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Vue3源码解读之patch

时间:2022-09-28 11:22:57浏览次数:81  
标签:isSVG parentComponent patch parentSuspense 源码 Vue3 n1 n2 null

例子代码

本篇将要讲解dom diff,那么咱们结合下面的例子来进行讲解,这个例子是在上一篇文章的基础上,加了一个数据变更,也就是list的值发生了改变。html中增加了一个按钮change,通过点击change按钮来调用change函数,来改变list的值。例子位于源代码/packages/vue/examples/classic/目录下,下面是例子的代码:

const app = Vue.createApp({
    data() {
        return {
            list: ['a', 'b', 'c', 'd']
        }
    },
    methods: {
        change() {
            this.list = ['a', 'd', 'e', 'b']
        }
    }
});
app.mount('#demo')

<!DOCTYPE html>
<html>
<head>
    <meta name="viewport"
          content="initial-scale=1.0,maximum-scale=1.0,minimum-scale=1.0,user-scalable=no,target-densitydpi=medium-dpi,viewport-fit=cover"
    />
    <title>Vue3.js hello example</title>

    <script src="../../dist/vue.global.js"></script>
</head>
<body>
<div id="demo">
    <ul>
        <li v-for="item in list" :key="item">
            {{item}}
        </li>
    </ul>
    <button @click="change">change</button>
</div>
<script src="./hello.js"></script>
</body>
</html>

Vue3源码视频讲解:进入学习

源码解读

关于Vue3中数据发生变更,最终影响到页面发生变化的过程,我们本篇文章只对componentEffect以及以后的代码进行讲解,对于数据变更后,是如何执行到componentEffect函数,以及为何会执行componentEffect,后面的文章再进行讲解。

componentEffect

来看下componentEffect更新部分的代码:

  // @file packages/runtime-core/src/renderer.ts
  function componentEffect() {
    if (!instance.isMounted) {
        // first render
    } else {
        let {next, bu, u, parent, vnode} = instance
        let originNext = next
        let vnodeHook: VNodeHook | null | undefined

        if (next) {
            updateComponentPreRender(instance, next, optimized)
        } else {
            next = vnode
        }
        next.el = vnode.el

        // beforeUpdate hook
        if (bu) {
            invokeArrayFns(bu)
        }
        // onVnodeBeforeUpdate
        if ((vnodeHook = next.props && next.props.onVnodeBeforeUpdate)) {
            invokeVNodeHook(vnodeHook, parent, next, vnode)
        }
        const nextTree = renderComponentRoot(instance)
        const prevTree = instance.subTree
        instance.subTree = nextTree

        if (instance.refs !== EMPTY_OBJ) {
            instance.refs = {}
        }
        patch(
            prevTree,
            nextTree,
            hostParentNode(prevTree.el!)!,
            getNextHostNode(prevTree),
            instance,
            parentSuspense,
            isSVG
        )
        next.el = nextTree.el
        if (originNext === null) {
            updateHOCHostEl(instance, nextTree.el)
        }
        // updated hook
        if (u) {
            queuePostRenderEffect(u, parentSuspense)
        }
        // onVnodeUpdated
        if ((vnodeHook = next.props && next.props.onVnodeUpdated)) {
            queuePostRenderEffect(() => {
                invokeVNodeHook(vnodeHook!, parent, next!, vnode)
            }, parentSuspense)
        }
    }
  }

当数据发生变化的时候,最终会走到上面的else的逻辑部分。

  • 默认情况下next是null,父组件调用processComponent触发当前调用的时候会是VNode,此时next为null;
  • 调用当前实例beforeUpdate钩子函数;调用要更新的Vnode(next)的父组件的beforeUpdate钩子函数;
  • 获取当前实例的vNode => prevTree;获取要更新的vNode=> nextTree;然后调用patch;

调用patch函数的过程,也就是根据VNode的type,走不同的支流的过程;点击change按钮:n1的值: n2的值: 根据这个值,可以知晓,会走到processFragment函数;

processFragment

调用processFragment(n1, n2, container, anchor, parentComponent, parentSuspense, isSVG, optimized)函数,参数的值:

  • 此时n1和n2如上图;
  • container为#demo;
  • anchor为null;
  • parentComponent为instance实例;
  • parentSuspense为null;
  • isSVG为false;
  • optimized为false;

