/* @flow */
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/* globals MutationObserver */
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import { noop } from 'shared/util'
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import { handleError } from './error'
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import { isIE, isIOS, isNative } from './env'
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export let isUsingMicroTask = false
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const callbacks = []
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let pending = false
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function flushCallbacks () {
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pending = false
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const copies = callbacks.slice(0)
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callbacks.length = 0
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for (let i = 0; i < copies.length; i++) {
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copies[i]()
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}
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}
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// Here we have async deferring wrappers using microtasks.
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// In 2.5 we used (macro) tasks (in combination with microtasks).
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// However, it has subtle problems when state is changed right before repaint
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// (e.g. #6813, out-in transitions).
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// Also, using (macro) tasks in event handler would cause some weird behaviors
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// that cannot be circumvented (e.g. #7109, #7153, #7546, #7834, #8109).
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// So we now use microtasks everywhere, again.
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// A major drawback of this tradeoff is that there are some scenarios
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// where microtasks have too high a priority and fire in between supposedly
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// sequential events (e.g. #4521, #6690, which have workarounds)
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// or even between bubbling of the same event (#6566).
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let timerFunc
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// The nextTick behavior leverages the microtask queue, which can be accessed
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// via either native Promise.then or MutationObserver.
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// MutationObserver has wider support, however it is seriously bugged in
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// UIWebView in iOS >= 9.3.3 when triggered in touch event handlers. It
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// completely stops working after triggering a few times... so, if native
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// Promise is available, we will use it:
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/* istanbul ignore next, $flow-disable-line */
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if (typeof Promise !== 'undefined' && isNative(Promise)) {
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const p = Promise.resolve()
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timerFunc = () => {
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p.then(flushCallbacks)
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// In problematic UIWebViews, Promise.then doesn't completely break, but
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// it can get stuck in a weird state where callbacks are pushed into the
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// microtask queue but the queue isn't being flushed, until the browser
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// needs to do some other work, e.g. handle a timer. Therefore we can
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// "force" the microtask queue to be flushed by adding an empty timer.
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if (isIOS) setTimeout(noop)
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}
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isUsingMicroTask = true
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} else if (!isIE && typeof MutationObserver !== 'undefined' && (
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isNative(MutationObserver) ||
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// PhantomJS and iOS 7.x
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MutationObserver.toString() === '[object MutationObserverConstructor]'
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)) {
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// Use MutationObserver where native Promise is not available,
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// e.g. PhantomJS, iOS7, Android 4.4
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// (#6466 MutationObserver is unreliable in IE11)
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let counter = 1
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const observer = new MutationObserver(flushCallbacks)
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const textNode = document.createTextNode(String(counter))
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observer.observe(textNode, {
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characterData: true
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})
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timerFunc = () => {
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counter = (counter + 1) % 2
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textNode.data = String(counter)
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}
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isUsingMicroTask = true
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} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
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// Fallback to setImmediate.
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// Technically it leverages the (macro) task queue,
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// but it is still a better choice than setTimeout.
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timerFunc = () => {
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setImmediate(flushCallbacks)
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}
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} else {
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// Fallback to setTimeout.
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timerFunc = () => {
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setTimeout(flushCallbacks, 0)
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}
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}
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export function nextTick (cb?: Function, ctx?: Object) {
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let _resolve
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callbacks.push(() => {
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if (cb) {
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try {
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cb.call(ctx)
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} catch (e) {
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handleError(e, ctx, 'nextTick')
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}
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} else if (_resolve) {
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_resolve(ctx)
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}
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})
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if (!pending) {
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pending = true
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timerFunc()
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}
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// $flow-disable-line
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if (!cb && typeof Promise !== 'undefined') {
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return new Promise(resolve => {
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_resolve = resolve
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})
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}
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}
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