"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