Introduce LegacyEthereumProvider interface

src/index.ts

@@ -1,3 +1,3 @@
+export * from './legacy-ethereum-provider';
 export * from './provider-from-engine';
 export * from './provider-from-middleware';
-export type { SafeEventEmitterProvider } from './safe-event-emitter-provider';

src/legacy-ethereum-provider.ts

@@ -0,0 +1,36 @@
+import { JsonRpcRequest } from '@metamask/json-rpc-engine';
+
+/**
+ * The interface for a legacy Ethereum provider.
+ *
+ * This provider follows conventions that pre-date
+ * [EIP-1193](https://eips.ethereum.org/EIPS/eip-1193). It is not compliant with
+ * any Ethereum provider standard.
+ */
+export type LegacyEthereumProvider = {
+  /**
+   * Send a provider request asynchronously.
+   *
+   * @param req - The request to send.
+   * @param callback - A function that is called upon the success or failure of the request.
+   */
+  sendAsync(
+    req: JsonRpcRequest<unknown>,
+    callback: (error: unknown, providerRes?: any) => void,
+  ): void;
+
+  /**
+   * Send a provider request asynchronously.
+   *
+   * This method serves the same purpose as `sendAsync`. It only exists for
+   * legacy reasons.
+   *
+   * @deprecated Use `sendAsync` instead.
+   * @param req - The request to send.
+   * @param callback - A function that is called upon the success or failure of the request.
+   */
+  send(
+    req: JsonRpcRequest<unknown>,
+    callback: (error: unknown, providerRes?: any) => void,
+  ): void;
+};

src/safe-event-emitter-provider.ts

@@ -2,13 +2,21 @@
 import SafeEventEmitter from '@metamask/safe-event-emitter';
 import type { JsonRpcRequest } from '@metamask/utils';
 
+import { JsonRpcProvider } from './json-rpc-provider';
+
 /**
- * An Ethereum provider.
+ * SafeEventEmitterProvider implements the JSON-RPC provider interface by using
+ * a [JSON-RPC engine](https://github.com/MetaMask/json-rpc-engine) to make
+ * requests.
  *
- * This provider loosely follows conventions that pre-date EIP-1193.
- * It is not compliant with any Ethereum provider standard.
+ * This provider follows conventions that pre-date
+ * [EIP-1193](https://eips.ethereum.org/EIPS/eip-1193). It is not compliant with
+ * any Ethereum provider standard.
  */
-export class SafeEventEmitterProvider extends SafeEventEmitter {
+export class SafeEventEmitterProvider
+  extends SafeEventEmitter
+  implements JsonRpcProvider
+{
   #engine: JsonRpcEngine;
 
   /**