feat(@angular-devkit/core): add a fallback registry

Its get method goes through an array of registries and return the first
one found.

Author: hansl

packages/angular_devkit/core/src/experimental/jobs/fallback-registry.ts

@@ -0,0 +1,43 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+import { Observable, from, of } from 'rxjs';
+import { concatMap, first } from 'rxjs/operators';
+import { JsonValue, schema } from '../../json';
+import { JobDescription, JobHandler, JobName, Registry, isJobHandler } from './api';
+import { JobNameAlreadyRegisteredException } from './exception';
+
+
+/**
+ * A simple job registry that keep a map of JobName => JobHandler internally.
+ */
+export class FallbackRegistry<
+  MinimumArgumentValueT extends JsonValue = JsonValue,
+  MinimumInputValueT extends JsonValue = JsonValue,
+  MinimumOutputValueT extends JsonValue = JsonValue,
+> implements Registry<MinimumArgumentValueT, MinimumInputValueT, MinimumOutputValueT> {
+  constructor(protected _fallbacks: Registry<
+    MinimumArgumentValueT,
+    MinimumInputValueT,
+    MinimumOutputValueT
+  >[] = []) {}
+
+  addFallback(registry: Registry) {
+    this._fallbacks.push(registry);
+  }
+
+  get<
+    A extends MinimumArgumentValueT = MinimumArgumentValueT,
+    I extends MinimumInputValueT = MinimumInputValueT,
+    O extends MinimumOutputValueT = MinimumOutputValueT,
+  >(name: JobName): Observable<JobHandler<A, I, O> | null> {
+    return from(this._fallbacks).pipe(
+      concatMap(fb => fb.get<A, I, O>(name)),
+      first(x => x !== null, null),
+    );
+  }
+}

packages/angular_devkit/core/src/experimental/jobs/index.ts

@@ -9,6 +9,7 @@ export * from './api';
 export * from './create-job-handler';
 export * from './exception';
 export * from './dispatcher';
+export * from './fallback-registry';
 export * from './simple-registry';
 export * from './simple-scheduler';
 export * from './strategy';