refactor(router): create pipeable setupPreactivation function (#25740)

PR Close #25740

packages/router/src/operators/setup_preactivation.ts

@@ -0,0 +1,30 @@
+/**
+ * @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 {Injector} from '@angular/core';
+import {Observable, OperatorFunction} from 'rxjs';
+import {map} from 'rxjs/operators';
+
+import {Event} from '../events';
+import {PreActivation} from '../pre_activation';
+import {ChildrenOutletContexts} from '../router_outlet_context';
+import {RouterStateSnapshot} from '../router_state';
+
+export function setupPreactivation(
+    rootContexts: ChildrenOutletContexts, currentSnapshot: RouterStateSnapshot,
+    moduleInjector: Injector,
+    forwardEvent?: (evt: Event) => void): OperatorFunction<RouterStateSnapshot, PreActivation> {
+  return function(source: Observable<RouterStateSnapshot>) {
+    return source.pipe(map(snapshot => {
+      const preActivation =
+          new PreActivation(snapshot, currentSnapshot, moduleInjector, forwardEvent);
+      preActivation.initialize(rootContexts);
+      return preActivation;
+    }));
+  };
+}