I was going to respond via github to this issue comment, but I figured the
answer probably was of interest to a broader audience. So go and read
https://github.com/Raynos/observ-hash/issues/2#issuecomment-35857671 and then
continue.
Rev22 of the ES6 draft introduced the concepts of Tasks and TaskQueues:
http://people.mozilla.org/~jorendorff/es6-draft.html#sec-tasks-and-task-queues.
A Task is a non-interruptable, run to completion activity within the ECMAScript
execution environment that typically includes the evaluation of ECMAScript
code. A TaskQueue is a FIFO queue of Tasks that are waiting to run. Tasks were
added to ES6 in support of Promises. They are also useful for describing the
execution sequences of top level scripts and hopefully other features that may
be added post ES6.
Rev22 defines two built-in tasks queues A queue for promise tasks and a queue
for script evaluation tasks. Other task queues may be added in the future and
an host platform may defined additional task queues and new kinds of tasks.
The ES6 spec. does not define a complete scheduling algorithm for tasks,
instead it says:
The PendingTask records from a single Task Queue are always initiated in FIFO
order. This specification does not define the order in which multiple Task
Queues are serviced. An ECMAScript implementation may interweave the FIFO
evaluation of the PendingTask records of a Task Queue with the evaluation of
the PendingTask records of one or more other Task Queues. An implementation
must define what occurs when there are no running execution context and all
Task Queues are empty.
Are Tasks the same as HTML Micro-tasks? Why are these called Tasks and not
micro-tasks.
I intentionally didn't use the term "micro-task" to avoid confusion with HTML's
use of that term. It's probably best to talk about the things called "Tasks"
in the ES6 spec. as "ECMAScript Tasks". "Micro-task" (and "Task", I believe)
have specific meaning and semantics within the context of HTML. I don't want
to build the HTML semantics into the ES spec. because ES is also used in other
platforms besides the browser/HTML environment. The expectation is that such
platforms can use ECMAScript Tasks, TaskQueues, and platform specific
scheduling rules to define the behavior of activities they push to the ES
execution environment.
Are ECMAScript 6 Tasks insufficient support the HTML or other browser
requirements?
Not as far as I know, but please let me know if you think something interferes
with those requirements. ES Tasks are not intended to support all activity
scheduling that might take place in a complex platform but only activities that
involve the synchronous (to the activity) execution of ES code. Task queues
are used to enforce necessary relative serialization of certain activities.
The set of available task queues, their sizes (could be 1), and their relative
scheduling order are left completely up to the host platform. Also, there is
nothing that says a platform can't have another higher level scheduling
mechanism (I'm thinking about mutator events here) that is used to populate ES
Task Queues. ES Tasks/TaskQueues are so basic it hard for me to see how they
would limit a host platform. But, let me know if you see a problem.
Do ES Tasks have a ES API?
Not currently. It would be easy enough to define an API for enqueing the
invocation of an ES function as a ES Task. Also you can imagine APIs for
defining TaskQueues and setting scheduling policies for TaskQueues. That is
all post ES6 work and requires somebody to develop proposals and champion them.
If HTML (or any other platform) is considering defining a capability like that
(which isn't in other ways semantically couple to that platform) it would be
nice to see it done as a ES feature proposal.
Allen
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