Preloading Strategy issue for forChild routes - angular7

Question

My application is a bit large in size. It contains many feature modules and most are lazily loaded. I want to preload a lazily loaded module, which is included in forChild routes.
For this, I have referred to Angular documentation and followed their steps. I have provided a custom preloading strategy service mentioned below.
This is my custom preloading strategy file:
#Injectable()
export class CustomPreloadingWithDelayStrategy implements PreloadingStrategy {
preload(route: Route, load: () => Observable<any>): Observable<any> {
if (route.data && route.data['preload']) {
return load();
} else {
return Observable.of(null);
}
}
}
app-routing file,
const routes: Routes =
[
XXX,
{
path: '',
data: {
base: true
},
component: MyComp,
children: [
{
path: 'page1/:id',
loadChildren: 'XXXXXXX'
},
{
path: 'page2',
loadChildren: 'XXXXXXXX'
},
{
path: 'page3',
loadChildren: 'app/feature-modules/folder1/my-folder1-module#Folder1Module'
}];
#NgModule({
imports: [RouterModule.forRoot(routes, {useHash: true, preloadingStrategy: CustomPreloadingWithDelayStrategy})],
exports: [RouterModule],
entryComponents: [ ]
})
export class AppRoutingModule {}
My Folder1Module's routing file:
const routes: Routes = [{
path: 'sub-page1/:data1/:data2',
loadChildren: 'app/feature-modules/sub-pages/pages/sub-page1.module#SubPage1Module'
}, {
path: 'sub-page2/:data1',
loadChildren: 'app/feature-modules/sub-pages/pages/sub-page2.module#SubPage2Module',
data: {preload: true}
}];
#NgModule({
imports: [RouterModule.forChild(routes)],
exports: [RouterModule]
})
export class Folder1RoutingModule {
}
So when I open this route /page3/sub-page1/data1/data2, SubPage2Module is to be preloaded. But that is not happening.

Answer 1

I have spent nearly 2 hours to understand why the module is not pre-loaded even when I was doing everything right.
The problem lies with children, any route that is defined and further distributed with children is considered as a normal route even if you've defined your modules within the children with the help of loadChildren. The preloadingStrategy just takes modules to preload and not path/routes. The moment you've defined your routes under children it considers it as normal route and moves on to scan other routes that are provided with loadChildren. Following is the interpretation of Angular with your routes:
const routes: Routes =
[
XXX, // Normal path
{
path: '',
data: {
base: true
},
component: MyComp,
children: [ // Normal path (no module) as children is used, move on
{
path: 'page1/:id',
loadChildren: 'XXXXXXX'
},
{
path: 'page2',
loadChildren: 'XXXXXXXX'
},
{
path: 'page3',
loadChildren: 'app/feature-modules/folder1/my-folder1-module#Folder1Module'
},
],
},
{ path: 'abc', loadChildren: '../path/to/module#AbcModule', data: { preload: true }} // Module found, preload it!
];
If you debug closely in your custom CustomPreloadingWithDelayStrategy then you will observe that your route /page3/sub-page1/data1/data2 can't even make up to the route parameter of preload() method because preloading is all about loading module and not about loading routes. However, our route abc does make an appearance! Hope it helps :)