vue2源码解析（二）

一、理解Vue批量异步更新策略　　

异步更新队列：Vue高效的秘诀是批量、异步的更新策略

首先来了解下Event Loop事件循环机制

 事件循环Event Loop:浏览器为了协调事件处理、脚本执行、网络请求和渲染等任务而制定的工作机制

看源码执行过程

在Object.defineReactive()中的set方法，set方法中通过Dep大管家通知更新（dep.notify()）

dep.notify()中执行的代码,其中的subs就是dep的[watcher]，最后执行subs[i].update()，进入到改update中

notify () {
    // stabilize the subscriber list first
    const subs = this.subs.slice()
    if (process.env.NODE_ENV !== 'production' && !config.async) {
      // subs aren't sorted in scheduler if not running async
      // we need to sort them now to make sure they fire in correct
      // order
      subs.sort((a, b) => a.id - b.id)
    }
    // 遍历关联所有watcher
    for (let i = 0, l = subs.length; i < l; i++) {
      subs[i].update()
    }
  }

update方法是将watcher加入到队列中

update () {
    /* istanbul ignore else */
    if (this.lazy) {
      this.dirty = true
    } else if (this.sync) {
      this.run()
    } else {
      // watcher入队
      queueWatcher(this)
    }
  }

　　

export function queueWatcher (watcher: Watcher) {
  const id = watcher.id
  // 去重：单个watcher只入队一次
  if (has[id] == null) {
    has[id] = true
    if (!flushing) {
      queue.push(watcher)
    } else {
      // if already flushing, splice the watcher based on its id
      // if already past its id, it will be run next immediately.
      let i = queue.length - 1
      while (i > index && queue[i].id > watcher.id) {
        i--
      }
      queue.splice(i + 1, 0, watcher)
    }
    // queue the flush
    if (!waiting) {
      waiting = true

      if (process.env.NODE_ENV !== 'production' && !config.async) {
        flushSchedulerQueue()
        return
      }
　　　　nextTick的作用
      // 异步方式将flushSchedulerQueue放入队列
      nextTick(flushSchedulerQueue)
    }
  }
}

接下来看nextTick方法

// 此方法就是我们平时使用的nextTick方法
export function nextTick (cb?: Function, ctx?: Object) {
  let _resolve
  callbacks.push(() => {
    if (cb) {
      try {
        cb.call(ctx)
      } catch (e) {
        handleError(e, ctx, 'nextTick')
      }
    } else if (_resolve) {
      _resolve(ctx)
    }
  })
  if (!pending) {
    pending = true
    // 异步执行callbacks中的任务
    timerFunc()  //看浏览器支持程度，promise.resolve().then(flushCallbacks) > MutationObserver 最后不得已浏览器不支持才将watcher派发给宏任务队列 => setTimeout(flushCallbacks,0)
  }
  // $flow-disable-line
  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      _resolve = resolve
    })
  }
}

// 先把flushCallbacks全部放到微任务队列中去，将callback中回调全部执行一遍

function flushCallbacks () {

  pending = false

  const copies = callbacks.slice(0)

  callbacks.length = 0

  for (let i = 0; i < copies.length; i++) {

    copies[i]()

  }

}

等到浏览器执行微任务的时候，会执行flushSchedulerQueue，冲洗所有的队列并且run watchers

function flushSchedulerQueue () {
  currentFlushTimestamp = getNow()
  flushing = true
  let watcher, id

  // Sort queue before flush.
  // This ensures that:
  // 1. Components are updated from parent to child. (because parent is always
  //    created before the child)
  // 2. A component's user watchers are run before its render watcher (because
  //    user watchers are created before the render watcher)
  // 3. If a component is destroyed during a parent component's watcher run,
  //    its watchers can be skipped.
  queue.sort((a, b) => a.id - b.id)

  // do not cache length because more watchers might be pushed
  // as we run existing watchers
  // 按id顺序执行watcher更新
  for (index = 0; index < queue.length; index++) {
    watcher = queue[index]
    if (watcher.before) {
      watcher.before()
    }
    id = watcher.id
    has[id] = null
    // 真正的更新函数
    watcher.run()
    // in dev build, check and stop circular updates.
    if (process.env.NODE_ENV !== 'production' && has[id] != null) {
      circular[id] = (circular[id] || 0) + 1
      if (circular[id] > MAX_UPDATE_COUNT) {
        warn(
          'You may have an infinite update loop ' + (
            watcher.user
              ? `in watcher with expression "${watcher.expression}"`
              : `in a component render function.`
          ),
          watcher.vm
        )
        break
      }
    }
  }

}　　

进入到watcher.run()方法 run方法会调用watcher的get方法，get方法会this.getter方法,this.getter指向updateComponent()方法

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

updateComponent()先执行vm._render()得到虚拟dom，再执行vm._update()将虚拟dom转化为真实dom

二、掌握虚拟DOM和diff算法

 细看vm.update()方法,初始化的时候拿真实dom走第一个方法，更新的时候有虚拟dom了就走第二个方法

if (!prevVnode) {
      // initial render
      //初始化渲染，vm.$el是真实dom
      vm.$el = vm.__patch__(vm.$el, vnode, hydrating, false /* removeOnly */)
    } else {
      // updates（有虚拟dom之后走更新函数）
      vm.$el = vm.__patch__(prevVnode, vnode)
    }　

