feat(router): add prioritizedGuardValue operator optimization and allowing UrlTree return from guard (#26478)

* If all guards return `true`, operator returns `true`
* `false` and `UrlTree` are now both valid returns from a guard
* Both these values wait for higher priority guards to resolve
* Highest priority `false` or `UrlTree` value will be returned

PR Close #26478

packages/router/src/operators/prioritized_guard_value.ts

@@ -0,0 +1,52 @@
+/**
+ * @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, OperatorFunction, combineLatest} from 'rxjs';
+import {filter, scan, startWith, switchMap, take} from 'rxjs/operators';
+
+import {UrlTree} from '../url_tree';
+
+const INITIAL_VALUE = Symbol('INITIAL_VALUE');
+declare type INTERIM_VALUES = typeof INITIAL_VALUE | boolean | UrlTree;
+
+export function prioritizedGuardValue():
+    OperatorFunction<Observable<boolean|UrlTree>[], boolean|UrlTree> {
+  return switchMap(obs => {
+    return combineLatest(
+               ...obs.map(o => o.pipe(take(1), startWith(INITIAL_VALUE as INTERIM_VALUES))))
+        .pipe(
+            scan(
+                (acc: INTERIM_VALUES, list: INTERIM_VALUES[]) => {
+                  let isPending = false;
+                  return list.reduce((innerAcc, val, i: number) => {
+                    if (innerAcc !== INITIAL_VALUE) return innerAcc;
+
+                    // Toggle pending flag if any values haven't been set yet
+                    if (val === INITIAL_VALUE) isPending = true;
+
+                    // Any other return values are only valid if we haven't yet hit a pending call.
+                    // This guarantees that in the case of a guard at the bottom of the tree that
+                    // returns a redirect, we will wait for the higher priority guard at the top to
+                    // finish before performing the redirect.
+                    if (!isPending) {
+                      // Early return when we hit a `false` value as that should always cancel
+                      // navigation
+                      if (val === false) return val;
+
+                      if (i === list.length - 1 || val instanceof UrlTree) {
+                        return val;
+                      }
+                    }
+
+                    return innerAcc;
+                  }, acc);
+                },
+                INITIAL_VALUE),
+            filter(item => item !== INITIAL_VALUE), take(1)) as Observable<boolean|UrlTree>;
+  });
+}