Fix context types to be compatible with WCCG context protocol types. (#…

…4614)

.changeset/fluffy-garlics-smash.md

@@ -0,0 +1,5 @@
+---
+'@lit/context': patch
+---
+
+Fix context types to be compatible with WCCG context protocol types.

packages/context/src/lib/context-request-event.ts

@@ -53,7 +53,7 @@ export class ContextRequestEvent<C extends Context<unknown, unknown>>
 {
   readonly context: C;
   readonly callback: ContextCallback<ContextType<C>>;
-  readonly subscribe: boolean;
+  readonly subscribe?: boolean;
 
   /**
    *

packages/context/src/test/protocol-types-test.ts

@@ -0,0 +1,26 @@
+/**
+ * @license
+ * Copyright 2021 Google LLC
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+import {ContextRequestEvent, Context} from './protocol-types.js';
+import {ContextRequestEvent as LitContextRequestEvent} from '../lib/context-request-event.js';
+import {Context as LitContext} from '../index.js';
+
+/*
+ * These function definitions check whether the Lit and protocol types are
+ * compatible.
+ */
+
+export const protocolFunction = (
+  e: ContextRequestEvent<Context<unknown, unknown>>
+) => {
+  litFunction(e);
+};
+
+export const litFunction = (
+  e: LitContextRequestEvent<LitContext<unknown, unknown>>
+) => {
+  protocolFunction(e);
+};

packages/context/src/test/protocol-types.ts

@@ -0,0 +1,68 @@
+// Copied from https://github.com/webcomponents-cg/community-protocols/blob/main/proposals/context.md#definitions
+// at 43f8158a75446af25164ff35d1a6b4f9e96e9d36
+
+/**
+ * A context key.
+ *
+ * A context key can be any type of object, including strings and symbols. The
+ *  Context type brands the key type with the `__context__` property that
+ * carries the type of the value the context references.
+ */
+export type Context<KeyType, ValueType> = KeyType & {__context__: ValueType};
+
+/**
+ * An unknown context type
+ */
+export type UnknownContext = Context<unknown, unknown>;
+
+/**
+ * A helper type which can extract a Context value type from a Context type
+ */
+export type ContextType<T extends UnknownContext> =
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
+  T extends Context<infer _, infer V> ? V : never;
+
+/**
+ * A function which creates a Context value object
+ */
+export const createContext = <ValueType>(key: unknown) =>
+  key as Context<typeof key, ValueType>;
+
+/**
+ * A callback which is provided by a context requester and is called with the value satisfying the request.
+ * This callback can be called multiple times by context providers as the requested value is changed.
+ */
+export type ContextCallback<ValueType> = (
+  value: ValueType,
+  unsubscribe?: () => void
+) => void;
+
+/**
+ * An event fired by a context requester to signal it desires a named context.
+ *
+ * A provider should inspect the `context` property of the event to determine if it has a value that can
+ * satisfy the request, calling the `callback` with the requested value if so.
+ *
+ * If the requested context event contains a truthy `subscribe` value, then a provider can call the callback
+ * multiple times if the value is changed, if this is the case the provider should pass an `unsubscribe`
+ * function to the callback which requesters can invoke to indicate they no longer wish to receive these updates.
+ */
+export class ContextRequestEvent<T extends UnknownContext> extends Event {
+  public constructor(
+    public readonly context: T,
+    public readonly callback: ContextCallback<ContextType<T>>,
+    public readonly subscribe?: boolean
+  ) {
+    super('context-request', {bubbles: true, composed: true});
+  }
+}
+
+declare global {
+  interface HTMLElementEventMap {
+    /**
+     * A 'context-request' event can be emitted by any element which desires
+     * a context value to be injected by an external provider.
+     */
+    'context-request': ContextRequestEvent<Context<unknown, unknown>>;
+  }
+}