接下来来看__patch__具体做了什么

 // 更新时调用的__patch__就是这个
  return function patch (oldVnode, vnode, hydrating, removeOnly) {
    // 新的不存在：删除
    if (isUndef(vnode)) {
      if (isDef(oldVnode)) invokeDestroyHook(oldVnode)
      return
    }

    let isInitialPatch = false
    const insertedVnodeQueue = []

    // 老的不存在：创建
    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
        // diff发生的地方（更新的时候才会触发）
        patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly)
      } else {
        // 初始化走这里
　　　　　isRealElement是否是真实的节点
        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是虚拟节点vnode，表明此时已经将真实节点转化为虚拟节点
          oldVnode = emptyNodeAt(oldVnode)
        }

        // replacing existing element
        const oldElm = oldVnode.elm
        const 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)) {
          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
  }
}　　

patch的实现

首先进行树级的比较，可能有三种情况：增删改

1、new vnode不存在就删

2、old vnode不存在就删

3、都存在就都执行diff执行更新

接下来具体看patch里运用到的diff算法，主要是再patchvnode（）（diff发生的地方）去实现的

 遵循下面规律：同层比较，深度优先 例如下面的图片

 patchVnode 比较两个虚拟dom

// 1.获取两个比较节点孩子
    const oldCh = oldVnode.children
    const ch = vnode.children
    // 2.属性更新
    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)
    }
    // 3.没有文本
    if (isUndef(vnode.text)) {
      // 双方均有孩子：比较子节点
      if (isDef(oldCh) && isDef(ch)) {
        if (oldCh !== ch) updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly)
      } else if (isDef(ch)) { //新的有孩子，老的没有
        if (process.env.NODE_ENV !== 'production') {
          checkDuplicateKeys(ch)
        }
        // 新增
        if (isDef(oldVnode.text)) nodeOps.setTextContent(elm, '')
        addVnodes(elm, null, ch, 0, ch.length - 1, insertedVnodeQueue)
      } else if (isDef(oldCh)) {
        // 删除
        removeVnodes(oldCh, 0, oldCh.length - 1)
      } else if (isDef(oldVnode.text)) {
        // 文本清空
        nodeOps.setTextContent(elm, '')
      }
    } else if (oldVnode.text !== vnode.text) {
      // 文本更新
      nodeOps.setTextContent(elm, vnode.text)
    }
    // 钩子
    if (isDef(data)) {
      if (isDef(i = data.hook) && isDef(i = i.postpatch)) i(oldVnode, vnode)
    }

updateChildren 对比新旧两个vnode的children

 

// 比较两组孩子节点
  function updateChildren (parentElm, oldCh, newCh, insertedVnodeQueue, removeOnly) {
    // 设置首尾4个游标以及相对应的节点
    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]
    // 后面进行查找时所需的变量
    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)
    }

    // 开始循环：结束条件开始游标不能超过结束游标
    while (oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx) {
      // 前两种情况是游标调整
      if (isUndef(oldStartVnode)) {
        oldStartVnode = oldCh[++oldStartIdx] // Vnode has been moved left
      } else if (isUndef(oldEndVnode)) {
        oldEndVnode = oldCh[--oldEndIdx]
      } else if (sameVnode(oldStartVnode, newStartVnode)) {
        // 两个开头相同
        patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
        // 游标向后移动
        oldStartVnode = oldCh[++oldStartIdx]
        newStartVnode = newCh[++newStartIdx]
      } else if (sameVnode(oldEndVnode, newEndVnode)) {
        // 两个结束
        patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx)
        // 游标向前移动
        oldEndVnode = oldCh[--oldEndIdx]
        newEndVnode = newCh[--newEndIdx]
      } else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved right
        // 老的开始和新的结束
        patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx)
        // 移动该节点到队尾
        canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm))
        oldStartVnode = oldCh[++oldStartIdx]
        newEndVnode = newCh[--newEndIdx]
      } else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved left
        // 老结束和新开始
        patchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
        // 移动到队首
        canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm)
        oldEndVnode = oldCh[--oldEndIdx]
        newStartVnode = newCh[++newStartIdx]
      } else {
        // 首尾没有找到相同的，从新的开头拿出一个节点，去老的数组查找
        if (isUndef(oldKeyToIdx)) oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx)
        idxInOld = isDef(newStartVnode.key)
          ? oldKeyToIdx[newStartVnode.key]
          : findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx)
        // 如果在老数组中没有找到
        if (isUndef(idxInOld)) { // New element
          // 新增
          createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx)
        } else {
          // 否则更新
          vnodeToMove = oldCh[idxInOld]
          if (sameVnode(vnodeToMove, newStartVnode)) {
            patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue, newCh, newStartIdx)
            oldCh[idxInOld] = undefined
            // 移动到队首
            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]
      }
    }

    // 清理工作：
    // 如果老的结束了，新数组中剩下的要批量新增
    if (oldStartIdx > oldEndIdx) {
      refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx + 1].elm
      addVnodes(parentElm, refElm, newCh, newStartIdx, newEndIdx, insertedVnodeQueue)
    } else if (newStartIdx > newEndIdx) {
      // 如果新的结束了，老数组中剩下的要批量删除
      removeVnodes(oldCh, oldStartIdx, oldEndIdx)
    }
  }　　

查找相同节点最重要的依赖依据：sameVnode(key是判断两个相同节点的必要条件)

function sameVnode (a, b) {
  return (
    // key是判断两个相同节点必要条件
    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)
      )
    )
  )
}