Expose derive staking.nextElected (#2042)

packages/api-derive/src/staking/validators.ts

@@ -12,16 +12,20 @@ import { StorageKey } from '@polkadot/types';
 
 import { memo } from '../util';
 
-function queryNextStakers (api: ApiInterfaceRx): Observable<AccountId[]> {
-  // only populate for next era in the last session, so track both here - entries are not
-  // subscriptions, so we need a trigger - currentIndex acts as that trigger to refresh
-  return api.derive.session.indexes().pipe(
-    switchMap(({ activeEra }): Observable<StorageKey[]> =>
-      api.query.staking.erasStakers.keys(activeEra.addn(1)).pipe(take(1))
-    ),
-    map((keys): AccountId[] =>
-      keys.map((key): AccountId => key.args[1] as AccountId)
-    )
+export function nextElected (api: ApiInterfaceRx): () => Observable<AccountId[]> {
+  return memo((): Observable<AccountId[]> =>
+    api.query.staking.erasStakers
+      ? api.derive.session.indexes().pipe(
+        // only populate for next era in the last session, so track both here - entries are not
+        // subscriptions, so we need a trigger - currentIndex acts as that trigger to refresh
+        switchMap(({ currentEra }): Observable<StorageKey[]> =>
+          api.query.staking.erasStakers.keys(currentEra).pipe(take(1))
+        ),
+        map((keys): AccountId[] =>
+          keys.map((key): AccountId => key.args[1] as AccountId)
+        )
+      )
+      : api.query.staking.currentElected<AccountId[]>()
   );
 }
 
@@ -38,9 +42,7 @@ export function validators (api: ApiInterfaceRx): () => Observable<DeriveStaking
         ? api.query.session.validators()
         : of([]),
       api.query.staking
-        ? api.query.staking.erasStakers
-          ? queryNextStakers(api)
-          : api.query.staking.currentElected<AccountId[]>()
+        ? api.derive.staking.nextElected()
         : of([])
     ]).pipe(
       map(([validators, nextElected]): DeriveStakingValidators => ({