keep-alive

它是一个抽象组件：它自身不会渲染任何DOM元素，也不会出现在组件的父组件链中。

默认情况下，一个组件实例被替换后就会被销毁掉，这会导致它丢失其中已经变化的状态，当这个组件再次显示时，会创建一个新的实例，它会回到初始化的状态。有的时候，我们希望组件在切换的时候不被销毁，可以保留当前的状态，这个时候就可以用到这个内置组件，将需要保存且状态已经有所变化的一些组件实例缓存起来。

默认情况下，<keep-alive>会缓存内部的所有组件实例，但是有时候我们可能并不需要缓存全部， 这时我们可以根据他的 include 和 exclude props 来定制行为，判断哪些是需要缓存的，哪些是需要销毁的。props 传参里面还有 max 属性，来限制可被缓存的最大组件实例数，如果超出，则最久没有被访问的缓存实例将被销毁，以便为新的组件实例腾出空间。

2.0

<keep-alive> 组件有一个 cache 属性 和 key 属性。

• cache:一个原型为空的对象，用来保存需要缓存的组件vnode。
• keys: 一个数组，用来表示缓存组件实例的长度。

1. keep-alive组件内置了一个默认的插槽slot, 会先拿到这个插槽的虚拟DOM(vnode or vdom),然后拿到这个虚拟DOM的componentOptions选项，如果没有，就直接返回这个vnode，不做缓存处理。
2. 通过componentOptions获取动态组件的name属性，如果设置了inclue和exclude, 就判断是否满足可以缓存的条件，如果不满足，就直接返回这个vnode，不做缓存处理。
3. 获取key，默认是vnode.key，如果不存在，就用componentOptions.Ctor.cid（组件实例id）和componentOptions.tag(如果有的话)拼接出一个key。
4. 根据key，查找cache对象是否已经有了该组件时间的缓存，如果没有，则将该vnode存储到cache对象，key为健，vnode为值。并将key值添加到keys数组里面。同时还要判断keys数组长度是否已经超过了设置的max（如果props有的话）最大值。如果超过了，就清除第一个值，同时找到cache缓存中的对应的value清空。
5. 如果存在，直接返回。同时也表示keys数组里面存在对应的name值，需要将这个name移动到keys数组的最后面，最前面的表示最长时间没有被访问的，最后面的表示最近被访问的。
6. 设置 vnode.data.keepAlive = true;组件切换时，下面两个生命周期会用到。

// 源码 node_modules/vue/src/core/components/keep-alive.js

import { isRegExp, remove } from 'shared/util'
import { getFirstComponentChild } from 'core/vdom/helpers/index'

type VNodeCache = { [key: string]: ?VNode };

function getComponentName (opts: ?VNodeComponentOptions): ?string {
  return opts && (opts.Ctor.options.name || opts.tag)
}

function matches (pattern: string | RegExp | Array<string>, name: string): boolean {
  if (Array.isArray(pattern)) {
    return pattern.indexOf(name) > -1
  } else if (typeof pattern === 'string') {
    return pattern.split(',').indexOf(name) > -1
  } else if (isRegExp(pattern)) {
    return pattern.test(name)
  }
  /* istanbul ignore next */
  return false
}

function pruneCache (keepAliveInstance: any, filter: Function) {
  const { cache, keys, _vnode } = keepAliveInstance
  for (const key in cache) {
    const cachedNode: ?VNode = cache[key]
    if (cachedNode) {
      const name: ?string = getComponentName(cachedNode.componentOptions)
      if (name && !filter(name)) {
        pruneCacheEntry(cache, key, keys, _vnode)
      }
    }
  }
}

function pruneCacheEntry (
  cache: VNodeCache,
  key: string,
  keys: Array<string>,
  current?: VNode
) {
  const cached = cache[key]
  if (cached && (!current || cached.tag !== current.tag)) {
    cached.componentInstance.$destroy()  //组件被销毁
  }
  cache[key] = null
  remove(keys, key)
}

const patternTypes: Array<Function> = [String, RegExp, Array]

