $scopedSlots等
    callHook(vm, 'beforeCreate');
    initInjections(vm); // resolve injections before data/props
    initState(vm);//利用數據劫持作響應式
    initProvide(vm); //resolve provide after data/props
    callHook(vm, 'created');

    /* istanbul ignore if */
    if ("development" !== 'production' && config.performance && mark) {
      vm._name = formatComponentName(vm, false);
      mark(endTag);
      measure(((vm._name) + " init"), startTag, endTag);
    }

    if (vm.$options.el) {
      vm.$mount(vm.$options.el);//若是有el配置相則主動掛載。觸發以後的compile.render
    }
  };

介紹了大概的_init函數，咱們繼續往下看程序的運行。完成了vue.component()以後。開始執行new vue()，建立實例。
對照_init函數。咱們知道它分別進行了對傳入參數的合併。初始化實例參數。建立響應的響應式。最後掛載：vm.$mount(vm.$options.el);
簡單說說掛載。好吧。咱們仍是往方法裏面看，掛載的時候發生了什麼：

// public mount method
Vue$3.prototype.$mount = function (
  el,
  hydrating
) {
  el = el && inBrowser ? query(el) : undefined;
  return mountComponent(this, el, hydrating)
};

var mount = Vue$3.prototype.$mount//緩存mount,用來觸發render
Vue$3.prototype.$mount = function (//核心mount用來構建render函數
  el,
  hydrating
) {
  el = el && query(el);

  /* istanbul ignore if */
  if (el === document.body || el === document.documentElement) {
    "development" !== 'production' && warn(
      "Do not mount Vue to <html> or <body> - mount to normal elements instead."／／檢測，排除不可掛載的元素
    );
    return this
  }

  var options = this.$options;
  // resolve template/el and convert to render function
  if (!options.render) {
    var template = options.template;／／假如輸入的是template模版時。
    if (template) {
      if (typeof template === 'string') {
        if (template.charAt(0) === '#') {
          template = idToTemplate(template);
          /* istanbul ignore if */
          if ("development" !== 'production' && !template) {
            warn(
              ("Template element not found or is empty: " + (options.template)),
              this
            );
          }
        }
      } else if (template.nodeType) {
        template = template.innerHTML;//輸入的是dom節點時
      } else {
        {
          warn('invalid template option:' + template, this);
        }
        return this
      }
    } else if (el) {
      template = getOuterHTML(el);／／若是是一個id，如這次初始化掛載的id=app，會取到id=app的html
    }
    if (template) {
      /* istanbul ignore if */
      if ("development" !== 'production' && config.performance && mark) {
        mark('compile');
      }

      var ref = compileToFunctions(template, {
        shouldDecodeNewlines: shouldDecodeNewlines,／／核心compile函數。用於生成render函數。這裏不細說
        delimiters: options.delimiters
      }, this);
      var render = ref.render;
      var staticRenderFns = ref.staticRenderFns;
      options.render = render;／／掛載render到實例options中。待調用
      options.staticRenderFns = staticRenderFns;／／靜態的元素區分開。提高性能，後續虛擬dom樹比較時，不會比較靜態節點

      /* istanbul ignore if */
      if ("development" !== 'production' && config.performance && mark) {
        mark('compile end');
        measure(((this._name) + " compile"), 'compile', 'compile end');
      }
    }
  }
  return mount.call(this, el, hydrating)／／利用緩存的mount調用準備好的render
};

$mount方法的核心其實就是準備好組件的render函數。這裏最核心的一個方法就是：

var ref = compileToFunctions(template, {
        shouldDecodeNewlines: shouldDecodeNewlines,／／核心compile函數。用於生成render函數。這裏不細說
        delimiters: options.delimiters
      }, this);

compileToFunctions這個函數中主要作了兩件事：
1:對模版進行compile（按標籤解析,生成ast（抽象語法樹）
2:利用generate(ast, options),生成render函數語法

咱們來看看最後實例生成的render函數：

沒有錯就是這個樣子，頗有感受。生成的render函數保存在options中，等待調用

好吧。開始調用吧。

mount.call(this, el, hydrating)＝》mountComponent(this, el, hydrating)＝》updateComponent = function () {
      vm._update(vm._render(), hydrating);
    };＝》vm._watcher = new Watcher(vm, updateComponent, noop);

new Watcher中會主動調用updateComponent去touch依賴（給頁面中引用過的data中的變量假如監聽）正式調用render函數。既然都說了。那就來看看render函數：

