vue2从数据变化到视图变化之diff算法图文详解

目录

• 引言
• 1、isUndef(oldStartVnode)
• 2、isUndef(oldEndVnode)
• 3、sameVnode(oldStartVnode, newStartVnode)
• 4、sameVnode(oldEndVnode, newEndVnode)
• 5、sameVnode(oldStartVnode, newEndVnode)
• 6、sameVnode(oldEndVnode, newStartVnode)
• 7、如果以上都不满足
• 小结

引言

vue数据的渲染会引入视图的重新渲染。

从数据到视图的渲染流程可以移步https://www.jb51.net/article/261839.htm，那么从数据的变化到视图的变化是怎样的?

vue在数据的初始化阶段会进行响应式的处理defineReactive：

/**
 * Define a reactive property on an Object.
 */
export function defineReactive (
  obj: Object,
  key: string,
  val: any,
  customSetter?: ?Function,
  shallow?: boolean
) {
  const dep = new Dep()
  const property = Object.getOwnPropertyDescriptor(obj, key)
  if (property && property.configurable === false) {
    return
  }
  // cater for pre-defined getter/setters
  const getter = property && property.get
  const setter = property && property.set
  if ((!getter || setter) && arguments.length === 2) {
    val = obj[key]
  }
  let childOb = !shallow && observe(val)
  Object.defineProperty(obj, key, {
    enumerable: true,
    configurable: true,
    get: function reactiveGetter () {
      const value = getter ? getter.call(obj) : val
      if (Dep.target) {
        dep.depend()
        if (childOb) {
          childOb.dep.depend()
          if (Array.isArray(value)) {
            dependArray(value)
          }
        }
      }
      return value
    },
    set: function reactiveSetter (newVal) {
      const value = getter ? getter.call(obj) : val
      /* eslint-disable no-self-compare */
      if (newVal === value || (newVal !== newVal && value !== value)) {
        return
      }
      /* eslint-enable no-self-compare */
      if (process.env.NODE_ENV !== 'production' && customSetter) {
        customSetter()
      }
      // #7981: for accessor properties without setter
      if (getter && !setter) return
      if (setter) {
        setter.call(obj, newVal)
      } else {
        val = newVal
      }
      childOb = !shallow && observe(newVal)
      dep.notify()
    }
  })
}

数据的变化会触发set方法，会让发布者dep执行 dep.notify，当vue所有的同步执行完后，在异步队列中按次序执行到vm的渲染流程，订阅者接收到发布者的通知后会执行到this.get()，指的是

updateComponent = () => {
  vm._update(vm._render(), hydrating)
}

vm._render()获取到vNode后，会执行vm._update视图的渲染:

Vue.prototype._update = function (vnode: VNode, hydrating?: boolean) {
    // ...
    const prevVnode = vm._vnode
    // ...
    if (!prevVnode) {
      // initial render
      vm.$el = vm.__patch__(vm.$el, vnode, hydrating, false /* removeOnly */)
    } else {
      // updates
      vm.$el = vm.__patch__(prevVnode, vnode)
    }
    // ...
  }

主要区别在于数据变化引起的视图变化有prevVnode，vm.__patch__(prevVnode, vnode)之后会执行到patch方法：

function patch (oldVnode, vnode, hydrating, removeOnly) {
    // ...
    if (isUndef(oldVnode)) {
      // empty mount (likely as component), create new root element
      isInitialPatch = true
      createElm(vnode, insertedVnodeQueue)
    } else {
      const isRealElement = isDef(oldVnode.nodeType)
      if (!isRealElement && sameVnode(oldVnode, vnode)) {
        // patch existing root node
        patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly)
      } else {
        // ...
        // 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)
        )
        // ...
        // destroy old node
        if (isDef(parentElm)) {
          removeVnodes([oldVnode], 0, 0)
        } else if (isDef(oldVnode.tag)) {
          invokeDestroyHook(oldVnode)
        }
      }
    }
    invokeInsertHook(vnode, insertedVnodeQueue, isInitialPatch)
    return vnode.elm
  }

