RouteConfigLoadStart event not firing if the navigation of a lazy loaded route was previously canceled

Which @angular/* package(s) are the source of the bug?

router

Is this a regression?

No

Description

If the navigation of a lazy loaded route was previously canceled, the RouteConfigLoadStart event is never fired again, although the RouteConfigLoadEnd fires.

It seams that the loader that contains the observable is returned immediately without triggering the RouteConfigLoadStart. This invalidates the possibility to track the loading progress of a route when the navigation is canceled and resumed sometime in the future (see code below).

I am not sure how I can provide a reproduction for this since the loading of the route is network dependent, but I think that by looking at the code below, is easy to understand the problem.

router_config_loader.ts

load(parentInjector: Injector, route: Route): Observable<LoadedRouterConfig> {
    if (route._loader$) {
      return route._loader$;
    }

    if (this.onLoadStartListener) {
      this.onLoadStartListener(route);
    }
...

Please provide a link to a minimal reproduction of the bug

No response

Please provide the exception or error you saw

No response

Please provide the environment you discovered this bug in (run ng version)

Angular CLI: 13.1.2
Node: 12.22.2
Package Manager: npm 6.14.13
OS: win32 x64

Angular: 13.1.1
... animations, common, compiler, compiler-cli, core, forms
... localize, platform-browser, platform-browser-dynamic, router

Package                         Version
---------------------------------------------------------
@angular-devkit/architect       0.1301.2
@angular-devkit/build-angular   13.1.2
@angular-devkit/core            13.1.2
@angular-devkit/schematics      13.1.2
@angular/cli                    13.1.2
@schematics/angular             13.1.2
ng-packagr                      13.0.8
rxjs                            7.4.0
typescript                      4.4.4

Anything else?

No response

[atscott]

This really does need a reproduction. The intent was to clear the loader if it was unsuccessful but this would not happen if it completes without an error https://github.com/angular/angular/blob/8ebc946c0e7bf80d26ec8268acb4ff0af9e5c34a/packages/router/src/router_config_loader.ts#L60-L63 I’m not sure exactly how that would happen, but this appears as though it could be the issue.

That said, the ordering of the event is correct and the fact that it’s not called if _loader$ already exists is also correct. The preloader and a navigation can both request a lazy loaded route to be loaded at the same time. The code prevents double loading and double listener notification. onLoadStartListener should not be called twice in this case.

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