forked from svrjs/svrjs
223 lines
9.4 KiB
JavaScript
223 lines
9.4 KiB
JavaScript
"use strict";
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// Use the fastest means possible to execute a task in its own turn, with
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// priority over other events including IO, animation, reflow, and redraw
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// events in browsers.
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//
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// An exception thrown by a task will permanently interrupt the processing of
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// subsequent tasks. The higher level `asap` function ensures that if an
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// exception is thrown by a task, that the task queue will continue flushing as
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// soon as possible, but if you use `rawAsap` directly, you are responsible to
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// either ensure that no exceptions are thrown from your task, or to manually
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// call `rawAsap.requestFlush` if an exception is thrown.
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module.exports = rawAsap;
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function rawAsap(task) {
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if (!queue.length) {
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requestFlush();
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flushing = true;
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}
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// Equivalent to push, but avoids a function call.
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queue[queue.length] = task;
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}
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var queue = [];
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// Once a flush has been requested, no further calls to `requestFlush` are
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// necessary until the next `flush` completes.
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var flushing = false;
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// `requestFlush` is an implementation-specific method that attempts to kick
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// off a `flush` event as quickly as possible. `flush` will attempt to exhaust
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// the event queue before yielding to the browser's own event loop.
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var requestFlush;
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// The position of the next task to execute in the task queue. This is
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// preserved between calls to `flush` so that it can be resumed if
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// a task throws an exception.
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var index = 0;
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// If a task schedules additional tasks recursively, the task queue can grow
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// unbounded. To prevent memory exhaustion, the task queue will periodically
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// truncate already-completed tasks.
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var capacity = 1024;
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// The flush function processes all tasks that have been scheduled with
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// `rawAsap` unless and until one of those tasks throws an exception.
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// If a task throws an exception, `flush` ensures that its state will remain
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// consistent and will resume where it left off when called again.
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// However, `flush` does not make any arrangements to be called again if an
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// exception is thrown.
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function flush() {
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while (index < queue.length) {
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var currentIndex = index;
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// Advance the index before calling the task. This ensures that we will
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// begin flushing on the next task the task throws an error.
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index = index + 1;
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queue[currentIndex].call();
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// Prevent leaking memory for long chains of recursive calls to `asap`.
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// If we call `asap` within tasks scheduled by `asap`, the queue will
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// grow, but to avoid an O(n) walk for every task we execute, we don't
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// shift tasks off the queue after they have been executed.
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// Instead, we periodically shift 1024 tasks off the queue.
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if (index > capacity) {
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// Manually shift all values starting at the index back to the
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// beginning of the queue.
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for (var scan = 0, newLength = queue.length - index; scan < newLength; scan++) {
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queue[scan] = queue[scan + index];
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}
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queue.length -= index;
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index = 0;
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}
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}
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queue.length = 0;
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index = 0;
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flushing = false;
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}
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// `requestFlush` is implemented using a strategy based on data collected from
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// every available SauceLabs Selenium web driver worker at time of writing.
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// https://docs.google.com/spreadsheets/d/1mG-5UYGup5qxGdEMWkhP6BWCz053NUb2E1QoUTU16uA/edit#gid=783724593
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// Safari 6 and 6.1 for desktop, iPad, and iPhone are the only browsers that
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// have WebKitMutationObserver but not un-prefixed MutationObserver.
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// Must use `global` or `self` instead of `window` to work in both frames and web
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// workers. `global` is a provision of Browserify, Mr, Mrs, or Mop.
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/* globals self */
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var scope = typeof global !== "undefined" ? global : self;
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var BrowserMutationObserver = scope.MutationObserver || scope.WebKitMutationObserver;
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// MutationObservers are desirable because they have high priority and work
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// reliably everywhere they are implemented.
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// They are implemented in all modern browsers.
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//
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// - Android 4-4.3
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// - Chrome 26-34
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// - Firefox 14-29
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// - Internet Explorer 11
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// - iPad Safari 6-7.1
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// - iPhone Safari 7-7.1
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// - Safari 6-7
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if (typeof BrowserMutationObserver === "function") {
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requestFlush = makeRequestCallFromMutationObserver(flush);
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// MessageChannels are desirable because they give direct access to the HTML
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// task queue, are implemented in Internet Explorer 10, Safari 5.0-1, and Opera
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// 11-12, and in web workers in many engines.
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// Although message channels yield to any queued rendering and IO tasks, they
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// would be better than imposing the 4ms delay of timers.
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// However, they do not work reliably in Internet Explorer or Safari.
