前端 | Vue2 diff 流程



在 mount 阶段， update 操作会将新老虚拟节点先对比，整合成一样后，再通过 createElm 生成真实 dom

判断旧节点类型是否为真的 dom 元素节点，
1.没有新虚拟节点，直接触发旧节点的 destroy 钩子
2.没有旧虚拟节点，说明是页面刚开始初始化，直接新建 createElm
3.新旧虚拟节点为同一节点，再进行比对 patchVnode
4.新旧虚拟节点不一样时，则直接创建新节点，删除旧节点

patch

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export function createPatchFunction (backend) {
  // ...

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

    let isInitialPatch = false // 初始打包
    const insertedVnodeQueue = []

    if (isUndef(oldVnode)) { // 旧节点未被定义
      // empty mount (likely as component), create new root element
      isInitialPatch = true
      createElm(vnode, insertedVnodeQueue)
    } else { // 旧节点已被定义
      const isRealElement = isDef(oldVnode.nodeType)
      // 判断旧节点类型是否为真的dom元素节点，且新老节点为同一节点，直接进行比对 patchVnode
      if (!isRealElement && sameVnode(oldVnode, vnode)) {
        // patch existing root node
        patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly)
      } else {
        if (isRealElement) { // 如为真实节点，将真实节点转化成虚拟节点
          // 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) // 将真是节点转化为空的虚拟节点
        }

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

        // create new node
        // 创建新的节点，通过传入的虚拟节点创建真实的 DOM 并插入到它的父节点中
        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)) {
          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)) {
          removeVnodes([oldVnode], 0, 0)
        } else if (isDef(oldVnode.tag)) {
          invokeDestroyHook(oldVnode)
        }
      }
    }

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

patchVode

检查新旧虚拟节点是否相等，若不相等说明有更新

先检测新旧节点是否为静态节点，并有相同的 key；

如果 vnode 不是文本节点或者注释节点，并且都有子节点：
1.新旧不一致则调用 updateChildren 进行更新操作；
2.如果只有新节点有子节点，则给老的 dom 添加 ch 子节点；
3.如果新节点没有子节点，而旧节点有子节点，则直接删除 oldCh；
4.如果老节点是文本节点，则直接清空 elm

如果新老虚拟节点都是文本节点或注释节点，且两个不一样，则给 elm 设置当前新节点的文本

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// patchVode

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);
  }

  // 使 vnode.elm 引用到现在的真实 dom，当 elm 修改时，vnode.elm 也会同步变化
  const 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;
  }

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

  const oldCh = oldVnode.children;
  const 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)) {
      if (process.env.NODE_ENV !== "production") {
        checkDuplicateKeys(ch);
      }
      if (isDef(oldVnode.text)) nodeOps.setTextContent(elm, "");
      addVnodes(elm, null, ch, 0, ch.length - 1, insertedVnodeQueue);
    } else if (isDef(oldCh)) {
      removeVnodes(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);
  }
}

updateChildren

对新旧虚拟节点的子节点数组进行对比遍历

整个 while 循环在不断移动两列数组的指针：

  • 当新老 vnode 节点的 start 相同，直接 patchVnode 处理两节点，索引递增
  • 当新老 vnode 节点的 end 相同，直接 patchVnode，索引递减
  • 当 oldStartVnode 和 newEndVnode 相同时，进入 patchVnode 处理，再将真实 dom 节点 oldStartVnode.elm 移动到 oldEndVnode.elm 之后（以新的为标准移动旧的），再将老节点索引递增，新节点索引递减
  • 当 oldEndVnode 和 newStartVnode 相同时，进入 patchVnode 处理，再将 oldEndVnode.elm 移动到 oldStartVnode.elm 之前，老节点索引增减，新节点索引递增
  • 当不满足以上四种情形，说明没有相同的节点可以复用，则会从 oldChildren 中寻找和 newStartVnode 具有相同 key 的 Vnode，如果没有找到那就直接新建节点 createElm；如果找到了将该节点与 newStartVnode 对比看是否为同一节点，如果是则进行 patchVnode，再将新找的 Vnode 移动到 oldStartVnode 前面，同时老节点索引递减，新节点索引递增；如果不是同一节点，直接创建一个新的 dom 节点放在 newStartIdx 
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// updateChildren

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)) {
      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(oldCh, oldStartIdx, oldEndIdx);
  }
}

sameVnode

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function sameVnode(a, b) {
  return (
    a.key === b.key &&
    ((a.tag === b.tag &&
      a.isComment === b.isComment &&
      isDef(a.data) === isDef(b.data) &&
      sameInputType(a, b)) ||
      (isTrue(a.isAsyncPlaceholder) &&
        a.asyncFactory === b.asyncFactory &&
        isUndef(b.asyncFactory.error)))
  );
}

参考

https://vue3js.cn/interview/vue/diff.html