Add chain.getBlockByNumber (#4598)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

packages/api-derive/src/chain/getBlockByNumber.ts

@@ -0,0 +1,19 @@
+// Copyright 2017-2022 @polkadot/api-derive authors & contributors
+// SPDX-License-Identifier: Apache-2.0
+
+import type { Observable } from 'rxjs';
+import type { AnyNumber } from '@polkadot/types/types';
+import type { SignedBlockExtended } from '../type/types';
+import type { DeriveApi } from '../types';
+
+import { switchMap } from 'rxjs';
+
+import { memo } from '../util';
+
+export function getBlockByNumber (instanceId: string, api: DeriveApi): (blockNumber: AnyNumber) => Observable<SignedBlockExtended | undefined> {
+  return memo(instanceId, (blockNumber: AnyNumber): Observable<SignedBlockExtended | undefined> =>
+    api.rpc.chain.getBlockHash(blockNumber).pipe(
+      switchMap((h) => api.derive.chain.getBlock(h))
+    )
+  );
+}

packages/api-derive/src/chain/index.ts

@@ -6,5 +6,6 @@ export * from './bestNumberFinalized';
 export * from './bestNumberLag';
 export * from './getHeader';
 export * from './getBlock';
+export * from './getBlockByNumber';
 export * from './subscribeNewBlocks';
 export * from './subscribeNewHeads';