Vue nextTick

Vue2 实现（Vue2.7）

Vue2 中，nextTick 的源码在 /src/core/util/next-tick.ts 下面：

// Here we have async deferring wrappers using microtasks.
// In 2.5 we used (macro) tasks (in combination with microtasks).
// However, it has subtle problems when state is changed right before repaint
// (e.g. #6813, out-in transitions).
// Also, using (macro) tasks in event handler would cause some weird behaviors
// that cannot be circumvented (e.g. #7109, #7153, #7546, #7834, #8109).
// So we now use microtasks everywhere, again.
// A major drawback of this tradeoff is that there are some scenarios
// where microtasks have too high a priority and fire in between supposedly
// sequential events (e.g. #4521, #6690, which have workarounds)
// or even between bubbling of the same event (#6566).
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)
  }
  isUsingMicroTask = true
} 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)
  }
  isUsingMicroTask = true
} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
  // Fallback to setImmediate.
  // Technically 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)
  }
}

总结一下，在 Vue2 中 nextTick 是使用微任务来实现的，在 2.5 中，nextTick 使用宏任务和微任务相结合的方式，但是会有一些奇怪的问题，因此在 2.5 之后再次在所有地方使用微任务。

Vue2 不支持 IE8 及以下版本，因为 Vue2 使用了 IE8 无法模拟的 ECMAScript 5 特性。但它支持所有兼容 ECMAScript 5 的浏览器。

降级流程如下：Promise -> MutationOvserver -> setImmediate -> setTimeout

Vue3 实现（Vue3.3.11）

最新版本的 Vue (3.x) 只支持原生支持 ES2015 的浏览器。这并不包括 IE11。Vue 3.x 使用的 ES2015 功能无法在旧版本的浏览器中进行兼容，如果你需要支持旧版本的浏览器，请使用 Vue 2.x 替代。

Vue3 中，由于浏览器兼容性更新，所以默认所有使用 Vue3 的环境都原生支持 ES2015（即 ES6）的特性。正是如此，Vue3 中的 nextTick 实现更为简单，直接使用了 Promise 来实现，没有其他的降级方法。

export function nextTick<T = void, R = void>(
  this: T,
  fn?: (this: T) => R
): Promise<Awaited<R>> {
  const p = currentFlushPromise || resolvedPromise
  return fn ? p.then(this ? fn.bind(this) : fn) : p
}