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II. Built-in API

The run-loop could be built into the browser and integrated closely with the browser’s event-loop. This would automatically move #4 into the browser and #3 becomes the platform exposed API. The API sketch follows.

We propose adding default task queues with three semantic priorities, i.e. enum TaskQueuePriority, can be one of these:

  • Immediate
  • Render-blocking
  • Default
  • Idle

Global set of Serial Task queues

Tasks are guaranteed to start and finish in the order submitted, i.e. a task does not start until the previous task has completed.

A set of global serial task queues will be made available to post work on main thread. There will be a global queue for each priority level.

API for posting & canceling tasks

NOTE: syntax is likely to change for compatibility for posting work off main thread variant (TODO: Link to repo).

function mytask() {
  ...
}

myQueue = TaskQueue.default("render-blocking")

returns the global task queue with priority “render-blocking”, for posting to main thread.

taskId = myQueue.postTask(myTask, <list of args>);

where myTask is a callback. The return value is a long integer, the task id, that uniquely identifies the entry in the queue.

myQueue.cancelTask(taskId);

taskId can be used to later cancel the task.

NOTE: The return type could be a promise instead of ID, for convenience and chaining of tasks. If so, then AbortController is a standard way to cancel pending promises.

User-defined task queues

Developers can define their own “virtual” serial task queues:

myQueue = new TaskQueue(‘myCustomQueue’, "default");
myQueue.postTask(task, <list of args for task>);

where task is a function.

User-defined task queues enables the app to manage a group of tasks, such as updating priority, canceling, draining (synchronously executing tasks) when the page is hidden etc.

Updating priority of entire queue: myQueue.updatePriority(<new priority>); Updating priority of specific task is equivalent to canceling that task and re-posting at a different priority. Flushing the queue: myQueue.flush();

TODO: supporting additional priorities, beyond small set of semantic priorities. Add API ideas for this.

Open Questions & Challenges

  • DOM read-write phase: enabling tasks to target read vs write phases
  • API for frame rate throttling: 30 vs. 60fps
  • handling 3P and non-cooperating script (directly embedded) in the page
  • lowering priority of event handlers (similar to “passive”)