来看下processFragment函数的源码:

// @file packages/runtime-core/src/renderer.ts
const processFragment = (
    n1: VNode | null,    n2: VNode,    container: RendererElement,    anchor: RendererNode | null,    parentComponent: ComponentInternalInstance | null,    parentSuspense: SuspenseBoundary | null,    isSVG: boolean,    optimized: boolean
) => {
    const fragmentStartAnchor = (n2.el = n1 ? n1.el : hostCreateText(''))!
    const fragmentEndAnchor = (n2.anchor = n1 ? n1.anchor : hostCreateText(''))!

    let {patchFlag, dynamicChildren} = n2
    if (patchFlag > 0) {
        optimized = true
    }

    if (n1 == null) {
        // first render的逻辑
    } else {
        if (
            patchFlag > 0 && patchFlag & PatchFlags.STABLE_FRAGMENT && dynamicChildren
        ) {
            patchBlockChildren(
                n1.dynamicChildren!,
                dynamicChildren,
                container,
                parentComponent,
                parentSuspense,
                isSVG
            )
            if (__DEV__ && parentComponent && parentComponent.type.__hmrId) {
                traverseStaticChildren(n1, n2)
            } else if (
                n2.key != null ||
                (parentComponent && n2 === parentComponent.subTree)
            ) {
                traverseStaticChildren(n1, n2, true /* shallow */)
            }
        } else {
            patchChildren(
                n1,
                n2,
                container,
                fragmentEndAnchor,
                parentComponent,
                parentSuspense,
                isSVG,
                optimized
            )
        }
    }
}

刨除掉first render的代码后,可以看到下面还是分为了两个分支;根据n1和n2可知,我们将会走if分支,执行patchBlockChildren。

patchBlockChildren

调用patchBlockChildren(n1.dynamicChildren, n2.dynamicChildren, container, parentComponent, parentSuspense, isSVG)函数,此时参数如下:

  • oldChildren:n1.dynamicChildren,也就是Symbol(Fragment) =>ul 和button两个元素组成的数组;
  • newChildren: n2.dynamicChildren,也就是Symbol(Fragment) =>ul 和button两个元素组成的数组;
  • fallbackContainer:container,也就是#demo;
  • parentComponent:instance实例;
  • parentSuspense:null;
  • isSVG:false。

来看下patchBlockChildren的源码:

// @file packages/runtime-core/src/renderer.ts
const patchBlockChildren: PatchBlockChildrenFn = (
    oldChildren,    newChildren,    fallbackContainer,    parentComponent,    parentSuspense,    isSVG
) => {
    for (let i = 0; i < newChildren.length; i++) {
        const oldVNode = oldChildren[i]
        const newVNode = newChildren[i]
        const container =
            oldVNode.type === Fragment ||
            !isSameVNodeType(oldVNode, newVNode) ||
            oldVNode.shapeFlag & ShapeFlags.COMPONENT ||
            oldVNode.shapeFlag & ShapeFlags.TELEPORT
                ? hostParentNode(oldVNode.el!)!
                : fallbackContainer
        patch(
            oldVNode,
            newVNode,
            container,
            null,
            parentComponent,
            parentSuspense,
            isSVG,
            true
        )
    }
}

可以看到patchBlockChildren是for循环调用patch函数,上面看到newChildren是一个长度为2的数组。循环遍历调用patch(oldVNode, newVNode, container, null, parentComponent, parentSuspense, isSVG, true);

  • 第一次循环:
    • oldVNode:老的ul数组生成的VNode对象;
    • newVNode:新的ul数组生成的VNode对象;
    • container:ul元素;
    • anchor:上面传递的是null;
    • parentComponent: instance实例;
    • parentSuspense: null;
    • isSVG: false;
    • optimized: true;
  • 第二次循环:
    • oldVNode: 老的change按钮构成的VNode对象;
    • newVNode:新的change按钮构成的VNode对象;
    • container:此时的container为#demo;
    • anchor:上面传递的是null;
    • parentComponent: instance实例;
    • parentSuspense: null;
    • isSVG: false;
    • optimized: true;

processElement

咱们先说第二次循环,第二次比较简单;上面说到调用patch函数,通过上面了解到第二次循环newVNode的type是button;则会走到processElement,参数全部是透传过来的:

const processElement = (
        n1: VNode | null,        n2: VNode,        container: RendererElement,        anchor: RendererNode | null,        parentComponent: ComponentInternalInstance | null,        parentSuspense: SuspenseBoundary | null,        isSVG: boolean,        optimized: boolean
    ) => {
        isSVG = isSVG || (n2.type as string) === 'svg'
        if (n1 == null) {
            // first render
        } else {
            patchElement(n1, n2, parentComponent, parentSuspense, isSVG, optimized)
        }
    }

