如有理解不到位的地方 欢迎指正

前置步骤

render.js,
1: 执行render方法，生成虚拟节点传给了lifeCycle中的update方法

  vnode = render.call(vm._renderProxy, vm.$createElement)

2:在lifecycle.js文件中
prevVnode保存上次保留的vm._vnode,第一次渲染的时候不存在就去调用vm.patch(vm.$el, vnode, hydrating, false /* removeOnly */)初始化，
后续有prevVnode就走vm.patch(prevVnode, vnode)去做更新（做diff）

export function lifecycleMixin (Vue: Class<Component>) {
  Vue.prototype._update = function (vnode: VNode, hydrating?: boolean) {
    const vm: Component = this
    const prevEl = vm.$el
    const prevVnode = vm._vnode // 第一次渲染 prevVnode不存在的
    const restoreActiveInstance = setActiveInstance(vm)
    vm._vnode = vnode // 首次渲染 会把vnode挂载到vm._vnode上
    // Vue.prototype.__patch__ is injected in entry points
    // based on the rendering backend used.
    if (!prevVnode) {
      // initial render  patch方法 做的是初始化渲染逻辑
      vm.$el = vm.__patch__(vm.$el, vnode, hydrating, false /* removeOnly */)
    } else {
      // updates
      vm.$el = vm.__patch__(prevVnode, vnode)
    }
  }
  //.....
}

正式进入比对流程

源码都在目录下： vdom/patch.js

（如果是真实的节点 并把内容塞到老的后边一个，删除老的）patch➡
（是相同根节点进入儿子情况的判断）➡patchVnode➡
（如果两个节点都有儿子）➡updateChildren

1：在createPatchFunction方法中return了一个patch方法

 return function patch (oldVnode, vnode, hydrating, removeOnly) {
    // 如果传入 patch(oldVnode,null); 进行卸载
    if (isUndef(vnode)) {
      if (isDef(oldVnode)) invokeDestroyHook(oldVnode)
      return
    }

    let isInitialPatch = false
    const insertedVnodeQueue = []

    if (isUndef(oldVnode)) { // patch(vnode); // 组件的挂载 只传一个参数
      // empty mount (likely as component), create new root element
      isInitialPatch = true
      createElm(vnode, insertedVnodeQueue)
    } else {
      const isRealElement = isDef(oldVnode.nodeType)
      //不是真实元素并且是相同节点 比较儿子的情况 （insertedVnodeQueue是做生命周期的）
      if (!isRealElement && sameVnode(oldVnode, vnode)) {
        // patch existing root node  核心：两个节点的比较
        patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly)
      } else {
        //如果是真实元素
        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 if (process.env.NODE_ENV !== 'production') {
              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
        const oldElm = oldVnode.elm
        const parentElm = nodeOps.parentNode(oldElm)

        //根据虚拟节点创造新的节点 并把内容塞到老的后边一个
        // 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 ? null : parentElm,
          nodeOps.nextSibling(oldElm)
        )

        //递归更新节点
        // update parent placeholder node element, recursively 
        if (isDef(vnode.parent)) {
          let ancestor = vnode.parent
          const patchable = isPatchable(vnode)
          while (ancestor) {
            for (let i = 0; i < cbs.destroy.length; ++i) {
              cbs.destroy[i](ancestor)
            }
            ancestor.elm = vnode.elm
            if (patchable) {
              for (let i = 0; i < cbs.create.length; ++i) {
                cbs.create[i](emptyNode, ancestor)
              }
              // #6513
              // invoke insert hooks that may have been merged by create hooks.
              // e.g. for directives that uses the "inserted" hook.
              const insert = ancestor.data.hook.insert
              if (insert.merged) {
                // start at index 1 to avoid re-invoking component mounted hook
                for (let i = 1; i < insert.fns.length; i++) {
                  insert.fns[i]()
                }
              }
            } else {
              registerRef(ancestor)
            }
            ancestor = ancestor.parent
          }
        }

        // destroy old node
        if (isDef(parentElm)) {
          //删除老的节点
          removeVnodes([oldVnode], 0, 0)
        } else if (isDef(oldVnode.tag)) {
          invokeDestroyHook(oldVnode)
        }
      }
    }

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

• 2 核心在于新老的节点都有儿子，进行儿子的比较 updateChildren

 function patchVnode ( 
    oldVnode,
    vnode,
    insertedVnodeQueue,
    ownerArray,
    index,
    removeOnly
  ) {
    if (oldVnode === vnode) { // 新的和老的一样 就不用比对了 基本上不会出现
      return
    }

    if (isDef(vnode.elm) && isDef(ownerArray)) {
      // clone reused vnode 虚拟节点copy 使用
      vnode = ownerArray[index] = cloneVNode(vnode)
    }

    const elm = vnode.elm = oldVnode.elm // 两个节点一样 节点的拷贝

    if (isTrue(oldVnode.isAsyncPlaceholder)) {//是不是异步组件占位符
      if (isDef(vnode.asyncFactory.resolved)) {
        hydrate(oldVnode.elm, vnode, insertedVnodeQueue)
      } else {
        vnode.isAsyncPlaceholder = true
      }
      return
    }

