feat(extensions): Introduce app extensions

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

src/components/markdown-renderer/hooks/use-extension-event-emitter.tsx

@@ -0,0 +1,51 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import type { PropsWithChildren } from 'react'
+import React, { createContext, useContext, useEffect, useMemo } from 'react'
+import EventEmitter2 from 'eventemitter2'
+
+export const eventEmitterContext = createContext<EventEmitter2 | undefined>(undefined)
+
+/**
+ * Provides the {@link EventEmitter2 event emitter} from the current {@link eventEmitterContext context}.
+ */
+export const useExtensionEventEmitter = () => {
+  return useContext(eventEmitterContext)
+}
+
+/**
+ * Creates a new {@link EventEmitter2 event emitter} and provides it as {@link eventEmitterContext context}.
+ *
+ * @param children The elements that should receive the context value
+ */
+export const ExtensionEventEmitterProvider: React.FC<PropsWithChildren> = ({ children }) => {
+  const eventEmitter = useMemo(() => new EventEmitter2(), [])
+  return <eventEmitterContext.Provider value={eventEmitter}>{children}</eventEmitterContext.Provider>
+}
+
+/**
+ * Registers a handler callback on the current {@link EventEmitter2 event emitter} that is provided in the {@link eventEmitterContext context}.
+ *
+ * @param eventName The name of the event which should be subscribed
+ * @param handler The callback that should be executed. If undefined the event will be unsubscribed.
+ */
+export const useExtensionEventEmitterHandler = <T,>(
+  eventName: string,
+  handler: ((values: T) => void) | undefined
+): void => {
+  const eventEmitter = useExtensionEventEmitter()
+
+  useEffect(() => {
+    if (!eventEmitter || !handler) {
+      return
+    }
+    eventEmitter.on(eventName, handler)
+    return () => {
+      eventEmitter.off(eventName, handler)
+    }
+  }, [eventEmitter, eventName, handler])
+}