export default {
  name: 'keep-alive',
  abstract: true,

  props: {
    include: patternTypes,
    exclude: patternTypes,
    max: [String, Number]
  },

  created () {
    this.cache = Object.create(null)
    this.keys = []
  },

  destroyed () {
    for (const key in this.cache) {
      pruneCacheEntry(this.cache, key, this.keys)
    }
  },

  mounted () {
    this.$watch('include', val => {
      pruneCache(this, name => matches(val, name))
    })
    this.$watch('exclude', val => {
      pruneCache(this, name => !matches(val, name))
    })
  },

  render () {
    const slot = this.$slots.default
    const vnode: VNode = getFirstComponentChild(slot)
    const componentOptions: ?VNodeComponentOptions = vnode && vnode.componentOptions
    if (componentOptions) {
      // check pattern
      const name: ?string = getComponentName(componentOptions)
      const { include, exclude } = this
      if (
        // not included
        (include && (!name || !matches(include, name))) ||
        // excluded
        (exclude && name && matches(exclude, name))
      ) {
        return vnode
      }

      const { cache, keys } = this
      const key: ?string = vnode.key == null
        // same constructor may get registered as different local components
        // so cid alone is not enough (#3269)
        ? componentOptions.Ctor.cid + (componentOptions.tag ? `::${componentOptions.tag}` : '')
        : vnode.key
      if (cache[key]) {
        vnode.componentInstance = cache[key].componentInstance
        // make current key freshest
        remove(keys, key)
        keys.push(key)
      } else {
        cache[key] = vnode
        keys.push(key)
        // prune oldest entry
        if (this.max && keys.length > parseInt(this.max)) {
          pruneCacheEntry(cache, keys[0], keys, this._vnode)
        }
      }

      vnode.data.keepAlive = true
    }
    return vnode || (slot && slot[0])
  }
}

当组件在 <keep-alive> 内被切换，它的 activated 和 deactivated 这两个生命周期钩子函数将会被对应执行。

在 2.2.0 及其更高版本中，activated 和 deactivated 将会在 <keep-alive> 树内的所有嵌套组件中触发。

var componentVNodeHooks = {
    init: ...,
    prepatch: ...,
    insert: function insert (vnode) {
    var context = vnode.context;
    var componentInstance = vnode.componentInstance;
    if (!componentInstance._isMounted) {
      componentInstance._isMounted = true;
      callHook(componentInstance, 'mounted');
    }
    if (vnode.data.keepAlive) {
      if (context._isMounted) {
        // vue-router#1212
        // During updates, a kept-alive component's child components may
        // change, so directly walking the tree here may call activated hooks
        // on incorrect children. Instead we push them into a queue which will
        // be processed after the whole patch process ended.
        queueActivatedComponent(componentInstance);
      } else {
        activateChildComponent(componentInstance, true /* direct */);
      }
    }
  },
  destroy: function destroy (vnode) {
    var componentInstance = vnode.componentInstance;
    if (!componentInstance._isDestroyed) {
      if (!vnode.data.keepAlive) {
        componentInstance.$destroy();
      } else {
        deactivateChildComponent(componentInstance, true /* direct */);
      }
    }
  }
}

 // 新旧组件进行 patch 时，组件切换时会触发挂载和卸载。
 // 如果换上来时缓存组件，如果不是首次挂在，不会发生真正的组件挂载，
 // 因为它并没有真正的卸载，触发 activated 。
 // 同时子组件也会深度优先遍历触发 activated。
 // 如果换下去的是缓存组件，并不会真的发生销毁，而是触发 deactivated，
 // 同时子组件也会深度优先遍历触发 deactivated。

// activated 回调函数
function activateChildComponent (vm, direct) {
  if (direct) {
    vm._directInactive = false;
    if (isInInactiveTree(vm)) {
      return
    }
  } else if (vm._directInactive) {
    return
  }
  if (vm._inactive || vm._inactive === null) {
    vm._inactive = false;
    for (var i = 0; i < vm.$children.length; i++) {
      activateChildComponent(vm.$children[i]);
    }
    callHook(vm, 'activated');
  }
}