Vue.prototype._render = function () {
    var vm = this;
    var ref = vm.$options;
    var render = ref.render;
    var staticRenderFns = ref.staticRenderFns;
    var _parentVnode = ref._parentVnode;

    if (vm._isMounted) {
      // clone slot nodes on re-renders
      for (var key in vm.$slots) {
        vm.$slots[key] = cloneVNodes(vm.$slots[key]);
      }
    }

    vm.$scopedSlots = (_parentVnode && _parentVnode.data.scopedSlots) || emptyObject;

    if (staticRenderFns && !vm._staticTrees) {
      vm._staticTrees = [];
    }
    // set parent vnode. this allows render functions to have access
    // to the data on the placeholder node.
    vm.$vnode = _parentVnode;
    // render self
    var vnode;
    try {
      vnode = render.call(vm._renderProxy, vm.$createElement);//核心函數，調用render
    } catch (e) {
      handleError(e, vm, "render function");
      // return error render result,
      // or previous vnode to prevent render error causing blank component
      /* istanbul ignore else */
      {
        vnode = vm.$options.renderError
          ? vm.$options.renderError.call(vm._renderProxy, vm.$createElement, e)
          : vm._vnode;
      }
    }
    // return empty vnode in case the render function errored out
    if (!(vnode instanceof VNode)) {
      if ("development" !== 'production' && Array.isArray(vnode)) {
        warn(
          'Multiple root nodes returned from render function. Render function ' +
          'should return a single root node.',
          vm
        );
      }
      vnode = createEmptyVNode();
    }
    // set parent
    vnode.parent = _parentVnode;
    return vnode
  };

_render()間接調用了vnode = render.call(vm._renderProxy, vm.$createElement);

而後結合render函數。看看發生了什麼。vm.$createElement是核心的建立虛擬dom的函數。

繼續看看核心構建虛擬dom函數：

function createElement (
  context,
  tag,
  data,
  children,//children是該元素下的全部子元素
  normalizationType,
  alwaysNormalize
) {
  if (Array.isArray(data) || isPrimitive(data)) {
    normalizationType = children;
    children = data;
    data = undefined;
  }
  if (isTrue(alwaysNormalize)) {
    normalizationType = ALWAYS_NORMALIZE;
  }
  return _createElement(context, tag, data, children, normalizationType)
}

function _createElement (
  context,
  tag,
  data,
  children,
  normalizationType
) {
  if (isDef(data) && isDef((data).__ob__)) {
    "development" !== 'production' && warn(
      "Avoid using observed data object as vnode data: " + (JSON.stringify(data)) + "\n" +
      'Always create fresh vnode data objects in each render!',
      context
    );
    return createEmptyVNode()
  }
  if (!tag) {
    // in case of component :is set to falsy value
    return createEmptyVNode()
  }
  // support single function children as default scoped slot
  if (Array.isArray(children) &&
      typeof children[0] === 'function') {
    data = data || {};
    data.scopedSlots = { default: children[0] };
    children.length = 0;
  }
  if (normalizationType === ALWAYS_NORMALIZE) {
    children = normalizeChildren(children);
  } else if (normalizationType === SIMPLE_NORMALIZE) {
    children = simpleNormalizeChildren(children);
  }
  var vnode, ns;
  if (typeof tag === 'string') {
    var Ctor;
    ns = config.getTagNamespace(tag);
    if (config.isReservedTag(tag)) {
      // platform built-in elements
      vnode = new VNode(
        config.parsePlatformTagName(tag), data, children,
        undefined, undefined, context
      );
    } else if (isDef(Ctor = resolveAsset(context.$options, 'components', tag))) {假如是組件則從上下文中取出組件的構造相關參數
      // component
      vnode = createComponent(Ctor, data, context, children, tag);
    } else {
      // unknown or unlisted namespaced elements
      // check at runtime because it may get assigned a namespace when its
      // parent normalizes children
      vnode = new VNode(
        tag, data, children,
        undefined, undefined, context
      );
    }
  } else {
    // direct component options / constructor
    vnode = createComponent(tag, data, context, children);
  }
  if (vnode !== undefined) {
    if (ns) { applyNS(vnode, ns); }
    return vnode
  } else {
    return createEmptyVNode()
  }
}

這裏其實咱們不難看出vue在構造虛擬dom時。遞歸的去調用createElement去生成虛擬dom樹。當children是組件或者是普通元素時。作不一樣的處理。這裏咱們關注的是。當子元素是組建時。這裏調用了
vnode = createComponent(tag, data, context, children);
細心的人能夠在去看看這個函數作了什麼。簡單來講這個函數將組件的構造參數取出來，放置在元素的componentOptions上。供後續建立真實dom時。標記該元素是組件。遞歸初始化。
跳過這些沉重的。咱們直接看看咱們的這個html生成的最終的虛擬dom長什麼樣。以下：

