vue-patch

Virtual Dom

vue2.0从引入了Virtual dom的概念作为优化dom操作的手段, 下面我们就详细介绍这玩意儿。
中所周知， VNode是virtual dom的主要角色， 先看看VNode的组成

VNode

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class VNode {
  tag: string | void;
  data: VNodeData | void;
  children: ?Array<VNode>;
  text: string | void;
  elm: Node | void;
  ns: string | void;
  context: Component | void; // rendered in this component's scope
  key: string | number | void;
  componentOptions: VNodeComponentOptions | void;
  componentInstance: Component | void; // component instance
  parent: VNode | void; // component placeholder node

  // strictly internal
  raw: boolean; // contains raw HTML? (server only)
  isStatic: boolean; // hoisted static node
  isRootInsert: boolean; // necessary for enter transition check
  isComment: boolean; // empty comment placeholder?
  isCloned: boolean; // is a cloned node?
  isOnce: boolean; // is a v-once node?
  asyncFactory: Function | void; // async component factory function
  asyncMeta: Object | void;
  isAsyncPlaceholder: boolean;
  ssrContext: Object | void;
  fnContext: Component | void; // real context vm for functional nodes
  fnOptions: ?ComponentOptions; // for SSR caching
  devtoolsMeta: ?Object; // used to store functional render context for devtools
  fnScopeId: ?string; // functional scope id support

  constructor (
    tag?: string,
    data?: VNodeData,
    children?: ?Array<VNode>,
    text?: string,
    elm?: Node,
    context?: Component,
    componentOptions?: VNodeComponentOptions,
    asyncFactory?: Function
  ) {
    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
  }

  // DEPRECATED: alias for componentInstance for backwards compat.
  /* istanbul ignore next */
  get child (): Component | void {
    return this.componentInstance
  }
}

以上是VNode的结构 这比真实dom少了很多属性 所以在通过比对VDom的变化在映射到真实dom的过程中会节省很多性能开销

Patch之前

本章主要讲vue在数据更新时是怎样及时的通过对比新旧virtual dom映射到真实dom的~
从源码来看， 有三个类极为重要 Observer类、Dep类和Watcher类
Observer实例负责给每个需要watch的对象添加getter和setter
Watcher实例则负责在数据变动的时候去收集依赖、触发patch等操作
而Dep实例则是observer和watch的桥梁 在数据变动的时候通知对应watcher执行更新操作

那么在数据更新的过程中都发生了什么？

observer.get -> dep.notify -> watcher.update -> flushSchedulerQueue ->
watcher.run -> watch.get -> updateComponent -> _render -> _update -> patch
-> patchVnode -> updateChildren

本章主要从patch的过程开始讲

Patch

diff

patch的代码

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function patch (oldVnode, vnode, hydrating, removeOnly) {
  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;
    createElm(vnode, insertedVnodeQueue);
  } else {
    var isRealElement = isDef(oldVnode.nodeType);
    if (!isRealElement && sameVnode(oldVnode, vnode)) {
      // patch existing root node
      patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, 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 = 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 ? null : parentElm,
        nodeOps.nextSibling(oldElm)
      );

      // update parent placeholder node element, recursively
      if (isDef(vnode.parent)) {
        var ancestor = vnode.parent;
        var patchable = isPatchable(vnode);
        while (ancestor) {
          for (var i = 0; i < cbs.destroy.length; ++i) {
            cbs.destroy[i](ancestor);
          }
          ancestor.elm = vnode.elm;
          if (patchable) {
            for (var i$1 = 0; i$1 < cbs.create.length; ++i$1) {
              cbs.create[i$1](emptyNode, ancestor);
            }
            // #6513
            // invoke insert hooks that may have been merged by create hooks.
            // e.g. for directives that uses the "inserted" hook.
            var insert = ancestor.data.hook.insert;
            if (insert.merged) {
              // start at index 1 to avoid re-invoking component mounted hook
              for (var i$2 = 1; i$2 < insert.fns.length; i$2++) {
                insert.fns[i$2]();
              }
            }
          } else {
            registerRef(ancestor);
          }
          ancestor = ancestor.parent;
        }
      }

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

  invokeInsertHook(vnode, insertedVnodeQueue, isInitialPatch);
  return vnode.elm
}
  1. 如果vnode不存在儿oldVnode存在 销毁oldVnode
  2. 如果oldVnode不存在而vnode存在 则调用createElm创建新的节点
  3. 如果oldVnode存在
    1. 如果oldVnode不是真实节点并且oldVnode和vnode是相同节点 则调用patchVnode进行比较
    2. 如果oldVnode是真实节点(说明是组件初始化的过程), 调用createElm创建新节点 并调用removeVnodes将oldVnode对应的老节点移除

patchVnode

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function patchVnode (
  oldVnode,
  vnode,
  insertedVnodeQueue,
  ownerArray,
  index,
  removeOnly
) {
  if (oldVnode === vnode) {
    return
  }
  if (isDef(vnode.elm) && isDef(ownerArray)) {
    // clone reused vnode
    vnode = ownerArray[index] = cloneVNode(vnode);
  }

  var elm = vnode.elm = oldVnode.elm;

  if (isTrue(oldVnode.isAsyncPlaceholder)) {
    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
  }

