Vue内部渲染视图的方法

var VNode = function VNode (
 tag,
 data,
 children,
 text,
 elm,
 context,
 componentOptions,
 asyncFactory
 ) {
 this.tag = tag;
 this.data = data;
 this.children = children;
 this.text = text;
 this.elm = elm;
 this.ns = undefined;
 this.context = context;
 this.fnContext = undefined;
 this.fnOptions = undefined;
 this.fnScopeId = undefined;
 this.key = data && data.key;
 this.componentOptions = componentOptions;
 this.componentInstance = undefined;
 this.parent = undefined;
 this.raw = false;
 this.isStatic = false;
 this.isRootInsert = true;
 this.isComment = false;
 this.isCloned = false;
 this.isOnce = false;
 this.asyncFactory = asyncFactory;
 this.asyncMeta = undefined;
 this.isAsyncPlaceholder = false;
 };

var createEmptyVNode = function (text) {
 if ( text === void 0 ) text = '';
 var node = new VNode();
 node.text = text;
 node.isComment = true;
 return node
 };

function createTextVNode (val) {
 return new VNode(undefined, undefined, undefined, String(val))
 }

function cloneVNode (vnode) {
 var cloned = new VNode(
  vnode.tag,
  vnode.data,
  // #7975
  // clone children array to avoid mutating original in case of cloning
  // a child.
  vnode.children && vnode.children.slice(),
  vnode.text,
  vnode.elm,
  vnode.context,
  vnode.componentOptions,
  vnode.asyncFactory
 );
 cloned.ns = vnode.ns;
 cloned.isStatic = vnode.isStatic;
 cloned.key = vnode.key;
 cloned.isComment = vnode.isComment;
 cloned.fnContext = vnode.fnContext;
 cloned.fnOptions = vnode.fnOptions;
 cloned.fnScopeId = vnode.fnScopeId;
 cloned.asyncMeta = vnode.asyncMeta;
 cloned.isCloned = true;
 return cloned
 }

{
 children: [VNode, VNode],
 context: {...},
 tag: 'div',
 data: {attr: {id: app}}
}

{ Ctor: Ctor, propsData: propsData, listeners: listeners, tag: tag, children: children }

new VNode(
  ("vue-component-" + (Ctor.cid) + (name "-" + name) : '')),
  data, undefined, undefined, undefined, context,
  { Ctor: Ctor, propsData: propsData, listeners: listeners, tag: tag, children: children },
  asyncFactory
 )

{
 componentOptions: {},
 componentInstance: {},
 tag: 'vue-component-1-child',
 data: {...},
 ...
}

vm._update(vm._render(), hydrating);

Vue.prototype._update = function (vnode, hydrating) {
  var vm = this;
  var prevEl = vm.$el;
  var prevVnode = vm._vnode;
  var restoreActiveInstance = setActiveInstance(vm);
  // 缓存vnode
  vm._vnode = vnode;
  // Vue.prototype.__patch__ is injected in entry points
  // based on the rendering backend used.
  // 第一次渲染，preVnode是不存在的
  if (!prevVnode) {
  // initial render
  vm.$el = vm.__patch__(vm.$el, vnode, hydrating, false /* removeOnly */);
  } else {
  // updates
  vm.$el = vm.__patch__(prevVnode, vnode);
  }
  restoreActiveInstance();
  // 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.
 };

if (isUndef(oldVnode)) {
 // empty mount (likely as component), create new root element
 isInitialPatch = true;
 createElm(vnode, insertedVnodeQueue);
}

if (isRealElement) {
 ...
 // either not server-rendered, or hydration failed.
 // create an empty node and replace it
 oldVnode = emptyNodeAt(oldVnode);
}

{
 asyncFactory: undefined,
 asyncMeta: undefined,
 children: [],
 componentInstance: undefined,
 componentOptions: undefined,
 context: undefined,
 data: {},
 elm: div#app,
 fnContext: undefined,
 fnOptions: undefined,
 fnScopeId: undefined,
 isAsyncPlaceholder: false,
 isCloned: false,
 isComment: false,
 isOnce: false,
 isRootInsert: true,
 isStatic: false,
 key: undefined,
 ns: undefined,
 parent: undefined,
 raw: false,
 tag: "div",
 text: undefined,
 child: undefined
}

// replacing existing element
var oldElm = oldVnode.elm;
var parentElm = nodeOps.parentNode(oldElm);

// create new node
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 "htmlcode">

