derive.chain.subscribeFinalizedHeads (#5063)

* derive.chain.subscribeFinalizedHeads

* Remove extra map

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

@@ -7,5 +7,6 @@ export * from './bestNumberLag';
 export * from './getHeader';
 export * from './getBlock';
 export * from './getBlockByNumber';
+export * from './subscribeFinalizedHeads';
 export * from './subscribeNewBlocks';
 export * from './subscribeNewHeads';

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

@@ -0,0 +1,53 @@
+// Copyright 2017-2022 @polkadot/api-derive authors & contributors
+// SPDX-License-Identifier: Apache-2.0
+
+import type { Observable } from 'rxjs';
+import type { Hash, Header } from '@polkadot/types/interfaces';
+import type { DeriveApi } from '../types';
+
+import { from, of, switchMap } from 'rxjs';
+
+import { memo } from '../util';
+
+export function _getHeaderRange (instanceId: string, api: DeriveApi): (startHash: Hash, endHash: Hash, prev?: Header[]) => Observable<Header[]> {
+  return memo(instanceId, (startHash: Hash, endHash: Hash, prev: Header[] = []): Observable<Header[]> =>
+    api.rpc.chain.getHeader(startHash).pipe(
+      switchMap((header) =>
+        header.parentHash.eq(endHash)
+          ? of([header, ...prev])
+          : api.derive.chain._getHeaderRange(header.parentHash, endHash, [header, ...prev])
+      )
+    )
+  );
+}
+
+/**
+ * @name subscribeFinalizedHeads
+ * @description An observable of the finalized block headers. Unlike the base
+ * chain.subscribeFinalizedHeads this does not skip any headers. Since finalization
+ * may skip specific blocks (finalization happens in terms of chains), this version
+ * of the derive tracks missing headers (since last  retrieved) and provides them
+ * to the caller
+ */
+export function subscribeFinalizedHeads (instanceId: string, api: DeriveApi): () => Observable<Header> {
+  return memo(instanceId, (): Observable<Header> => {
+    let prevHash: Hash | null = null;
+
+    return api.rpc.chain.subscribeFinalizedHeads().pipe(
+      switchMap((header) => {
+        const endHash = prevHash;
+        const startHash = header.parentHash;
+
+        prevHash = header.hash;
+
+        return endHash === null || startHash.eq(endHash)
+          ? of(header)
+          : api.derive.chain._getHeaderRange(startHash, endHash, [header]).pipe(
+            switchMap((headers) =>
+              from(headers)
+            )
+          );
+      })
+    );
+  });
+}