Simple plugin proof of concept.

src/components/common/messaging/MessageBox.tsx

@@ -36,6 +36,7 @@ import { takeError } from "../../../context/revoltjs/util";
 
 import IconButton from "../../ui/IconButton";
 
+import { PluginSingleton } from "../../../plugins";
 import AutoComplete, { useAutoComplete } from "../AutoComplete";
 import { PermissionTooltip } from "../Tooltip";
 import FilePreview from "./bars/FilePreview";
@@ -185,8 +186,11 @@ export default function MessageBox({ channel }: Props) {
             return;
 
         const content = draft?.trim() ?? "";
-        if (uploadState.type === "attached") return sendFile(content);
+        const target = { content };
-        if (content.length === 0) return;
+        if (PluginSingleton.emit("Message:Send", target)) return;
+
+        if (uploadState.type === "attached") return sendFile(target.content);
+        if (target.content.length === 0) return;
 
         stopTyping();
         setMessage();
@@ -203,7 +207,7 @@ export default function MessageBox({ channel }: Props) {
                 channel: channel._id,
                 author: client.user!._id,
 
-                content,
+                content: target.content,
                 replies,
             },
         });
@@ -217,7 +221,7 @@ export default function MessageBox({ channel }: Props) {
 
         try {
             await client.channels.sendMessage(channel._id, {
-                content,
+                content: target.content,
                 nonce,
                 replies,
             });

src/main.tsx

@@ -6,6 +6,7 @@ import { render } from "preact";
 import { internalEmit } from "./lib/eventEmitter";
 
 import { App } from "./pages/app";
+import "./plugins";
 
 export const updateSW = registerSW({
     onNeedRefresh() {

src/plugins/index.ts

@@ -0,0 +1,74 @@
+import { Children } from "../types/Preact";
+
+export interface PluginEvent<T> {
+    cancel: () => void;
+    target: T;
+}
+
+export interface PluginEvents {
+    "Message:Send": (
+        event: PluginEvent<{ content?: string; files?: File[] }>,
+    ) => void;
+    "Misc:Test": (event: PluginEvent<string>) => void;
+}
+
+export interface Plugin {
+    render?: () => Children;
+    events?: Partial<PluginEvents>;
+}
+
+class Singleton {
+    listeners: {
+        [key in keyof PluginEvents]?: PluginEvents[key][];
+    };
+
+    constructor() {
+        this.listeners = {};
+    }
+
+    private getListenerArray<K extends keyof PluginEvents>(
+        key: K,
+    ): PluginEvents[K][] {
+        const arr = this.listeners[key];
+        if (arr) return arr as PluginEvents[K][];
+
+        this.listeners[key] = [];
+        return this.listeners[key]! as PluginEvents[K][];
+    }
+
+    addListener<K extends keyof PluginEvents>(key: K, fn: PluginEvents[K]) {
+        this.getListenerArray(key).push(fn);
+    }
+
+    emit<K extends keyof PluginEvents>(
+        key: K,
+        target: Parameters<PluginEvents[K]>[0]["target"],
+    ): boolean {
+        let canceled = false;
+
+        const ev: PluginEvent<any> = {
+            cancel: () => (canceled = true),
+            target,
+        };
+
+        const arr = this.getListenerArray(key);
+        for (const listener of arr) {
+            listener(ev);
+            if (canceled) break;
+        }
+
+        return canceled;
+    }
+}
+
+export const PluginSingleton = new Singleton();
+
+PluginSingleton.addListener("Message:Send", (event) => {
+    const { content } = event.target;
+    if (content?.startsWith(";sus")) {
+        event.target.content = "baka!";
+    } else if (content?.startsWith(";amogus")) {
+        alert("fat");
+        event.cancel();
+    }
+});