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- /* @flow */
- /* globals MutationObserver */
- import { noop } from 'shared/util'
- import { handleError } from './error'
- import { isIE, isIOS, isNative } from './env'
- export let isUsingMicroTask = false
- const callbacks = []
- let pending = false
- function flushCallbacks () {
- pending = false
- const copies = callbacks.slice(0)
- callbacks.length = 0
- for (let i = 0; i < copies.length; i++) {
- copies[i]()
- }
- }
- // 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)
- }
- }
- 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
- })
- }
- }
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