refactor(router): add type guards for router guards (#26521)

PR Close #26521

packages/router/src/interfaces.ts

@@ -374,6 +374,7 @@ export interface Resolve<T> {
  *
  * @publicApi
  */
-export interface CanLoad {
-  canLoad(route: Route, segments: UrlSegment[]): Observable<boolean>|Promise<boolean>|boolean;
-}
+export interface CanLoad { canLoad: CanLoadFn; }
+
+export type CanLoadFn = (route: Route, segments: UrlSegment[]) =>
+    Observable<boolean>| Promise<boolean>| boolean;

packages/router/src/utils/type_guards.ts

@@ -0,0 +1,39 @@
+/**
+ * @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 {Type} from '@angular/core';
+import {CanActivate, CanActivateChild, CanDeactivate} from '../interfaces';
+
+/**
+ * Simple function check, but generic so type inference will flow. Example:
+ *
+ * function product(a: number, b: number) {
+ *   return a * b;
+ * }
+ *
+ * if (isFunction<product>(fn)) {
+ *   return fn(1, 2);
+ * } else {
+ *   throw "Must provide the `product` function";
+ * }
+ */
+export function isFunction<T>(v: any): v is T {
+  return typeof v === 'function';
+}
+
+export function isCanActivate(guard: any): guard is CanActivate {
+  return guard && isFunction<CanActivate>(guard.canActivate);
+}
+
+export function isCanActivateChild(guard: any): guard is CanActivateChild {
+  return guard && isFunction<CanActivateChild>(guard.canActivate);
+}
+
+export function isCanDeactivate(guard: any): guard is CanDeactivate<Type<any>> {
+  return guard && isFunction<CanDeactivate<Type<any>>>(guard.canDeactivate);
+}