Worklets Level 1

1. Introduction

1.1. Motivations

This section is not normative.

Allowing extension points defined in the document's global scope 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's global scope would restrict rendering engines to performing work in the same thread as the document's global scope. (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's global scope.

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 {
    attribute Console console;
};

A WorkletGlobalScope has an associated environment settings object.

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, and only its microtask queue is used (all other task queues are not used). The event loop is created by the create a WorkletGlobalScope algorithm.

2.1.2. Creating a WorkletGlobalScope

When a user agent is to create a WorkletGlobalScope, for a given worklet, it must run the following steps:

1. Let workletGlobalScopeType be the worklet’s worklet global scope type.

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 settingsObject be the result of set up a worklet environment settings object with realmExecutionContext.

4. Associate the settingsObject with workletGlobalScope.

5. For each resolvedModuleURL in the given worklet’s worklet’s resolved module URLs, run the following substeps:

   1. Let script be the result of fetch a module script tree given resolvedModuleURL, "anonymous" for the CORS setting attribute, and settingsObject.

   Note: Worklets follow web workers here in not allowing "use-credientials" for fetching resources.

   2. Run a module script given script.

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

   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(DOMString 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 list of the worklet’s resolved module URLs. Initially this list is empty; it is populated when module scripts resolved.

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. Run the following steps in parallel:

   1. Let resolvedModuleURL be the result of resolving the moduleURL relative to the API base URL specified by the entry settings object when the method was invoked.

   2. If this fails, reject promise with a SyntaxError exception and abort these steps.

   3. Add resolvedModuleURL to the list of worklet’s resolved module URLs.

   4. Ensure that there is at least one WorkletGlobalScope in the worklet’s WorkletGlobalScopes. If not create a WorkletGlobalScope given the current Worklet.

   5. For each WorkletGlobalScope in the worklet’s WorkletGlobalScopes, run these substeps:

      1. Let settings be the WorkletGlobalScope's associated environment settings object.

      2. Let script be the result of fetch a module script tree given resolvedModuleURL, "anonymous" for the CORS setting attribute, and settings.

      Note: Worklets follow web workers here in not allowing "use-credientials" for fetching resources.

      3. Run a module script given script.

   6. If all the steps above succeeded (in particular, if all of the scripts parsed and loaded into the global scopes), resolve promise.
      Otherwise, reject promise.

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

3. Return promise.

Need ability to load code into 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);
};

Each FakeWorkletGlobalScope has a map of the registered class constructors map.

When the registerAnArbitaryClass(type, classConstructor) method is called, the user agent will add the classConstructor of type to the map of registered class constructors map.

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.