Worklets Level 1

1. Introduction

1.1. Motivations

This section is not normative.

Allowing extension points defined in the document environment is difficult, as rendering engines would need to abandon previously held assumptions for what could happen in the middle of a phase.

For example, during the layout phase the rendering engine assumes that no DOM will be modified.

Additionally defining extension points in the document environment would restrict rendering engines to performing work in the same thread as the document environment. (Unless rendering engines added complex, high-overhead infrastructure to allow thread-safe APIs in addition to thread joining guarantees).

The worklet is designed to allow such extension points in rendering engines, while keeping guarantees which rendering engines rely currently on.

Worklets are similar to web workers however they:

• Are thread-agnostic. That is, they are not defined to run on a particular thread. Rendering engines may run them wherever they choose.

• Are able to have multiple duplicate instances of the global scope created for the purpose of parallelism.

• Are not event API based. Instead classes are registered on the global scope, whose methods are to be invoked by the user agent.

• Have a reduced API surface on the global scope.

• Have a lifetime for the global scope which is defined by subsequent specifications or user agents. They aren’t tied to the lifetime of the document.

As worklets have a relatively high overhead, they should be used sparingly. Due to this worklets are expected to be shared between separate scripts. This is similar to the document environment.

1.2. Code Idempotency

This section is not normative.

Multiple instances of WorkletGlobalScope can be created for each Worklet that they belong to. User agents may choose to do this in order to parallelize work over multiple threads, or to move work between threads as required.

Additionally different user agents may invoke a method on a class in a different order to other user agents.

As a result of this, to prevent this compatibility risk between user agents, authors who register classes on the global scope should make their code idempotent. That is, a method or set of methods on a class should produce the same output given a particular input.

The following techniques should be used in order to encourage authors to write code in an idempotent way:

• No reference to the global object, e.g. self on a DedicatedWorkerGlobalScope.

• Code is loaded as a module script which resulting in the code being executed in strict mode code without a shared this. This prevents two different module scripts sharing state be referencing shared objects on the global scope.

• User agents may choose to always have at least two WorkletGlobalScopes per Worklet and randomly assign a method or set of methods on a class to a particular global scope.

• User agents may create and destroy WorkletGlobalScopes at any time.

2. Infrastructure

2.1. The Global Scope

The WorkletGlobalScope object provides a worklet global scope which represents the global execution context of a Worklet.

[Exposed=Worklet]
interface WorkletGlobalScope {
};

Each WorkletGlobalScope has an associated environment settings object.

Each WorkletGlobalScope has a worklet global scope execution environment. This execution environment may be parallel (i.e. it may be on a separate thread, process, or other equivalent construct), or it may live on the same thread or process as the Worklet object it belongs to. Which thread or process it lives on is decided by the user agent.

Note: The WorkletGlobalScope has a limited global scope when compared to a DedicatedWorkerGlobalScope. It is expected that other specifications will extend WorkletGlobalScope with registerAClass methods which will allow authors to register classes for the user agent create and invoke methods on.

2.1.1. The event loop

Each WorkletGlobalScope object has a distinct event loop. This event loop has no associated browsing context. The event loop is created by the create a WorkletGlobalScope algorithm.

The event loop is run on the worklet global scope execution environment defined above.

It is expected that only tasks associated import(), the user agent invoking author defined callbacks, and microtasks will use this event loop.

Note: Even through the event loop processing model specifies that it loops continually, practically implementations aren’t expected to do this. The microtask queue is emptied while invoking callback functions provided by the author.

2.1.2. Creating a WorkletGlobalScope

When a user agent is to create a WorkletGlobalScope, given workletGlobalScopeType, moduleResponsesMap, and outsideSettings, it must run the following steps:

1. Create the worklet global scope execution environment and run the rest of these steps in that context.

2. Call the JavaScript InitializeHostDefinedRealm abstract operation with the following customizations:

   • For the global object, create a new workletGlobalScopeType object. Let workletGlobalScope be the created object.

   • Let realmExecutionContext be the created JavaScript execution context.

   • Do not obtain any source texts for scripts or modules.

3. Let insideSettings be the result of set up a worklet environment settings object with realmExecutionContext.

4. Associate the insideSettings with workletGlobalScope.

5. For each entry in the given moduleResponsesMap (in insertion order), run the following substeps:

   1. Let moduleURLRecord be entry’s key.

   2. Let script be the result of fetch a worklet script given moduleURLRecord, moduleResponsesMap, outsideSettings, and insideSettings when it asynchronously completes.

