Additional derive cleanups (#4308)

* Additional derive cleanups

* unwrapNumber

* Adjust

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

@@ -5,9 +5,7 @@ import type { Observable } from 'rxjs';
 import type { ApiInterfaceRx } from '@polkadot/api/types';
 import type { BlockNumber } from '@polkadot/types/interfaces';
 
-import { map } from 'rxjs';
-
-import { memo } from '../util';
+import { unwrapNumber } from './util';
 
 /**
  * @name bestNumber
@@ -22,8 +20,5 @@ import { memo } from '../util';
  * ```
  */
 export function bestNumber (instanceId: string, api: ApiInterfaceRx): () => Observable<BlockNumber> {
-  return memo(instanceId, (): Observable<BlockNumber> =>
-    api.derive.chain.subscribeNewHeads().pipe(
-      map((header) => header.number.unwrap())
-    ));
+  return unwrapNumber(instanceId, api.derive.chain.subscribeNewHeads);
 }

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

@@ -5,9 +5,7 @@ import type { Observable } from 'rxjs';
 import type { ApiInterfaceRx } from '@polkadot/api/types';
 import type { BlockNumber } from '@polkadot/types/interfaces';
 
-import { map } from 'rxjs';
-
-import { memo } from '../util';
+import { unwrapNumber } from './util';
 
 /**
  * @name bestNumberFinalized
@@ -23,8 +21,5 @@ import { memo } from '../util';
  * ```
  */
 export function bestNumberFinalized (instanceId: string, api: ApiInterfaceRx): () => Observable<BlockNumber> {
-  return memo(instanceId, (): Observable<BlockNumber> =>
-    api.rpc.chain.subscribeFinalizedHeads().pipe(
-      map((header) => header.number.unwrap())
-    ));
+  return unwrapNumber(instanceId, api.rpc.chain.subscribeFinalizedHeads);
 }

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

@@ -0,0 +1,14 @@
+// Copyright 2017-2021 @polkadot/api-derive authors & contributors
+// SPDX-License-Identifier: Apache-2.0
+
+import type { Observable } from 'rxjs';
+import type { Option } from '@polkadot/types';
+import type { BlockNumber } from '@polkadot/types/interfaces';
+
+import { map } from 'rxjs';
+
+import { memo } from '../util';
+
+export function unwrapNumber <T extends { number: Option<BlockNumber> }> (instanceId: string, fn: () => Observable<T>): () => Observable<BlockNumber> {
+  return memo(instanceId, () => fn().pipe(map((r) => r.number.unwrap())));
+}