browser-raw.js

"use strict";

// Use the fastest means possible to execute a task in its own turn, with
// priority over other events including IO, animation, reflow, and redraw
// events in browsers.
//
// An exception thrown by a task will permanently interrupt the processing of
// subsequent tasks. The higher level `asap` function ensures that if an
// exception is thrown by a task, that the task queue will continue flushing as
// soon as possible, but if you use `rawAsap` directly, you are responsible to
// either ensure that no exceptions are thrown from your task, or to manually
// call `rawAsap.requestFlush` if an exception is thrown.
module.exports = rawAsap;
function rawAsap(task) {
    if (!queue.length) {
        requestFlush();
        flushing = true;
    }
    // Equivalent to push, but avoids a function call.
    queue[queue.length] = task;
}

var queue = [];
// Once a flush has been requested, no further calls to `requestFlush` are
// necessary until the next `flush` completes.
var flushing = false;
// `requestFlush` is an implementation-specific method that attempts to kick
// off a `flush` event as quickly as possible. `flush` will attempt to exhaust
// the event queue before yielding to the browser's own event loop.
var requestFlush;
// The position of the next task to execute in the task queue. This is
// preserved between calls to `flush` so that it can be resumed if
// a task throws an exception.
var index = 0;
// If a task schedules additional tasks recursively, the task queue can grow
// unbounded. To prevent memory exhaustion, the task queue will periodically
// truncate already-completed tasks.
var capacity = 1024;

// The flush function processes all tasks that have been scheduled with
// `rawAsap` unless and until one of those tasks throws an exception.
// If a task throws an exception, `flush` ensures that its state will remain
// consistent and will resume where it left off when called again.
// However, `flush` does not make any arrangements to be called again if an
// exception is thrown.
function flush() {
    while (index < queue.length) {
        var currentIndex = index;
        // Advance the index before calling the task. This ensures that we will
        // begin flushing on the next task the task throws an error.
        index = index + 1;
        queue[currentIndex].call();
        // Prevent leaking memory for long chains of recursive calls to `asap`.
        // If we call `asap` within tasks scheduled by `asap`, the queue will
        // grow, but to avoid an O(n) walk for every task we execute, we don't
        // shift tasks off the queue after they have been executed.
        // Instead, we periodically shift 1024 tasks off the queue.
        if (index > capacity) {
            // Manually shift all values starting at the index back to the
            // beginning of the queue.
            for (var scan = 0, newLength = queue.length - index; scan < newLength; scan++) {
                queue[scan] = queue[scan + index];
            }
            queue.length -= index;
            index = 0;
        }
    }
    queue.length = 0;
    index = 0;
    flushing = false;
}

// `requestFlush` is implemented using a strategy based on data collected from
// every available SauceLabs Selenium web driver worker at time of writing.
// https://docs.google.com/spreadsheets/d/1mG-5UYGup5qxGdEMWkhP6BWCz053NUb2E1QoUTU16uA/edit#gid=783724593

// Safari 6 and 6.1 for desktop, iPad, and iPhone are the only browsers that
// have WebKitMutationObserver but not un-prefixed MutationObserver.
// Must use `global` or `self` instead of `window` to work in both frames and web
// workers. `global` is a provision of Browserify, Mr, Mrs, or Mop.

/* globals self */
var scope = typeof global !== "undefined" ? global : self;
var BrowserMutationObserver = scope.MutationObserver || scope.WebKitMutationObserver;

// MutationObservers are desirable because they have high priority and work
// reliably everywhere they are implemented.
// They are implemented in all modern browsers.
//
// - Android 4-4.3
// - Chrome 26-34
// - Firefox 14-29
// - Internet Explorer 11
// - iPad Safari 6-7.1
// - iPhone Safari 7-7.1
// - Safari 6-7
if (typeof BrowserMutationObserver === "function") {
    requestFlush = makeRequestCallFromMutationObserver(flush);

// MessageChannels are desirable because they give direct access to the HTML
// task queue, are implemented in Internet Explorer 10, Safari 5.0-1, and Opera
// 11-12, and in web workers in many engines.
// Although message channels yield to any queued rendering and IO tasks, they
// would be better than imposing the 4ms delay of timers.
// However, they do not work reliably in Internet Explorer or Safari.