    // reuse element for static trees.
    // note we only do this if the vnode is cloned -
    // if the new node is not cloned it means the render functions have been
    // reset by the hot-reload-api and we need to do a proper re-render.
    if (isTrue(vnode.isStatic) &&
      isTrue(oldVnode.isStatic) &&
      vnode.key === oldVnode.key &&
      (isTrue(vnode.isCloned) || isTrue(vnode.isOnce))
    ) {
      vnode.componentInstance = oldVnode.componentInstance
      return
    }

    let i
    const data = vnode.data
    //组件的更新流程
    if (isDef(data) && isDef(i = data.hook) && isDef(i = i.prepatch)) {
      i(oldVnode, vnode)
    }

    //复用完节点
    const oldCh = oldVnode.children // 老儿子
    const ch = vnode.children // 新的儿子 
    if (isDef(data) && isPatchable(vnode)) { 
      // 调用更新逻辑 包含 当前样式更新， 指令的更新 
      // cb以此调用比对方法，更新逻辑在运行时platform/modules下边的属性类等的比对方法
      for (i = 0; i < cbs.update.length; ++i) cbs.update[i](oldVnode, vnode)
      if (isDef(i = data.hook) && isDef(i = i.update)) i(oldVnode, vnode)
    }
    if (isUndef(vnode.text)) { // 如果不是文本 并且两个人都有内容
      if (isDef(oldCh) && isDef(ch)) { // 两个儿子的比对
        if (oldCh !== ch) updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly)
      } else if (isDef(ch)) { // 新的有儿子 那么老的就没儿子
        if (process.env.NODE_ENV !== 'production') {
          checkDuplicateKeys(ch)
        }
        // 将文本清空 添加新儿子
        if (isDef(oldVnode.text)) nodeOps.setTextContent(elm, '') 
        // 添加儿子
        addVnodes(elm, null, ch, 0, ch.length - 1, insertedVnodeQueue)
      } else if (isDef(oldCh)) { // 如果老的有儿子 ，新的没儿子
        removeVnodes(oldCh, 0, oldCh.length - 1) // 删除老儿子
      } else if (isDef(oldVnode.text)) { //老的是文本清空
        nodeOps.setTextContent(elm, '')
      }
    } else if (oldVnode.text !== vnode.text) {
      nodeOps.setTextContent(elm, vnode.text) // 更新文本内容
    }
    if (isDef(data)) { // 如果是组件 会去调用组件的postpatch 方法
      if (isDef(i = data.hook) && isDef(i = i.postpatch)) i(oldVnode, vnode)
    }
  }

• 3 diff双指针比较
  首先是特殊情况头头比对；尾尾对比；头尾对比：并且把相同的节点移动到右边；尾头对比：并且把相同的移动到左边
  最后乱序比对，对比到相同的移动，并且原来的位置重置为undefined(否则会造成数组塌陷)

//核心 双指针比较
  function updateChildren (parentElm, oldCh, newCh, insertedVnodeQueue, removeOnly) {
    let oldStartIdx = 0
    let newStartIdx = 0
    let oldEndIdx = oldCh.length - 1
    let oldStartVnode = oldCh[0]
    let oldEndVnode = oldCh[oldEndIdx]
    let newEndIdx = newCh.length - 1
    let newStartVnode = newCh[0]
    let newEndVnode = newCh[newEndIdx]
    let oldKeyToIdx, idxInOld, vnodeToMove, refElm

    // removeOnly is a special flag used only by <transition-group>
    // to ensure removed elements stay in correct relative positions
    // during leaving transitions
    const canMove = !removeOnly

    if (process.env.NODE_ENV !== 'production') {
      checkDuplicateKeys(newCh)
    }

    while (oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx) {
      if (isUndef(oldStartVnode)) {
        oldStartVnode = oldCh[++oldStartIdx] // Vnode has been moved left
      } else if (isUndef(oldEndVnode)) {
        oldEndVnode = oldCh[--oldEndIdx]
      } else if (sameVnode(oldStartVnode, newStartVnode)) { // 头头比对
        patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
        oldStartVnode = oldCh[++oldStartIdx]
        newStartVnode = newCh[++newStartIdx]
      } else if (sameVnode(oldEndVnode, newEndVnode)) { // 尾尾比对
        patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx)
        oldEndVnode = oldCh[--oldEndIdx]
        newEndVnode = newCh[--newEndIdx]
        // ABCD
        // BCDA
      } else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved right  头尾比对
        patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx)
        canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm))
        oldStartVnode = oldCh[++oldStartIdx]
        newEndVnode = newCh[--newEndIdx]
        // ABCD
        // DABC
      } 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]
      } else {
        // 乱序比对
        // 先创建索引
        if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)
        idxInOld = isDef(newStartVnode.key)
          ? oldKeyToIdx[newStartVnode.key]
          : findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)
        if (isUndef(idxInOld)) { // New element 新创建塞入
          createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)
        } else {
          vnodeToMove = oldCh[idxInOld]
          //判断是不是相同节点
          if (sameVnode(vnodeToMove, newStartVnode)) {
            //相同 比对 并且移动和插入 置空
            patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
            oldCh[idxInOld] = undefined // 做一个占位符
            canMove && nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm)
          } else {
            // 如果key一样 标签不一样也是一个新元素
            // same key but different element. treat as new element
            createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)
          }
        }
        //不同移动新的节点
        newStartVnode = newCh[++newStartIdx] // 不停的移动新的节点
      }
    }
    //
    if (oldStartIdx > oldEndIdx) {
      refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx + 1].elm
      addVnodes(parentElm, refElm, newCh, newStartIdx, newEndIdx, insertedVnodeQueue)
    } else if (newStartIdx > newEndIdx) {
      //老的多
      removeVnodes(oldCh, oldStartIdx, oldEndIdx)
    }
  }