   3. Run a module script given script.

   Note: Fetch a worklet script won’t actually perform a network request as it will hit the worklet’s module responses map. It also won’t have a parsing error as at this point it should have successfully been parsed by another worklet global scope. I.e. script should never be null here.

6. Run the responsible event loop specified by insideSettings.

2.1.3. Script settings for worklets

When a user agent is to set up a worklet environment settings object, given a executionContext, it must run the following steps:

1. Let inheritedResponsibleBrowsingContext be the responsible browsing context specified by the incumbent settings object.

2. Let inheritedOrigin be the origin specified by the incumbent settings object.

3. Let inheritedAPIBaseURL be the API base URL specified by the incumbent settings object.

4. Let workletEventLoop be a newly created event loop.

5. Let workletGlobalScope be executionContext’s global object.

6. Let settingsObject be a new environment settings object whose algorithms are defined as follows:

   The realm execution context

   Return executionContext.

   The global object

   Return workletGlobalScope.

   The responsible browsing context

   Return inheritedResponsibleBrowsingContext.

   The responsible event loop

   Return workletEventLoop.

   The responsible document

   Not applicable (the responsible event loop is not a browsing context event loop).

   The API URL character encoding

   Return UTF-8.

   The API base URL

   Return inheritedAPIBaseURL.

   The origin and effective script origin

   Return inheritedOrigin.

   The creation URL

   Not applicable.

   The HTTPS state

   Return workletGlobalScope’s HTTPS state.

7. Return settingsObject.

Merge this with https://html.spec.whatwg.org/multipage/workers.html#set-up-a-worker-environment-settings-object

2.2. Worklet

The Worklet object provides the capability to import module scripts into its associated WorkletGlobalScopes. The user agent can then create classes registered on the WorkletGlobalScopes and invoke their methods.

interface Worklet {
    [NewObject] Promise<void> import(USVString moduleURL);
};

A Worklet has a worklet global scope type. This is used for creating new WorkletGlobalScope and the type must inherit from WorkletGlobalScope.

Note: As an example the worklet global scope type might be a PaintWorkletGlobalScope.

A Worklet has a list of the worklet’s WorkletGlobalScopes. Initially this list is empty; it is populated when the user agent chooses to create its WorkletGlobalScope.

A Worklet has a module responses map. This is a ordered map of module URLs to values that are a fetch responses. The map’s entries are ordered based on their insertion order. Access to this map should be thread-safe.

The module responses map exists to ensure that WorkletGlobalScopes created at different times contain the same set of script source text and have the same behaviour. The creation of additional WorkletGlobalScopes should be transparent to the author.

A pending tasks struct is a struct consiting of:

• A counter.

When the import(moduleURL) method is called on a Worklet object, the user agent must run the following steps:

1. Let promise be a new promise.

2. Let worklet be the current Worklet.

3. Let moduleURLRecord be the result of parsing the moduleURL argument relative to the relevant settings object of this.

4. If moduleURLRecord is failure, then reject promise with a "SyntaxError" DOMException and return promise.

5. Return promise, and then continue running this algorithm in parallel.

6. Let outsideSettings be the relevant settings object of this.

7. Let moduleResponsesMap be worklet’s module responses map.

8. Let workletGlobalScopeType be worklet’s worklet global scope type.

9. If the worklet’s WorkletGlobalScopes is empty, run the following steps:

   1. Create a WorkletGlobalScope given workletGlobalScopeType, moduleResponsesMap, and outsideSettings.

   2. Add the WorkletGlobalScope to worklet’s WorkletGlobalScopes.

   The user agent may also create additional WorkletGlobalScopes at this time.

   Wait for this step to complete before continuing.

10. Let pendingTaskStruct be a new pending tasks struct with counter initialized to the length of worklet’s WorkletGlobalScopes.

11. For each workletGlobalScope in the worklet’s WorkletGlobalScopes, queue a task on the workletGlobalScope to fetch and invoke a worklet script given workletGlobalScope, moduleURLRecord, moduleResponsesMap, outsideSettings, pendingTaskStruct, and promise.

Note: The rejecting and resolving of the promise occurs within the fetch and invoke a worklet script algorithm.

When the user agent is to fetch and invoke a worklet script given workletGlobalScope, moduleURLRecord, moduleResponsesMap, outsideSettings, pendingTaskStruct, and promise, the user agent must run the following steps:

Note: This algorithm is to be run within the worklet global scope execution environment.

1. Let insideSettings be the workletGlobalScope’s associated environment settings object.

2. Let script by the result of fetch a worklet script given moduleURLRecord, moduleResponsesMap, outsideSettings, and insideSettings when it asynchronously completes.