咱們在來看看咱們的my-component組件長什麼樣子：

componentOptios上存着初始化組件須要的參數。

構建好虛擬dom後。vue進入update階段：
這個階段vue會判斷先前有無該元素。是否爲第一次渲染。假如是第一次。那麼直接建立。若是不是有先前的ovnode,則比較差別。最小化更新。看看具體函數：

nction lifecycleMixin (Vue) {
  Vue.prototype._update = function (vnode, hydrating) {
    var vm = this;
    if (vm._isMounted) {
      callHook(vm, 'beforeUpdate');
    }
    var prevEl = vm.$el;
    var prevVnode = vm._vnode;//取出緩存的久的虛擬dom
    var prevActiveInstance = activeInstance;
    activeInstance = vm;
    vm._vnode = vnode;//緩存當前vnode，供下次更新使用
    // Vue.prototype.__patch__ is injected in entry points
    // based on the rendering backend used.
    if (!prevVnode) {//假如第一次渲染。直接建立
      // initial render
      vm.$el = vm.__patch__(
        vm.$el, vnode, hydrating, false /* removeOnly */,
        vm.$options._parentElm,
        vm.$options._refElm
      );
    } else {
      // updates
      vm.$el = vm.__patch__(prevVnode, vnode);//更新時。會比較差別
    }
    activeInstance = prevActiveInstance;
    // update __vue__ reference
    if (prevEl) {
      prevEl.__vue__ = null;
    }
    if (vm.$el) {
      vm.$el.__vue__ = vm;
    }
    // if parent is an HOC, update its $el as well
    if (vm.$vnode && vm.$parent && vm.$vnode === vm.$parent._vnode) {
      vm.$parent.$el = vm.$el;
    }
    // updated hook is called by the scheduler to ensure that children are
    // updated in a parent's updated hook.
  };

__patch__函數咱們就不細看了。算了看一下：

return function patch (oldVnode, vnode, hydrating, removeOnly, parentElm, refElm) {
    if (isUndef(vnode)) {
      if (isDef(oldVnode)) { invokeDestroyHook(oldVnode); }
      return
    }

    var isInitialPatch = false;
    var insertedVnodeQueue = [];

    if (isUndef(oldVnode)) {
      // empty mount (likely as component), create new root element
      isInitialPatch = true;//假如第一次渲染。直接建立
      createElm(vnode, insertedVnodeQueue, parentElm, refElm);
    } else {
      var isRealElement = isDef(oldVnode.nodeType);
      if (!isRealElement && sameVnode(oldVnode, vnode)) {／／假如更新而且先後虛擬dom類似，這裏類似有本身的一個算法。好比tag,key必需一致。纔會去diff比較
        // patch existing root node
        patchVnode(oldVnode, vnode, insertedVnodeQueue, removeOnly);
      } else {
        if (isRealElement) {
          // mounting to a real element
          // check if this is server-rendered content and if we can perform
          // a successful hydration.
          if (oldVnode.nodeType === 1 && oldVnode.hasAttribute(SSR_ATTR)) {
            oldVnode.removeAttribute(SSR_ATTR);
            hydrating = true;
          }
          if (isTrue(hydrating)) {
            if (hydrate(oldVnode, vnode, insertedVnodeQueue)) {
              invokeInsertHook(vnode, insertedVnodeQueue, true);
              return oldVnode
            } else {
              warn(
                'The client-side rendered virtual DOM tree is not matching ' +
                'server-rendered content. This is likely caused by incorrect ' +
                'HTML markup, for example nesting block-level elements inside ' +
                '<p>, or missing <tbody>. Bailing hydration and performing ' +
                'full client-side render.'
              );
            }
          }
          // either not server-rendered, or hydration failed.
          // create an empty node and replace it
          oldVnode = emptyNodeAt(oldVnode);
        }
        // replacing existing element
        var oldElm = oldVnode.elm;
        var parentElm$1 = nodeOps.parentNode(oldElm);
        createElm(
          vnode,
          insertedVnodeQueue,
          // extremely rare edge case: do not insert if old element is in a
          // leaving transition. Only happens when combining transition +
          // keep-alive + HOCs. (#4590)
          oldElm._leaveCb ? null : parentElm$1,
          nodeOps.nextSibling(oldElm)
        );