// Internet Explorer 10 is the only browser that has setImmediate but does
// not have MutationObservers.
// Although setImmediate yields to the browser's renderer, it would be
// preferrable to falling back to setTimeout since it does not have
// the minimum 4ms penalty.
// Unfortunately there appears to be a bug in Internet Explorer 10 Mobile (and
// Desktop to a lesser extent) that renders both setImmediate and
// MessageChannel useless for the purposes of ASAP.
// https://github.com/kriskowal/q/issues/396

// Timers are implemented universally.
// We fall back to timers in workers in most engines, and in foreground
// contexts in the following browsers.
// However, note that even this simple case requires nuances to operate in a
// broad spectrum of browsers.
//
// - Firefox 3-13
// - Internet Explorer 6-9
// - iPad Safari 4.3
// - Lynx 2.8.7
} else {
    requestFlush = makeRequestCallFromTimer(flush);
}

// `requestFlush` requests that the high priority event queue be flushed as
// soon as possible.
// This is useful to prevent an error thrown in a task from stalling the event
// queue if the exception handled by Node.js’s
// `process.on("uncaughtException")` or by a domain.
rawAsap.requestFlush = requestFlush;

// To request a high priority event, we induce a mutation observer by toggling
// the text of a text node between "1" and "-1".
function makeRequestCallFromMutationObserver(callback) {
    var toggle = 1;
    var observer = new BrowserMutationObserver(callback);
    var node = document.createTextNode("");
    observer.observe(node, {characterData: true});
    return function requestCall() {
        toggle = -toggle;
        node.data = toggle;
    };
}

// The message channel technique was discovered by Malte Ubl and was the
// original foundation for this library.
// http://www.nonblocking.io/2011/06/windownexttick.html

// Safari 6.0.5 (at least) intermittently fails to create message ports on a
// page's first load. Thankfully, this version of Safari supports
// MutationObservers, so we don't need to fall back in that case.

// function makeRequestCallFromMessageChannel(callback) {
//     var channel = new MessageChannel();
//     channel.port1.onmessage = callback;
//     return function requestCall() {
//         channel.port2.postMessage(0);
//     };
// }

// For reasons explained above, we are also unable to use `setImmediate`
// under any circumstances.
// Even if we were, there is another bug in Internet Explorer 10.
// It is not sufficient to assign `setImmediate` to `requestFlush` because
// `setImmediate` must be called *by name* and therefore must be wrapped in a
// closure.
// Never forget.

// function makeRequestCallFromSetImmediate(callback) {
//     return function requestCall() {
//         setImmediate(callback);
//     };
// }

// Safari 6.0 has a problem where timers will get lost while the user is
// scrolling. This problem does not impact ASAP because Safari 6.0 supports
// mutation observers, so that implementation is used instead.
// However, if we ever elect to use timers in Safari, the prevalent work-around
// is to add a scroll event listener that calls for a flush.

// `setTimeout` does not call the passed callback if the delay is less than
// approximately 7 in web workers in Firefox 8 through 18, and sometimes not
// even then.

function makeRequestCallFromTimer(callback) {
    return function requestCall() {
        // We dispatch a timeout with a specified delay of 0 for engines that
        // can reliably accommodate that request. This will usually be snapped
        // to a 4 milisecond delay, but once we're flushing, there's no delay
        // between events.
        var timeoutHandle = setTimeout(handleTimer, 0);
        // However, since this timer gets frequently dropped in Firefox
        // workers, we enlist an interval handle that will try to fire
        // an event 20 times per second until it succeeds.
        var intervalHandle = setInterval(handleTimer, 50);

        function handleTimer() {
            // Whichever timer succeeds will cancel both timers and
            // execute the callback.
            clearTimeout(timeoutHandle);
            clearInterval(intervalHandle);
            callback();
        }
    };
}

// This is for `asap.js` only.
// Its name will be periodically randomized to break any code that depends on
// its existence.
rawAsap.makeRequestCallFromTimer = makeRequestCallFromTimer;

// ASAP was originally a nextTick shim included in Q. This was factored out
// into this ASAP package. It was later adapted to RSVP which made further
// amendments. These decisions, particularly to marginalize MessageChannel and
// to capture the MutationObserver implementation in a closure, were integrated
// back into ASAP proper.
// https://github.com/tildeio/rsvp.js/blob/cddf7232546a9cf858524b75cde6f9edf72620a7/lib/rsvp/asap.js