上一节vue 响应式原理解析我们提到当数据发生变化时，会触发所有组件Watcher的update方法，下面我们进入源码分析
update () {
/* istanbul ignore else */
if (this.lazy) {
this.dirty = true
} else if (this.sync) {
this.run()
} else {
queueWatcher(this)
}
}
这是在Watcher类中定义的update方法，数据修改时，执行queueWatcher函数，我们进入queueWatcher，
export function queueWatcher (watcher: Watcher) {
const id = watcher.id
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(flushSchedulerQueue)
}
}
}
可以看到，函数开头会对当前组件Watcher进行判断，当我们在写vue代码时我们可能会有疑问，如果我们连续改变data中的某个值两次，或者我们连续改变几个data中的值，组件会重新渲染几次呢，在这里我们可以看到，只会渲染一次吗，因为之后的此组件重新渲染会在这里被阻断掉。
进行重新渲染时会把所有的组件Watcher和用户定义的Watcher都添加到queue队列中，最后执行nextTick(flushSchedulerQueue)，通过nextTick我们可以想到常用的$nextTick函数就是这个函数，我们进入nextTick函数
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
timerFunc()
}
// $flow-disable-line
if (!cb && typeof Promise !== 'undefined') {
return new Promise(resolve => {
_resolve = resolve
})
}
}
重点是tmierFunc的执行，我们来看timerFunc的定义
let timerFunc
// The nextTick behavior leverages the microtask queue, which can be accessed
// via either native Promise.then or MutationObserver.
// MutationObserver has wider support, however it is seriously bugged in
// UIWebView in iOS >= 9.3.3 when triggered in touch event handlers. It
// completely stops working after triggering a few times... so, if native
// Promise is available, we will use it:
/* istanbul ignore next, $flow-disable-line */
if (typeof Promise !== 'undefined' && isNative(Promise)) {
const p = Promise.resolve()
timerFunc = () => {
p.then(flushCallbacks)
// In problematic UIWebViews, Promise.then doesn't completely break, but
// it can get stuck in a weird state where callbacks are pushed into the
// microtask queue but the queue isn't being flushed, until the browser
// needs to do some other work, e.g. handle a timer. Therefore we can
// "force" the microtask queue to be flushed by adding an empty timer.
if (isIOS) setTimeout(noop)
}
} else if (!isIE && typeof MutationObserver !== 'undefined' && (
isNative(MutationObserver) ||
// PhantomJS and iOS 7.x
MutationObserver.toString() === '[object MutationObserverConstructor]'
)) {
// Use MutationObserver where native Promise is not available,
// e.g. PhantomJS, iOS7, Android 4.4
// (#6466 MutationObserver is unreliable in IE11)
let counter = 1
const observer = new MutationObserver(flushCallbacks)
const textNode = document.createTextNode(String(counter))
observer.observe(textNode, {
characterData: true
})
timerFunc = () => {
counter = (counter + 1) % 2
textNode.data = String(counter)
}
} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
// Fallback to setImmediate.
// Techinically it leverages the (macro) task queue,
// but it is still a better choice than setTimeout.
timerFunc = () => {
setImmediate(flushCallbacks)
}
} else {
// Fallback to setTimeout.
timerFunc = () => {
setTimeout(flushCallbacks, 0)
}
}
可以看到这时一个执行顺序的问题，我们知道在浏览器js运行环境中有microtask和macrotask，两种方式都是在执行完js环境中代码后最后运行，mic在mac前面，这里就是把nextTick执行的代码放入到这两个其中一个环境中，具体哪个和当前使用的浏览器有关，总之是所有其他同步代码执行完毕后执行，最后看赋值给timerFunc的flushCallbacks
function flushCallbacks () {
pending = false
const copies = callbacks.slice(0)
callbacks.length = 0
for (let i = 0; i < copies.length; i++) {
copies[i]()
}
}
将多次调用nextTick中传入callbacks数组的函数循环执行，执行的也就是queueWatcher中的flushSchedulerQueue
我们进入flushSchedulerQueue的定义
function flushSchedulerQueue () {
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
for (index = 0; index < queue.length; index++) {
watcher = queue[index]
if (watcher.before) {
watcher.before()
}
id = watcher.id
has[id] = null
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
}
}
}
// keep copies of post queues before resetting state
const activatedQueue = activatedChildren.slice()
const updatedQueue = queue.slice()
resetSchedulerState()
// call component updated and activated hooks
callActivatedHooks(activatedQueue)
callUpdatedHooks(updatedQueue)
// devtool hook
/* istanbul ignore if */
if (devtools && config.devtools) {
devtools.emit('flush')
}
}
对update中的Watcher队列进行排序，排序原因注释也写得很清楚，执行组件Watcher的beforeUpdate钩子，执行Watcher.run方法，
run方法中执行Wathcer的get方法，get方法执行getter函数，又回到了之前vm._update(vm._render(), hydrating) 这里就会触发一系列的render函数，patch，createElem等等完成组件的重新渲染，
通过这两节的介绍我们可以清楚的看到vue 响应式原理