// 标记是否是根节点
 vnode.isRootInsert = !nested; // for transition enter check
 // 这个函数如果vnode有componentInstance属性，会创建子组件，后续具体介绍，否则不做处理
if (createComponent(vnode, insertedVnodeQueue, parentElm, refElm)) {
 return
}


接着在对子节点处理


var data = vnode.data;
 var children = vnode.children;
 var tag = vnode.tag;
 if (isDef(tag)) {
 ...
 vnode.elm = vnode.ns
  "htmlcode">

function createChildren (vnode, children, insertedVnodeQueue) {
 if (Array.isArray(children)) {
  for (var i = 0; i < children.length; ++i) {
   createElm(children[i], insertedVnodeQueue, vnode.elm, null, true, children, i);
  }
 } else if (isPrimitive(vnode.text)) {
  // 如果vnode是文本直接挂载
  nodeOps.appendChild(vnode.elm, nodeOps.createTextNode(String(vnode.text)));
 }
}


遍历children，递归createElm方法创建子元素节点


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);
}


如果是评论节点，直接创建评论节点，并将其插入到父节点上，其他的创建文本节点，并将其插入到父节点parentElm(刚创建的div)上去。 触发钩子，更新节点属性，将其插入到parentElm('#app'元素节点)上


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


最后将老的节点删掉


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




function removeAndInvokeRemoveHook (vnode, rm) {
 if (isDef(rm) || isDef(vnode.data)) {
  var i;
  var listeners = cbs.remove.length + 1;
  ...
  // recursively invoke hooks on child component root node
  if (isDef(i = vnode.componentInstance) && isDef(i = i._vnode) && isDef(i.data)) {
   removeAndInvokeRemoveHook(i, rm);
  }
  for (i = 0; i < cbs.remove.length; ++i) {
   cbs.remove[i](vnode, rm);
  }
  if (isDef(i = vnode.data.hook) && isDef(i = i.remove)) {
   i(vnode, rm);
  } else {
   // 删除id为app的老节点
   rm();
  }
 } else {
  removeNode(vnode.elm);
 }
}


初次渲染结束。
更新节点过程
为了更好地测试，模板选用


<div id="app">{{ message }}<button @click="update">更新</button></div>


点击按钮，会更新message，重新渲染视图，生成的VNode为


{
 asyncFactory: undefined,
 asyncMeta: undefined,
 children: [VNode, VNode],
 componentInstance: undefined,
 componentOptions: undefined,
 context: Vue实例,
 data: {attrs: {id: "app"}},
 elm: undefined,
 fnContext: undefined,
 fnOptions: undefined,
 fnScopeId: undefined,
 isAsyncPlaceholder: false,
 isCloned: false,
 isComment: false,
 isOnce: false,
 isRootInsert: true,
 isStatic: false,
 key: undefined,
 ns: undefined,
 parent: undefined,
 raw: false,
 tag: "div",
 text: undefined,
 child: undefined
}


在组件更新的时候,preVnode和vnode都是存在的,执行


vm.$el = vm.__patch__(prevVnode, vnode);


实际上是运行以下函数


patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly);


该函数首先判断oldVnode和vnode是否相等，相等则立即返回


if (oldVnode === vnode) {
 return
}


如果两者均为静态节点且key值相等，且vnode是被克隆或者具有isOnce属性时，vnode的组件实例componentInstance直接赋值


if (isTrue(vnode.isStatic) &&
 isTrue(oldVnode.isStatic) &&
 vnode.key === oldVnode.key &&
 (isTrue(vnode.isCloned) || isTrue(vnode.isOnce))
) {
 vnode.componentInstance = oldVnode.componentInstance;
 return
}


接着对两者的属性值作对比，并更新


var oldCh = oldVnode.children;
var ch = vnode.children;
if (isDef(data) && isPatchable(vnode)) {
 for (i = 0; i < cbs.update.length; ++i) {  // 以vnode为准更新oldVnode的不同属性
  cbs.update[i](oldVnode, vnode); 
 }
 if (isDef(i = data.hook) && isDef(i = i.update)) { 
  i(oldVnode, vnode); 
 }
}


vnode和oldVnode的对比以及相应的DOM操作具体如下:


