Add destroy method

src/JsonRpcEngine.ts

@@ -29,12 +29,15 @@ export type JsonRpcEngineEndCallback = (
   error?: JsonRpcEngineCallbackError,
 ) => void;
 
-export type JsonRpcMiddleware<Params, Result> = (
-  req: JsonRpcRequest<Params>,
-  res: PendingJsonRpcResponse<Result>,
-  next: JsonRpcEngineNextCallback,
-  end: JsonRpcEngineEndCallback,
-) => void;
+export interface JsonRpcMiddleware<Params, Result> {
+  (
+    req: JsonRpcRequest<Params>,
+    res: PendingJsonRpcResponse<Result>,
+    next: JsonRpcEngineNextCallback,
+    end: JsonRpcEngineEndCallback,
+  ): void;
+  destroy?: () => void | Promise<void>;
+}
 
 /**
  * A JSON-RPC request and response processor.
@@ -57,6 +60,26 @@ export class JsonRpcEngine extends SafeEventEmitter {
     this._middleware.push(middleware as JsonRpcMiddleware<unknown, unknown>);
   }
 
+  /**
+   * Calls the `destroy()` function of any middleware with that property and clears
+   * the middleware array.
+   */
+  cleanup(): void {
+    this._middleware.forEach(
+      (middleware: JsonRpcMiddleware<unknown, unknown>) => {
+        if (
+          // `in` walks the prototype chain, which is probably the desired
+          // behavior here.
+          'destroy' in middleware &&
+          typeof middleware.destroy === 'function'
+        ) {
+          middleware.destroy();
+        }
+      },
+    );
+    this._middleware = [];
+  }
+
   /**
    * Handle a JSON-RPC request, and return a response.
    *

src/engine.test.ts

@@ -5,7 +5,7 @@ import {
   isJsonRpcSuccess,
 } from '@metamask/utils';
 import { ethErrors } from 'eth-rpc-errors';
-import { JsonRpcEngine } from '.';
+import { JsonRpcEngine, JsonRpcMiddleware } from '.';
 
 const jsonrpc = '2.0' as const;
 
@@ -528,4 +528,28 @@ describe('JsonRpcEngine', () => {
 
     await expect(engine.handle([{}] as any)).rejects.toThrow('foo');
   });
+
+  it('cleanup middleware test', async () => {
+    const engine = new JsonRpcEngine();
+
+    engine.push((_req, res, next, _end) => {
+      res.result = 42;
+      next();
+    });
+
+    const destroyMock = jest.fn();
+    const destroyableMiddleware: JsonRpcMiddleware<unknown, unknown> = (
+      _req,
+      _res,
+      _next,
+      end,
+    ) => {
+      end();
+    };
+    destroyableMiddleware.destroy = destroyMock;
+    engine.push(destroyableMiddleware);
+
+    engine.cleanup();
+    expect(destroyMock).toHaveBeenCalledTimes(1);
+  });
 });