Scheduler Polyfill

This is a polyfill for the Prioritized Task Scheduling API. Documentation can be found on MDN.

The polyfill includes implementations of Scheduler, exposed through self.scheduler, as well as TaskController and TaskPriorityChangeEvent classes.

scheduler.postTask()

The implementation uses a combination of setTimeout, MessageChannel, and requestIdleCallback to implement task scheduling, falling back to setTimeout when other APIs are not available.

The polyfill, like the native implementation, runs scheduler tasks in descending priority order ('user-blocking' > 'user-visible' > 'background'). But there are some differences in the relative order of non-scheduler tasks:

• "background" tasks are scheduled using requestIdleCallback on browsers that support it, which provides similar scheduling as scheduler. For browsers that don't support it, these tasks do not have low/idle event loop priority.

• "user-blocking" tasks have the same event loop scheduling prioritization as "user-visible" (similar to setTimeout()), meaning these tasks do not have a higher event loop priority.

scheduler.yield()

The polyfill does not support priority or signal inheritance, so all continuations are scheduled with "user-visible" continuation priority. The scheduling behavior of this depends on whether the browser supports scheduler.postTask() (i.e. older Chrome versions):

• For browsers that support scheduler.postTask(), scheduler.yield() is polyfilled with "user-blocking" scheduler.postTask() tasks. This means they typically have a higher event loop priority than other tasks (consistent with yield()), but they can be interleaved with other "user-blocking" tasks.

• On browsers that don't support scheduler.postTask(), the same event loop prioritization as the postTask() polyfill applies (see above), but continuations run between "user-blocking" and "user-visible" tasks.

Requirements

A browser that supports ES6 is required for this polyfill.

Usage

Include via npm and a bundler

npm install scheduler-polyfill

Import to populate the task-scheduling global variables, if not already available in the executing browser:

import 'scheduler-polyfill';

Include via unpkg

<script src="https://unpkg.com/scheduler-polyfill"></script>

Building from source

git clone https://github.com/GoogleChromeLabs/scheduler-polyfill
cd scheduler-polyfill
npm i
npm test        # Tests should pass
npm run build   # Outputs minified polyfill to dist/

<script src="/path_to_polyfill/scheduler-polyfill.js"></script>

License

Apache 2.0