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// Internet Explorer 10 is the only browser that has setImmediate but does
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// not have MutationObservers.
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// Although setImmediate yields to the browser's renderer, it would be
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// preferrable to falling back to setTimeout since it does not have
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// the minimum 4ms penalty.
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// Unfortunately there appears to be a bug in Internet Explorer 10 Mobile (and
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// Desktop to a lesser extent) that renders both setImmediate and
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// MessageChannel useless for the purposes of ASAP.
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// https://github.com/kriskowal/q/issues/396
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// Timers are implemented universally.
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// We fall back to timers in workers in most engines, and in foreground
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// contexts in the following browsers.
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// However, note that even this simple case requires nuances to operate in a
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// broad spectrum of browsers.
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//
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// - Firefox 3-13
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// - Internet Explorer 6-9
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// - iPad Safari 4.3
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// - Lynx 2.8.7
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} else {
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requestFlush = makeRequestCallFromTimer(flush);
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}
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// `requestFlush` requests that the high priority event queue be flushed as
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// soon as possible.
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// This is useful to prevent an error thrown in a task from stalling the event
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// queue if the exception handled by Node.js’s
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// `process.on("uncaughtException")` or by a domain.
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rawAsap.requestFlush = requestFlush;
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// To request a high priority event, we induce a mutation observer by toggling
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// the text of a text node between "1" and "-1".
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function makeRequestCallFromMutationObserver(callback) {
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var toggle = 1;
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var observer = new BrowserMutationObserver(callback);
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var node = document.createTextNode("");
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observer.observe(node, {characterData: true});
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return function requestCall() {
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toggle = -toggle;
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node.data = toggle;
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};
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}
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// The message channel technique was discovered by Malte Ubl and was the
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// original foundation for this library.
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// http://www.nonblocking.io/2011/06/windownexttick.html
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// Safari 6.0.5 (at least) intermittently fails to create message ports on a
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// page's first load. Thankfully, this version of Safari supports
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// MutationObservers, so we don't need to fall back in that case.
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// function makeRequestCallFromMessageChannel(callback) {
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// var channel = new MessageChannel();
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// channel.port1.onmessage = callback;
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// return function requestCall() {
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// channel.port2.postMessage(0);
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// };
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// }
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// For reasons explained above, we are also unable to use `setImmediate`
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// under any circumstances.
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// Even if we were, there is another bug in Internet Explorer 10.
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// It is not sufficient to assign `setImmediate` to `requestFlush` because
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// `setImmediate` must be called *by name* and therefore must be wrapped in a
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// closure.
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// Never forget.
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// function makeRequestCallFromSetImmediate(callback) {
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// return function requestCall() {
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// setImmediate(callback);
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// };
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// }
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// Safari 6.0 has a problem where timers will get lost while the user is
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// scrolling. This problem does not impact ASAP because Safari 6.0 supports
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// mutation observers, so that implementation is used instead.
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// However, if we ever elect to use timers in Safari, the prevalent work-around
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// is to add a scroll event listener that calls for a flush.
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// `setTimeout` does not call the passed callback if the delay is less than
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// approximately 7 in web workers in Firefox 8 through 18, and sometimes not
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// even then.
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function makeRequestCallFromTimer(callback) {
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return function requestCall() {
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// We dispatch a timeout with a specified delay of 0 for engines that
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// can reliably accommodate that request. This will usually be snapped
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// to a 4 milisecond delay, but once we're flushing, there's no delay
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// between events.
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var timeoutHandle = setTimeout(handleTimer, 0);
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// However, since this timer gets frequently dropped in Firefox
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// workers, we enlist an interval handle that will try to fire
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// an event 20 times per second until it succeeds.
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var intervalHandle = setInterval(handleTimer, 50);
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function handleTimer() {
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// Whichever timer succeeds will cancel both timers and
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// execute the callback.
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clearTimeout(timeoutHandle);
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clearInterval(intervalHandle);
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callback();
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}
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};
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}
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// This is for `asap.js` only.
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// Its name will be periodically randomized to break any code that depends on
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// its existence.
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rawAsap.makeRequestCallFromTimer = makeRequestCallFromTimer;
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// ASAP was originally a nextTick shim included in Q. This was factored out
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// into this ASAP package. It was later adapted to RSVP which made further
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// amendments. These decisions, particularly to marginalize MessageChannel and
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// to capture the MutationObserver implementation in a closure, were integrated
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// back into ASAP proper.
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// https://github.com/tildeio/rsvp.js/blob/cddf7232546a9cf858524b75cde6f9edf72620a7/lib/rsvp/asap.js
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