// deactivated 回调函数
function deactivateChildComponent (vm, direct) {
  if (direct) {
    vm._directInactive = true;
    if (isInInactiveTree(vm)) {
      return
    }
  }
  if (!vm._inactive) {
    vm._inactive = true;
    for (var i = 0; i < vm.$children.length; i++) {
      deactivateChildComponent(vm.$children[i]);
    }
    callHook(vm, 'deactivated');
  }
}

3.0

const KeepAliveImpl = {
    name: `KeepAlive`,
    // Marker for special handling inside the renderer. We are not using a ===
    // check directly on KeepAlive in the renderer, because importing it directly
    // would prevent it from being tree-shaken.
    __isKeepAlive: true,  //标志这个组件是 keepAlice 组件
    props: {
        include: [String, RegExp, Array],
        exclude: [String, RegExp, Array],
        max: [String, Number]
    },
    setup(props, { slots }) {
        const instance = getCurrentInstance();
        // KeepAlive communicates with the instantiated renderer via the
        // ctx where the renderer passes in its internals,
        // and the KeepAlive instance exposes activate/deactivate implementations.
        // The whole point of this is to avoid importing KeepAlive directly in the
        // renderer to facilitate tree-shaking.
        const sharedContext = instance.ctx;
        // if the internal renderer is not registered, it indicates that this is server-side rendering,
        // for KeepAlive, we just need to render its children
        if (!sharedContext.renderer) {
            return () => {
                const children = slots.default && slots.default();
                return children && children.length === 1 ? children[0] : children;
            };
        }
        const cache = new Map();
        const keys = new Set();
        let current = null;
        if ((process.env.NODE_ENV !== 'production') || __VUE_PROD_DEVTOOLS__) {
            instance.__v_cache = cache;
        }
        const parentSuspense = instance.suspense;
        const { renderer: { p: patch, m: move, um: _unmount, o: { createElement } } } = sharedContext;
        const storageContainer = createElement('div');
        sharedContext.activate = (vnode, container, anchor, isSVG, optimized) => {
            const instance = vnode.component;
            move(vnode, container, anchor, 0 /* ENTER */, parentSuspense);
            // in case props have changed
            patch(instance.vnode, vnode, container, anchor, instance, parentSuspense, isSVG, vnode.slotScopeIds, optimized);
            queuePostRenderEffect(() => {
                instance.isDeactivated = false;
                if (instance.a) {
                    invokeArrayFns(instance.a);
                }
                const vnodeHook = vnode.props && vnode.props.onVnodeMounted;
                if (vnodeHook) {
                    invokeVNodeHook(vnodeHook, instance.parent, vnode);
                }
            }, parentSuspense);
            if ((process.env.NODE_ENV !== 'production') || __VUE_PROD_DEVTOOLS__) {
                // Update components tree
                devtoolsComponentAdded(instance);
            }
        };
        sharedContext.deactivate = (vnode) => {
            const instance = vnode.component;
            move(vnode, storageContainer, null, 1 /* LEAVE */, parentSuspense);
            queuePostRenderEffect(() => {
                if (instance.da) {
                    invokeArrayFns(instance.da);
                }
                const vnodeHook = vnode.props && vnode.props.onVnodeUnmounted;
                if (vnodeHook) {
                    invokeVNodeHook(vnodeHook, instance.parent, vnode);
                }
                instance.isDeactivated = true;
            }, parentSuspense);
            if ((process.env.NODE_ENV !== 'production') || __VUE_PROD_DEVTOOLS__) {
                // Update components tree
                devtoolsComponentAdded(instance);
            }
        };
        function unmount(vnode) {
            // reset the shapeFlag so it can be properly unmounted
            resetShapeFlag(vnode);
            _unmount(vnode, instance, parentSuspense, true);
        }
        function pruneCache(filter) {
            cache.forEach((vnode, key) => {
                const name = getComponentName(vnode.type);
                if (name && (!filter || !filter(name))) {
                    pruneCacheEntry(key);
                }
            });
        }
        function pruneCacheEntry(key) {
            const cached = cache.get(key);
            if (!current || cached.type !== current.type) {
                unmount(cached);
            }
            else if (current) {
                // current active instance should no longer be kept-alive.
                // we can't unmount it now but it might be later, so reset its flag now.
