Pass online/offline status indicators with staking accounts (#1265)

* staking online status

* restrict onlineStatus.online to validators

* get onlineStatus for stash/validator only

* tests

* fixes

* Add offline

* Revert stash, change to stashId, linting

* Heartbeat signature

* Make CodeClimate (slightly) happier

* keep compatibility

* change status type

* rv null possible

* cleanup

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

@@ -0,0 +1,5 @@
+// Copyright 2017-2019 @polkadot/api-derive authors & contributors
+// This software may be modified and distributed under the terms
+// of the Apache-2.0 license. See the LICENSE file for details.
+
+export * from './receivedHeartbeats';

packages/api-derive/src/imOnline/receivedHeartbeats.ts

@@ -0,0 +1,37 @@
+// Copyright 2017-2019 @polkadot/api-derive authors & contributors
+// This software may be modified and distributed under the terms
+// of the Apache-2.0 license. See the LICENSE file for details.
+
+import { AccountId, SessionIndex } from '@polkadot/types/interfaces';
+
+import { of, Observable } from 'rxjs';
+import { map, switchMap } from 'rxjs/operators';
+import { ApiInterfaceRx } from '@polkadot/api/types';
+import { Bytes } from '@polkadot/types';
+
+import { drr } from '../util/drr';
+
+/**
+ * @description Return a boolean array indicating whether the passed accounts had received heartbeats in the current session
+ */
+export function receivedHeartbeats (api: ApiInterfaceRx): (addresses: (AccountId | string)[]) => Observable<boolean[]> {
+  return (addresses: (AccountId | string)[]): Observable<boolean[]> => {
+    return api.query.imOnline && api.query.imOnline.receivedHeartbeats
+      ? api.query.session
+        .currentIndex<SessionIndex>()
+        .pipe(
+          switchMap((currentIndex): Observable<Bytes[]> =>
+            api.query.imOnline.receivedHeartbeats.multi(
+              addresses.map((address): [SessionIndex, AccountId | string] =>
+                [currentIndex, address]
+              )
+            )
+          ),
+          map((heartbeats): boolean[] =>
+            heartbeats.map((heartbeat): boolean => !heartbeat.isEmpty)
+          ),
+          drr()
+        )
+      : of([...new Array(addresses.length).keys()].map((): boolean => false));
+  };
+}