在数据变化引起的patch过程中isRealElement显然为false，新旧节点是否相同的另一个判断条件是sameVnode：

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)
      )
    )
  )
}

如果sameVnode(oldVnode, vnode)为false，则执行createElm以及后续流程，该流程可以参考模板渲染的流程(请移步https://www.jb51.net/article/261850.htm )。

sameVnode(oldVnode, vnode)为true的时候，执行到patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly)：

function patchVnode (
    oldVnode,
    vnode,
    insertedVnodeQueue,
    ownerArray,
    index,
    removeOnly
  ) {
    // ...
    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)
    }
  }

ch = vnode.children和oldCh = oldVnode.children分别获取到新旧vnode的子元素，ch和oldCh都存在时会执行到updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly)：

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)
    }
  }

这里定义了四个索引oldStartIdx、newStartIdx、oldEndIdx和newEndIdx，也可以称之为指针，通过while循环，进行四个指针的移动：

1、isUndef(oldStartVnode)

如果oldStartVnode不存在，执行oldStartVnode = oldCh[++oldStartIdx]，将oldStartIdx指针向右移动一位，进行下次循环。

2、isUndef(oldEndVnode)

如果oldEndVnode不存在，执行oldEndVnode = oldCh[--oldEndIdx]，将oldEndIdx指针向左移动一位，进行下次循环。

3、sameVnode(oldStartVnode, newStartVnode)

如果满足sameVnode(oldStartVnode, newStartVnode)，执行patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)开始递归执行，结束后oldStartIdx和newStartIdx分别向右移动一位。

4、sameVnode(oldEndVnode, newEndVnode)

如果满足sameVnode(oldEndVnode, newEndVnode)，执行patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue, newCh, newStartIdx)开始递归执行，结束后oldEndIdx和newEndIdx分别向左移动一位。

5、sameVnode(oldStartVnode, newEndVnode)

如果满足sameVnode(oldStartVnode, newEndVnode)，执行patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue, newCh, newStartIdx)开始递归执行，结束后oldStartVnode向右移动一位，newEndIdx向左移动一位。
并且通过nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm))的方式将oldStartVnode.elm插入到oldEndVnode.elm节点之后。

6、sameVnode(oldEndVnode, newStartVnode)

如果满足sameVnode(oldEndVnode, newStartVnode)，执行patchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)开始递归执行，结束后newStartIdx向右移动一位，oldEndIdx向左移动一位。
并且通过nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm)的方式将 oldEndVnode.elm插入到oldStartVnode.elm节点之前。

7、如果以上都不满足

如果新旧vNode首首、首尾、尾首和尾尾对比都没找到相同的，则在旧vNode的oldStartIdx和oldEndIdx之间去找。 oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)创建以旧vNode的key为key值，位置索引为value的map映射:

function createKeyToOldIdx (children, beginIdx, endIdx) {
  let i, key
  const map = {}
  for (i = beginIdx; i <= endIdx; ++i) {
    key = children[i].key
    if (isDef(key)) map[key] = i
  }
  return map
}

如果通过createKeyToOldIdx找不到，则通过findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)和旧vNode的方式去进行比对，并返回位置索引：

function findIdxInOld (node, oldCh, start, end) {
    for (let i = start; i < end; i++) {
      const c = oldCh[i]
      if (isDef(c) && sameVnode(node, c)) return i
    }
}

通过oldKeyToIdx[newStartVnode.key]和findIdxInOld (node, oldCh, start, end)的查询会有两种结果：

1、没找到如果没有找到，则以newStartVnode为渲染vNode通过createElm去进行节点的创建。

2、找到了如果找到了，通过vnodeToMove = oldCh[idxInOld]获取到介于oldStartIdx和oldEndIdx之间的可以比对的vnode, 执行完patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)后将当前位置的oldCh[idxInOld] = undefined。

通过nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm)将vnodeToMove.elm移动到oldStartVnode.elm之前。

小结

diff算法从两端进行比对，找不到再从中间寻找，是一种 “滑动窗口” 算法的使用，以达到通过节点移动来实现原地复用的目的。

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