如上代码,会直接调用patchElement,此时参数为:

  • n1: 老的change按钮构成的VNode对象;
  • n2:新的change按钮构成的VNode对象;
  • parentComponent: instance实例;
  • parentSuspense: null;
  • isSVG: false;
  • optimized: true;

patchChildren

现在再来说第一次循环,执行patch的时候,newVNode的type为Symbol(Fragment) => ul,此时还是会走到processFragment函数,不过此时的dynamicChildren为空,会继续运行到patchChildren函数。

patchChildren

此时运行到patchChildren函数,我们来看下运行到此时的参数:

  • n1:老的ul数组生成的VNode对象;
  • n2:新的ul数组生成的VNode对象;
  • container:ul元素;
  • anchor:ul结尾生成的对象;
  • parentComponent:instance实例;
  • parentSuspense:null
  • isSVG:false;
  • optimized:true;

下面看下patchChildren的源码:

const patchChildren: PatchChildrenFn = (
    n1,    n2,    container,    anchor,    parentComponent,    parentSuspense,    isSVG,    optimized = false
) => {
    const c1 = n1 && n1.children
    const prevShapeFlag = n1 ? n1.shapeFlag : 0
    const c2 = n2.children

    const {patchFlag, shapeFlag} = n2
    if (patchFlag > 0) {
        if (patchFlag & PatchFlags.KEYED_FRAGMENT) {
            patchKeyedChildren(
                c1 as VNode[],
                c2 as VNodeArrayChildren,
                container,
                anchor,
                parentComponent,
                parentSuspense,
                isSVG,
                optimized
            )
            return
        } else if (patchFlag & PatchFlags.UNKEYED_FRAGMENT) {
            // patchUnkeyedChildren
            return
        }
    }

    // other ......
}

此时patchFlag的值为128,同时我们的list渲染是有key的,so 会运行patchKeyedChildren函数,c1为四个li组成的数组(a,b,c,d);c2为新的li组成的数组(a,d,e,b);其他值透传到patchKeyedChildren。

patchKeyedChildren

上面对patchKeyedChildren函数的参数已经进行了说明,在这里我们再回顾下:

  • c1:四个li组成的数组(a,b,c,d);
  • c2:新的li组成的数组(a,d,e,b);
  • container:ul元素;
  • parentAnchor:ul结尾生成的对象;
  • parentComponent:instance实例;
  • parentSuspense:null
  • isSVG:false;
  • optimized:true;

接下来看下patchKeyedChildren函数的源码:

const patchKeyedChildren = (
    c1: VNode[],    c2: VNodeArrayChildren,    container: RendererElement,    parentAnchor: RendererNode | null,    parentComponent: ComponentInternalInstance | null,    parentSuspense: SuspenseBoundary | null,    isSVG: boolean,    optimized: boolean
) => {
    let i = 0
    const l2 = c2.length
    let e1 = c1.length - 1 
    let e2 = l2 - 1 

    while (i <= e1 && i <= e2) {
        const n1 = c1[i]
        const n2 = (c2[i] = optimized ? cloneIfMounted(c2[i] as VNode) : normalizeVNode(c2[i]))
        if (isSameVNodeType(n1, n2)) {
               patch(n1,n2,container,null,parentComponent,parentSuspense,isSVG,optimized)
        } else {
            break
        }
        i++
    }

    while (i <= e1 && i <= e2) {
        const n1 = c1[e1]
        const n2 = (c2[e2] = optimized ? cloneIfMounted(c2[e2] as VNode) : normalizeVNode(c2[e2]))
        if (isSameVNodeType(n1, n2)) {
            patch(n1,n2,container,null,parentComponent,parentSuspense,isSVG,optimized)
        } else {
            break
        }
        e1--
        e2--
    }