// vnode不存在text属性的情况
if (isUndef(vnode.text)) {
 if (isDef(oldCh) && isDef(ch)) {
 // 子节点不相等时，更新
 if (oldCh !== ch) { 
  updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly); }
 } else if (isDef(ch)) {
  {
  checkDuplicateKeys(ch);
  }
  // 只存在vnode的子节点，如果oldVnode存在text属性，则将元素的文本内容清空，并新增elm节点
  if (isDef(oldVnode.text)) {    nodeOps.setTextContent(elm, ''); 
  }
  addVnodes(elm, null, ch, 0, ch.length - 1, insertedVnodeQueue);
 } else if (isDef(oldCh)) {
  // 如果只存在oldVnode的子节点，则删除DOM的子节点
  removeVnodes(elm, oldCh, 0, oldCh.length - 1);
 } else if (isDef(oldVnode.text)) {
  // 只存在oldVnode有text属性，将元素的文本清空
  nodeOps.setTextContent(elm, '');
 }
} else if (oldVnode.text !== vnode.text) {
 // node和oldVnode的text属性都存在且不一致时，元素节点内容设置为vnode.text
 nodeOps.setTextContent(elm, vnode.text);
}


对于子节点的对比，先分别定义oldVnode和vnode两数组的前后两个指针索引


var oldStartIdx = 0;
var newStartIdx = 0;
var oldEndIdx = oldCh.length - 1;
var oldStartVnode = oldCh[0];
var oldEndVnode = oldCh[oldEndIdx];
var newEndIdx = newCh.length - 1;
var newStartVnode = newCh[0];
var newEndVnode = newCh[newEndIdx];
var oldKeyToIdx, idxInOld, vnodeToMove, refElm;


如下图:

接下来是一个while循环，在这过程中，oldStartIdx、newStartIdx、oldEndIdx 以及 newEndIdx 会逐渐向中间靠拢


while (oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx) 


当oldStartVnode或者oldEndVnode为空时，两中间移动


if (isUndef(oldStartVnode)) {
 oldStartVnode = oldCh[++oldStartIdx]; // Vnode has been moved left
} else if (isUndef(oldEndVnode)) {
 oldEndVnode = oldCh[--oldEndIdx];
} 


接下来这一块，是将 oldStartIdx、newStartIdx、oldEndIdx 以及 newEndIdx 两两比对的过程，共四种:


else if (sameVnode(oldStartVnode, newStartVnode)) {
 patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx);
 oldStartVnode = oldCh[++oldStartIdx];
 newStartVnode = newCh[++newStartIdx];
} else if (sameVnode(oldEndVnode, newEndVnode)) {
 patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx);
 oldEndVnode = oldCh[--oldEndIdx];
 newEndVnode = newCh[--newEndIdx];
} else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved right
 patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx);
 canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm));
 oldStartVnode = oldCh[++oldStartIdx];
 newEndVnode = newCh[--newEndIdx];
} else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved left
 patchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx);
 canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm);
 oldEndVnode = oldCh[--oldEndIdx];
 newStartVnode = newCh[++newStartIdx];
}


第一种: 前前相等比较

如果相等，则oldStartVnode.elm和newStartVnode.elm均向后移一位，继续比较。 第二种: 后后相等比较

如果相等，则oldEndVnode.elm和newEndVnode.elm均向前移一位，继续比较。 第三种: 前后相等比较

将oldStartVnode.elm节点直接移动到oldEndVnode.elm节点后面，然后将oldStartIdx向后移一位，newEndIdx向前移动一位。 第四种: 后前相等比较

将oldEndVnode.elm节点直接移动到oldStartVnode.elm节点后面，然后将oldEndIdx向前移一位，newStartIdx向后移动一位。 如果以上均不满足，则


else {
 if (isUndef(oldKeyToIdx)) { 
   oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx); 
 }
 idxInOld = isDef(newStartVnode.key)
  "htmlcode">

createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx);


插入到oldStartVnode.elm节点前面，否则，如果找到了节点，并符合sameVnode，将两个节点patchVnode，并将该位置的老节点置为undefined，同时将vnodeToMove.elm移到oldStartVnode.elm的前面，以及newStartIdx往后移一位，示意图如下:
 
如果不符合sameVnode，只能创建一个新节点插入到 parentElm 的子节点中，newStartIdx 往后移动一位。 最后如果，oldStartIdx > oldEndIdx，说明老节点比对完了，但是新节点还有多的，需要将新节点插入到真实 DOM 中去，调用 addVnodes 将这些节点插入即可；如果满足 newStartIdx > newEndIdx 条件，说明新节点比对完了，老节点还有多，将这些无用的老节点通过 removeVnodes 批量删除即可。到这里这个过程基本结束。
总结
以上所述是小编给大家介绍的Vue内部渲染视图的方法,希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对网站的支持！

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