3. If script is null, then queue a task on outsideSettings’s responsible event loop to run these steps:

   1. If pendingTaskStruct’s counter is not -1, then run these steps:

      1. Set pendingTaskStruct’s counter to -1.

      2. Reject promise with an "AbortError" DOMException.

4. Run a module script given script.

5. Queue a task on outsideSettings’s responsible event loop to run these steps:

   1. If pendingTaskStruct’s counter is not -1, then run these steps:

      1. Decrement pendingTaskStruct’s counter by 1.

      2. If pendingTaskStruct’s counter is 0, then resolve promise.

When the user agent is to fetch a worklet script given moduleURLRecord, moduleResponsesMap, outsideSettings, and insideSettings, the user agent must run the following steps:

Note: This algorithm is to be run within the worklet global scope execution environment.

1. Fetch a module worker script graph given moduleURLRecord, outsideSettings, "script", "omit", and insideSettings.

   To perform the request given request, perform the following steps:

   1. Let cache be the moduleResponsesMap.

   2. Let url be request’s url.

   3. If cache contains an entry with key url whose value is "fetching", wait until that entry’s value changes, then proceed to the next step.

   4. If cache contains an entry with key url, asynchronously complete this algorithm with that entry’s value, and abort these steps.

   5. Create an entry in cache with key url and value "fetching".

   6. Fetch request.

   7. Let response be the result of fetch when it asynchronously completes.

   8. Set the value of the entry in cache whose key is url to response, and asynchronously complete this algorithm with response.

2. Return the result of fetch a module worker script graph when it asynchronously completes.

Note: Specifically, if a script fails to parse or fails to load over the network, it will reject the promise. If the script throws an error while first evaluating the promise it will resolve as a classes may have been registered correctly.

// script.js
console.log('Hello from a WorkletGlobalScope!');

// main.js
await CSS.paintWorklet.import('script.js');

[paintWorklet#1] Hello from a WorkletGlobalScope!
[paintWorklet#4] Hello from a WorkletGlobalScope!
[paintWorklet#2] Hello from a WorkletGlobalScope!
[paintWorklet#3] Hello from a WorkletGlobalScope!

Need ability to load code into a WorkletGlobalScope declaratively. <https://github.com/w3c/css-houdini-drafts/issues/47>

2.3. Lifetime of the Worklet

The lifetime of a Worklet is tied to the object it belongs to, for example the Window.

The lifetime of a WorkletGlobalScope should be defined by subsequent specifications which inherit from WorkletGlobalScope.

Subsequent specifications may define that a WorkletGlobalScope can be terminated at any time particularly if there are no pending operations, or detects abnormal operation such as infinite loops and callbacks exceeding imposed time limits.

3. Security Considerations

Need to decide if to allow worklets for unsecure context, etc. <https://github.com/w3c/css-houdini-drafts/issues/92>

4. Examples

This section is not normative.

For these examples we’ll use a fake worklet on window.

partial interface Window {
  [SameObject] readonly attribute Worklet fakeWorklet1;
  [SameObject] readonly attribute Worklet fakeWorklet2;
};

callback Function = any (any... arguments);

[Global=(Worklet,FakeWorklet),Exposed=FakeWorklet]
interface FakeWorkletGlobalScope : WorkletGlobalScope {
    void registerAnArbitaryClass(DOMString type, Function classConstructor);
};

4.1. Loading scripts into a worklet.

window.fakeWorklet1.import('script1.js');
window.fakeWorklet1.import('script2.js');

// Assuming no other calls to fakeWorklet1 valid script loading orderings are:
// 1. 'script1.js', 'script2.js'
// 2. 'script2.js', 'script1.js'

4.2. Loading scripts into multiple worklets.

Promise.all([
    window.fakeWorklet1.import('script1.js'),
    window.fakeWorklet2.import('script2.js')
]).then(function() {
    // Both scripts now have loaded code, can do a task which relies on this.
});

4.3. Create a registered class and invoke a method.

// Inside FakeWorkletGlobalScope
registerAnArbitaryClass('key', class FooClass {
    process(arg) {
        return !arg;
    }
});

As an example, if the user agent wants to invoke "process" on a new class instance, the user agent could follow the following steps:

1. Let workletGlobalScope be a FakeWorkletGlobalScope from the list of worklet’s WorkletGlobalScopes from the fake Worklet.

   The user agent may also create a WorkletGlobalScope given the fake Worklet and use that.

2. Let classCtor be the result of performing a lookup in registered class constructors map with "key" as the key.

3. Let classInstance be the result of Construct(classCtor).

4. Let result be the result of Invoke(O=classInstance, P="process", Arguments=["true"]).

5. Return result.