    if (i > e1) {
        if (i <= e2) {
            const nextPos = e2 + 1
            const anchor = nextPos < l2 ? (c2[nextPos] as VNode).el : parentAnchor
            while (i <= e2) {
                patch(
                    null,
                    (c2[i] = optimized
                        ? cloneIfMounted(c2[i] as VNode)
                        : normalizeVNode(c2[i])),
                    container,
                    anchor,
                    parentComponent,
                    parentSuspense,
                    isSVG
                )
                i++
            }
        }
    }

    else if (i > e2) {
        while (i <= e1) {
            unmount(c1[i], parentComponent, parentSuspense, true)
            i++
        }
    }

    else {
        const s1 = i
        const s2 = i 
        for (i = s2; i <= e2; i++) {
            const nextChild = (c2[i] = optimized ? cloneIfMounted(c2[i] as VNode) : normalizeVNode(c2[i]))
            if (nextChild.key != null) {
                keyToNewIndexMap.set(nextChild.key, i)
            }
        }

        let j
        let patched = 0
        const toBePatched = e2 - s2 + 1
        let moved = false
        let maxNewIndexSoFar = 0
        const newIndexToOldIndexMap = new Array(toBePatched)
        for (i = 0; i < toBePatched; i++) newIndexToOldIndexMap[i] = 0

        for (i = s1; i <= e1; i++) {
            const prevChild = c1[i]
            if (patched >= toBePatched) {
                unmount(prevChild, parentComponent, parentSuspense, true)
                continue
            }
            let newIndex
            if (prevChild.key != null) {
                newIndex = keyToNewIndexMap.get(prevChild.key)
            } else {
                for (j = s2; j <= e2; j++) {
                    if (
                        newIndexToOldIndexMap[j - s2] === 0 &&
                        isSameVNodeType(prevChild, c2[j] as VNode)
                    ) {
                        newIndex = j
                        break
                    }
                }
            }
            if (newIndex === undefined) {
                unmount(prevChild, parentComponent, parentSuspense, true)
            } else {
                newIndexToOldIndexMap[newIndex - s2] = i + 1
                if (newIndex >= maxNewIndexSoFar) {
                    maxNewIndexSoFar = newIndex
                } else {
                    moved = true
                }
                patch(
                    prevChild,
                    c2[newIndex] as VNode,
                    container,
                    null,
                    parentComponent,
                    parentSuspense,
                    isSVG,
                    optimized
                )
                patched++
            }
        }

        const increasingNewIndexSequence = moved
            ? getSequence(newIndexToOldIndexMap)
            : EMPTY_ARR
        j = increasingNewIndexSequence.length - 1
        for (i = toBePatched - 1; i >= 0; i--) {
            const nextIndex = s2 + i
            const nextChild = c2[nextIndex] as VNode
            const anchor =
                nextIndex + 1 < l2 ? (c2[nextIndex + 1] as VNode).el : parentAnchor
            if (newIndexToOldIndexMap[i] === 0) {
                patch(
                    null,
                    nextChild,
                    container,
                    anchor,
                    parentComponent,
                    parentSuspense,
                    isSVG
                )
            } else if (moved) {
                if (j < 0 || i !== increasingNewIndexSequence[j]) {
                    move(nextChild, container, anchor, MoveType.REORDER)
                } else {
                    j--
                }
            }
        }
    }
}

上面代码包含的有两个while循环和两对if-else;

  • i=0,循环开始下标;e1、e2为c1和c2的长度;l2为新的children的长度;
  • 第一个while循环,从头开始对列表进行遍历:
    • 当nodeType一样的时候,调用patch;
    • 当nodeType不一样的时候,跳出循环;
  • 第二个while循环,当第一个while循环对c1和c2都没有遍历完的时候,从尾部开始对其进行遍历:
    • 当nodeType一样的时候,调用patch;
    • 当nodeType不一样的时候,跳出循环;
  • 第一个if,i>e1证明c1已经遍历完,i<=e2证明c2还没遍历完,对剩余的c2继续遍历,调用patch;
  • 第二个else-if,i>e2证明c2已经遍历完,i<=e1证明c1还没遍历完,对剩余的c1继续遍历,因为c1为老的列表,则调用unmount把无用的列表内容卸载掉:
  • 第二个else:c1和c2至少有一个没有遍历完,走到最后一个else的逻辑:
    • for (i = s2; i <= e2; i++)for循环遍历剩余c2,收集每个c2的元素的key,构成map => keyToNewIndexMap;
    • for (i = 0; i < toBePatched; i++)for循环遍历剩余c2部分长度来生成映射,并赋值为0;
    • for (i = s1; i <= e1; i++) for循环遍历剩余c1,使用key进行直接获取(for循环剩余c2进行获取)newIndex,此处证明还是要绑定好key,唯一性很重要;newIndex有值说明c2中存在当前老的元素在c1中,老的preChild,在c2中还需要,则调用patch;如果newIndex为undefined,则说明老的preChild在c2中不需要了,调用unmount,把当前preChild卸载掉;
    • 遍历完剩余c1后,再倒着遍历剩余c2:for (i = toBePatched - 1; i >= 0; i--);如果(newIndexToOldIndexMap[i] === 0则证明当前nextChild为新的节点,调用patch;否则判断之前是否发生了移动moved,经过逻辑判断,调用move;

