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

$3

``console
npm install scheduler-polyfill
`

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

`js
import 'scheduler-polyfill';
`

$3

`html

`

$3

`console
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/
`

`html

``

License

Apache 2.0