  var i;
  var data = vnode.data;
  if (isDef(data) && isDef(i = data.hook) && isDef(i = i.prepatch)) {
    i(oldVnode, vnode);
  }

  var oldCh = oldVnode.children;
  var ch = vnode.children;
  if (isDef(data) && isPatchable(vnode)) {
    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)) {
    if (isDef(oldCh) && isDef(ch)) {
      if (oldCh !== ch) { updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly); }
    } else if (isDef(ch)) {
      {
        checkDuplicateKeys(ch);
      }
      if (isDef(oldVnode.text)) { nodeOps.setTextContent(elm, ''); }
      addVnodes(elm, null, ch, 0, ch.length - 1, insertedVnodeQueue);
    } else if (isDef(oldCh)) {
      removeVnodes(elm, oldCh, 0, oldCh.length - 1);
    } else if (isDef(oldVnode.text)) {
      nodeOps.setTextContent(elm, '');
    }
  } else if (oldVnode.text !== vnode.text) {
    nodeOps.setTextContent(elm, vnode.text);
  }
  if (isDef(data)) {
    if (isDef(i = data.hook) && isDef(i = i.postpatch)) { i(oldVnode, vnode); }
  }
}
  1. 如果oldVnode完全等于vnode return
  2. 如果oldVnode和vnode都是静态节点，且vnode的key等于oldVnode的key，且vnode是克隆节点或者v-once控制的节点的时候 只需要把
    oldVnode的componentInstance复制给vnode即可
  3. 如果vnode不是文本节点
    1. 如果oldVnode存在children且vnode存在children 且两个children 不完全相等 执行updateChildren
    2. 如果只有vnode存在children子节点， 调用addVnodes添加子节点
    3. 如果只有oldVnode存在子节点， 调用removeVnodes移除这些子节点
    4. 如果vnode和oldVnode都没有子节点，但是oldVnode是文本节点 则把oldVnode对应的真实dom的文本内容清空
  4. 如果vnode是文本节点且文本内容和oldnode的文本内容不一样， 则把vnode的文本内容赋值给oldVnode对应dom节点的文本

updateChildren

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function updateChildren (parentElm, oldCh, newCh, insertedVnodeQueue, removeOnly) {
  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;

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

  {
    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)) {
      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];
    } else {
      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
        createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx);
      } else {
        vnodeToMove = oldCh[idxInOld];
        if (sameVnode(vnodeToMove, newStartVnode)) {
          patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue, newCh, newStartIdx);
          oldCh[idxInOld] = undefined;
          canMove && nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm);
        } else {
          // same key but different element. treat as new element
          createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx);
        }
      }
      newStartVnode = newCh[++newStartIdx];
    }
  }
  if (oldStartIdx > oldEndIdx) {
    refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx + 1].elm;
    addVnodes(parentElm, refElm, newCh, newStartIdx, newEndIdx, insertedVnodeQueue);
  } else if (newStartIdx > newEndIdx) {
    removeVnodes(parentElm, oldCh, oldStartIdx, oldEndIdx);
  }
}

updateChildren方法主要通过while循环去对比新旧两棵树的子节点来更新dom

  1. 如果oldStartVnode不存在 则将oldStartVnode设置为下一个子节点 oldStartIdx也指向这个子节点
  2. 如果oldEndVnode不存在 oldEndVnode指向上一个子节点 oldEndIdx也指向这个子节点
  3. 如果oldStartVnode和newStartVnode是同一个子节点 调用patchVnode 进行比较 并且将oldStartVnode和newStartVnode设置为下一个子节点
  4. 如果oldEndVnode和newEndVnode是同一个节点， 调用patchVnode进行比对， 同时将他们设置为上一个节点
  5. 如果oldStartVnode和newEndVnode是同一个子节点， 调用patchVnode进行比对 将这个节点移到oldEndVnode下一个兄弟节点的前面 并将oldStartVnode指向下一个 newEndVnode指向下一个
  6. 如果oldEndVnode和newStartVnode是同一个子节点 调用patchVnode进行比对 将这个节点移到oldStartVnode对应节点的前面 并将oldEndVnode指向前一个节点 newStartVnode指向下一个节点
  7. 如果以上都不成立 根据newStartVnode的key值在oldVnodeChildren中进行查找
    1. 如果oldVnodeChildren中存在key值一样
    2. 如果oldVnodeChildren中找到的节点和newStartVnode是相同节点，调用patchVnode的比对并将目标节点移至oldStartVnode的前面
    3. 如果不是相同节点 则调用createElm创建新的节点
    4. 如果找不到和newStartVnode的key值一样的节点 则调用createElm创建新的节点

跳出while循环后 还要针对oldStartIdx和oldEndIdx进行操作

  1. 如果oldStartIdx 大于 oldEndIdx 说明newVnodesChildren没有遍历完 执行addVnodes
  2. 如果newStartIdx > newEndIdx 说明oldVnodeChildren没有遍历完 执行removeVnodes

以上就是整个patch过程， 尤其是updateChildren的逻辑还是很精妙的 通过不断缩减两条边界线去判断有无相同节点 不管是空间复杂度还是时间复杂度都减少了很多