patchKeyedChildren 例子

根据咱们上面的例子,由old: ['a', 'b', 'c', 'd']变更为new: ['a', 'd', 'e', 'b']的过程如下:

  • 首先进入第一个while循环,此时i为0,l2为4,e1为3,e2为3;
    • 第一次循环,old-a与new-a是相同的,调用patch,不发生变化;
    • 第二次循环,old-b与new-b是不相同的,break;
    • 跳出循环,从头开始的循环结束;
  • 进入第二个while循环,此时i为1,l2为4,e1为3,e2为3;
    • 第一次循环,old-d与new-b是不相同的,break;
    • 跳出循环,从尾部开始的循环结束;
  • 进入第一个if判断为false,进入第二个else-if判断为false,进入else;
  • for循环收集每个c2的元素的key,keyToNewIndexMap = ['d' => 1, 'e' => 2, 'b' => 3];
  • 建立长度为剩余c2长度的数组newIndexToOldIndexMap = [0, 0 ,0];
  • 此时进入for (i = s1; i <= e1; i++) for循环遍历剩余c1阶段,此时i为1,s1为1,s2为1:
    • 第一次循环:遍历的元素为old-b,发现在new中存在,通过keyToNewIndexMap获得在new中的index为3;调用patch;
    • 第二次循环:遍历的元素为old-c,在new中不存在,调用unmount卸载当前old-c,改变后c1为['a', 'b', 'd']
    • 第三次循环:遍历的元素为old-d,在new中存在,通过keyToNewIndexMap获得在new中的index为1;调用patch;
    • 跳出循环,遍历c1剩余阶段结束;
  • 此时进入for (i = toBePatched - 1; i >= 0; i--)倒着遍历剩余c2阶段,此时i为2,j为0,s1为1,s2为1,newIndexToOldIndexMap为[4, 0, 2]:
    • 第一次循环,判断当前nextChild(new-b)存不存在,通过newIndexToOldIndexMap发现nextChild存在,并且在old里面的索引值为2,j--,此时j为-1;i--,i为1;
    • 第二次循环,判断当前nextChild(new-e)存不存在,通过newIndexToOldIndexMap发现nextChild的索引值为0,表示不存在,则调用patch;i--,i为0;改变后c1为['a', 'e', 'b', 'd'];
    • 第三次循环,判断当前nextChild(new-d)存不存在,通过newIndexToOldIndexMap发现nextChild的索引值为4,表示存在,则调用move;i--,i为-1;改变后c1为['a',, 'd' 'e', 'b'];
    • 此时i为-1,跳出循环,循环结束
  • 遍历结束,结果变更为了new: ['a', 'd', 'e', 'b']

isSameVNodeType

大家可以看下下面isSameVNodeType的代码,大家在写代码的时候,为了能够提高页面性能,dom diff的速度,如果没有发生变更的元素,key一定要保持一样,不要v-for="(item, index) in list" :key="index"这样来写,因为当只有数组内部元素发生了位置移动而元素未发生改变时,index的值是变更的,这样在dom diff的时候就会使程序发生误解。key的唯一性很重要

export function isSameVNodeType(n1: VNode, n2: VNode): boolean {
    return n1.type === n2.type && n1.key === n2.key
}

标签:isSVG,parentComponent,patch,parentSuspense,源码,Vue3,n1,n2,null
From: https://www.cnblogs.com/yyzzabc123/p/16737338.html

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