Add Module Composer API (#33284)

* Spec composer API

* Add composer api implementation

* Tests

* Copyright

* update sigs

* cleanup

* a snap

* cleanup

* linting

* Tidy up

* Adjust

packages/module-api/src/api/composer.ts

@@ -0,0 +1,20 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+/**
+ * API to interact with the message composer.
+ * @alpha Likely to change
+ */
+export interface ComposerApi {
+    /**
+     * Insert plaintext into the current composer.
+     * @param plaintext - The plain text to insert
+     * @returns Returns immediately, does not await action.
+     * @alpha Likely to change
+     */
+    insertPlaintextIntoComposer(plaintext: string): void;
+}

packages/module-api/src/api/index.ts

@@ -23,6 +23,7 @@ import { type ClientApi } from "./client.ts";
 import { type WidgetLifecycleApi } from "./widget-lifecycle.ts";
 import { type WidgetApi } from "./widget.ts";
 import { type CustomisationsApi } from "./customisations.ts";
+import { type ComposerApi } from "./composer.ts";
 
 /**
  * Module interface for modules to implement.
@@ -159,6 +160,12 @@ export interface Api
      */
     readonly customisations: CustomisationsApi;
 
+    /**
+     * Allows modules to customise the message composer.
+     * @alpha Subject to change.
+     */
+    readonly composer: ComposerApi;
+
     /**
      * Create a ReactDOM root for rendering React components.
      * Exposed to allow modules to avoid needing to bundle their own ReactDOM.

packages/module-api/src/index.ts

@@ -11,6 +11,7 @@ export type { Config, ConfigApi } from "./api/config";
 export type { I18nApi, Variables, Translations, SubstitutionValue, Tags } from "./api/i18n";
 export type * from "./models/event";
 export type * from "./models/Room";
+export type * from "./api/composer";
 export type * from "./api/custom-components";
 export type * from "./api/extras";
 export type * from "./api/legacy-modules";