                resetShapeFlag(current);
            }
            cache.delete(key);
            keys.delete(key);
        }
        // prune cache on include/exclude prop change
        watch(() => [props.include, props.exclude], ([include, exclude]) => {
            include && pruneCache(name => matches(include, name));
            exclude && pruneCache(name => !matches(exclude, name));
        }, 
        // prune post-render after `current` has been updated
        { flush: 'post', deep: true });
        // cache sub tree after render
        let pendingCacheKey = null;
        const cacheSubtree = () => {
            // fix #1621, the pendingCacheKey could be 0
            if (pendingCacheKey != null) {
                cache.set(pendingCacheKey, getInnerChild(instance.subTree));
            }
        };
        onMounted(cacheSubtree);
        onUpdated(cacheSubtree);
        onBeforeUnmount(() => {
            cache.forEach(cached => {
                const { subTree, suspense } = instance;
                const vnode = getInnerChild(subTree);
                if (cached.type === vnode.type) {
                    // current instance will be unmounted as part of keep-alive's unmount
                    resetShapeFlag(vnode);
                    // but invoke its deactivated hook here
                    const da = vnode.component.da;
                    da && queuePostRenderEffect(da, suspense);
                    return;
                }
                unmount(cached);
            });
        });
        return () => {
            pendingCacheKey = null;
            if (!slots.default) {
                return null;
            }
            const children = slots.default();
            const rawVNode = children[0];
            if (children.length > 1) {
                if ((process.env.NODE_ENV !== 'production')) {
                    warn(`KeepAlive should contain exactly one component child.`);
                }
                current = null;
                return children;
            }
            else if (!isVNode(rawVNode) ||
                (!(rawVNode.shapeFlag & 4 /* STATEFUL_COMPONENT */) &&
                    !(rawVNode.shapeFlag & 128 /* SUSPENSE */))) {
                current = null;
                return rawVNode;
            }
            let vnode = getInnerChild(rawVNode);
            const comp = vnode.type;
            // for async components, name check should be based in its loaded
            // inner component if available
            const name = getComponentName(isAsyncWrapper(vnode)
                ? vnode.type.__asyncResolved || {}
                : comp);
            const { include, exclude, max } = props;
            if ((include && (!name || !matches(include, name))) ||
                (exclude && name && matches(exclude, name))) {
                current = vnode;
                return rawVNode;
            }
            const key = vnode.key == null ? comp : vnode.key;
            const cachedVNode = cache.get(key);
            // clone vnode if it's reused because we are going to mutate it
            if (vnode.el) {
                vnode = cloneVNode(vnode);
                if (rawVNode.shapeFlag & 128 /* SUSPENSE */) {
                    rawVNode.ssContent = vnode;
                }
            }
            // #1513 it's possible for the returned vnode to be cloned due to attr
            // fallthrough or scopeId, so the vnode here may not be the final vnode
            // that is mounted. Instead of caching it directly, we store the pending
            // key and cache `instance.subTree` (the normalized vnode) in
            // beforeMount/beforeUpdate hooks.
            pendingCacheKey = key;
            if (cachedVNode) {
                // copy over mounted state
                vnode.el = cachedVNode.el;
                vnode.component = cachedVNode.component;
                if (vnode.transition) {
                    // recursively update transition hooks on subTree
                    setTransitionHooks(vnode, vnode.transition);
                }
                // avoid vnode being mounted as fresh
                vnode.shapeFlag |= 512 /* COMPONENT_KEPT_ALIVE */;
                // make this key the freshest
                keys.delete(key);
                keys.add(key);
            }
            else {
                keys.add(key);
                // prune oldest entry
                if (max && keys.size > parseInt(max, 10)) {
                    pruneCacheEntry(keys.values().next().value);
                }
            }
            // avoid vnode being unmounted
            vnode.shapeFlag |= 256 /* COMPONENT_SHOULD_KEEP_ALIVE */;
            current = vnode;
            return isSuspense(rawVNode.type) ? rawVNode : vnode;
        };
    }
};