        if (isDef(vnode.parent)) {
          // component root element replaced.
          // update parent placeholder node element, recursively
          var ancestor = vnode.parent;
          while (ancestor) {
            ancestor.elm = vnode.elm;
            ancestor = ancestor.parent;
          }
          if (isPatchable(vnode)) {
            for (var i = 0; i < cbs.create.length; ++i) {
              cbs.create[i](emptyNode, vnode.parent);
            }
          }
        }

        if (isDef(parentElm$1)) {
          removeVnodes(parentElm$1, [oldVnode], 0, 0);
        } else if (isDef(oldVnode.tag)) {
          invokeDestroyHook(oldVnode);
        }
      }
    }

    invokeInsertHook(vnode, insertedVnodeQueue, isInitialPatch);
    return vnode.elm
  }

patch方法中核心的是createElm：看懂這個函數很是重要代碼以下

function createElm (vnode, insertedVnodeQueue, parentElm, refElm, nested) {
    vnode.isRootInsert = !nested; // for transition enter check
    if (createComponent(vnode, insertedVnodeQueue, parentElm, refElm)) {//根據以前保存的componentoptions來識別是否爲組件。如果。則進這個邏輯
      return
    }

    var data = vnode.data;
    var children = vnode.children;
    var tag = vnode.tag;
    if (isDef(tag)) {
      {
        if (data && data.pre) {
          inPre++;
        }
        if (
          !inPre &&
          !vnode.ns &&
          !(config.ignoredElements.length && config.ignoredElements.indexOf(tag) > -1) &&
          config.isUnknownElement(tag)
        ) {
          warn(
            'Unknown custom element: <' + tag + '> - did you ' +
            'register the component correctly? For recursive components, ' +
            'make sure to provide the "name" option.',
            vnode.context
          );
        }
      }
      vnode.elm = vnode.ns
        ? nodeOps.createElementNS(vnode.ns, tag)
        : nodeOps.createElement(tag, vnode);
      setScope(vnode);

      /* istanbul ignore if */
      {
        createChildren(vnode, children, insertedVnodeQueue);
        if (isDef(data)) {
          invokeCreateHooks(vnode, insertedVnodeQueue);
        }
        insert(parentElm, vnode.elm, refElm);
      }

      if ("development" !== 'production' && data && data.pre) {
        inPre--;
      }
    } else if (isTrue(vnode.isComment)) {
      vnode.elm = nodeOps.createComment(vnode.text);
      insert(parentElm, vnode.elm, refElm);
    } else {
      vnode.elm = nodeOps.createTextNode(vnode.text);
      insert(parentElm, vnode.elm, refElm);
    }
  }

咱們這邊仍是先關注本身的組件部分。當children是組件元素時，很顯然調用了createComponent(vnode, insertedVnodeQueue, parentElm, refElm)；

var componentVNodeHooks = {
  init: function init (
    vnode,
    hydrating,
    parentElm,
    refElm
  ) {
    if (!vnode.componentInstance || vnode.componentInstance._isDestroyed) {
      var child = vnode.componentInstance = createComponentInstanceForVnode(
        vnode,
        activeInstance,
        parentElm,//調用了組件內部的_init方法遞歸建立子組件。正式進入子組件的生命週期
        refElm
      );
      child.$mount(hydrating ? vnode.elm : undefined, hydrating);觸發子組件的掛載。出發子組件的編譯和render。又從新來一遍／直到子組件徹底渲染好。再開始creelem下一個child
    } else if (vnode.data.keepAlive) {
      // kept-alive components, treat as a patch
      var mountedNode = vnode; // work around flow
      componentVNodeHooks.prepatch(mountedNode, mountedNode);
    }
  },

這裏就是遞歸建立子組件的核心部分.

總結： 第一次寫這個vue。失敗了。切模塊切的不夠細。組件機制感受用了好多東西。這個面太大了。本身講的時候也不知道該細講仍是。。。

總的來講：vue在建立虛擬dom的時候，若是元素是組件。則準備好組件的構造參數。包括模版和數據等等。組件中的元素如slot，和child放在組件元素的children下。供後面的內容分發用組件中的元素也是在父組件的做用域內編譯的。看—_render（）函數就知道。而後在vue須要將虛擬dom變爲真實dom時。遇到組件元素時。開始遞歸初始化。直到把組件compile,render構建完後。開始構建下一個元素。最後添加到真實id=app上。而且把舊的刪了。哈哈。隨便寫了