feat(router): deprecate loadChildren:string (#30073)

The proposed ES dynamic import() is now supported by the Angular CLI and the
larger toolchain. This renders the `loadChildren: string` API largely
redundant, as import() is far more natural, is less error-prone, and is
standards compliant. This commit deprecates the `string` form of
`loadChildren` in favor of dynamic import().

DEPRECATION:

When defining lazy-loaded route, Angular previously offered two options for
configuring the module to be loaded, both via the `loadChildren` parameter
of the route. Most Angular developers are familiar withthe `string` form of
this API. For example, the following route definition configures Angular to
load a `LazyModule` NgModule from `lazy-route/lazy.module.ts`:

```
[{
  path: 'lazy',
  loadChildren: 'lazy-route/lazy.module#LazyModule',
}]
```

This "magic string" configuration was previously necessary as there was
no dynamic module loading standard on the web. This has changed with the
pending standardization of dynamic `import()` expressions, which are now
supported in the Angular CLI and in web tooling in general. `import()`
offers a more natural and robust solution to dynamic module loading. The
above example can be rewritten to use dynamic `import()`:

```
[{
  path: 'lazy',
  loadChildren: () => import('./lazy-route/lazy.module').then(mod => mod.LazyModule),
}]
```

This form of lazy loading offers significant advantages in terms of:

* type checking via TypeScript
* simplicity of generated code
* future potential to run natively in supporting browsers
  (see: [caniuse: dynamic import()](https://caniuse.com/#feat=es6-module-dynamic-import))

As a result, Angular is deprecating the `loadChildren: string` syntax in
favor of ES dynamic `import()`. An automatic migration will run during
`ng upgrade` to convert your existing Angular code to the new syntax.

PR Close #30073

packages/core/src/linker/ng_module_factory_loader.ts

@@ -17,6 +17,8 @@ import {NgModuleFactory} from './ng_module_factory';
  * Used to load ng module factories.
  *
  * @publicApi
+ * @deprecated the `string` form of `loadChildren` is deprecated, and `NgModuleFactoryLoader` is
+ * part of its implementation. See `LoadChildren` for more details.
  */
 export abstract class NgModuleFactoryLoader {
   abstract load(path: string): Promise<NgModuleFactory<any>>;

packages/core/src/linker/system_js_ng_module_factory_loader.ts

@@ -24,6 +24,8 @@ declare var System: any;
  * token.
  *
  * @publicApi
+ * @deprecated the `string` form of `loadChildren` is deprecated, and `SystemJsNgModuleLoaderConfig`
+ * is part of its implementation. See `LoadChildren` for more details.
  */
 export abstract class SystemJsNgModuleLoaderConfig {
   /**
@@ -47,6 +49,8 @@ const DEFAULT_CONFIG: SystemJsNgModuleLoaderConfig = {
 /**
  * NgModuleFactoryLoader that uses SystemJS to load NgModuleFactory
  * @publicApi
+ * @deprecated the `string` form of `loadChildren` is deprecated, and `SystemJsNgModuleLoader` is
+ * part of its implementation. See `LoadChildren` for more details.
  */
 @Injectable()
 export class SystemJsNgModuleLoader implements NgModuleFactoryLoader {

packages/router/src/config.ts

@@ -94,6 +94,18 @@ export type ResolveData = {
  *
  * A function that is called to resolve a collection of lazy-loaded routes.
  * 
+ * Often this function will be implemented using an ES dynamic `import()` expression. For example:
+ * 
+ * ```
+ * [{
+ *   path: 'lazy',
+ *   loadChildren: () => import('./lazy-route/lazy.module').then(mod => mod.LazyModule),
+ * }];
+ * ```
+ * 
+ * This function _must_ match the form above: an arrow function of the form
+ * `() => import('...').then(mod => mod.MODULE)`.
+ *
  * @see `Route#loadChildren`.
  * @publicApi
  */
@@ -105,10 +117,24 @@ export type LoadChildrenCallback = () => Type<any>| NgModuleFactory<any>| Observ
  * A string of the form `path/to/file#exportName` that acts as a URL for a set of routes to load,
  * or a function that returns such a set.
  *
+ * The string form of `LoadChildren` is deprecated (see `DeprecatedLoadChildren`). The function
+ * form (`LoadChildrenCallback`) should be used instead.
+ *
  * @see `Route#loadChildren`.
  * @publicApi
  */
-export type LoadChildren = string | LoadChildrenCallback;
+export type LoadChildren = LoadChildrenCallback | DeprecatedLoadChildren;
+
+/**
+ * A string of the form `path/to/file#exportName` that acts as a URL for a set of routes to load.
+ *
+ * @see `Route#loadChildren`
+ * @publicApi
+ * @deprecated the `string` form of `loadChildren` is deprecated in favor of the proposed ES dynamic
+ * `import()` expression, which offers a more natural and standards-based mechanism to dynamically
+ * load an ES module at runtime.
+ */
+export type DeprecatedLoadChildren = string;
 
 /**
  *

packages/router/src/index.ts

@@ -7,7 +7,7 @@
  */
 
 
-export {Data, LoadChildren, LoadChildrenCallback, ResolveData, Route, Routes, RunGuardsAndResolvers, UrlMatchResult, UrlMatcher} from './config';
+export {Data, DeprecatedLoadChildren, LoadChildren, LoadChildrenCallback, ResolveData, Route, Routes, RunGuardsAndResolvers, UrlMatchResult, UrlMatcher} from './config';
 export {RouterLink, RouterLinkWithHref} from './directives/router_link';
 export {RouterLinkActive} from './directives/router_link_active';
 export {RouterOutlet} from './directives/router_outlet';

tools/public_api_guard/core/core.d.ts

@@ -586,6 +586,7 @@ export declare abstract class NgModuleFactory<T> {
     abstract create(parentInjector: Injector | null): NgModuleRef<T>;
 }
 
+/** @deprecated */
 export declare abstract class NgModuleFactoryLoader {
     abstract load(path: string): Promise<NgModuleFactory<any>>;
 }
@@ -1297,11 +1298,13 @@ export interface SkipSelfDecorator {
 
 export declare type StaticProvider = ValueProvider | ExistingProvider | StaticClassProvider | ConstructorProvider | FactoryProvider | any[];
 
+/** @deprecated */
 export declare class SystemJsNgModuleLoader implements NgModuleFactoryLoader {
     constructor(_compiler: Compiler, config?: SystemJsNgModuleLoaderConfig);
     load(path: string): Promise<NgModuleFactory<any>>;
 }
 
+/** @deprecated */
 export declare abstract class SystemJsNgModuleLoaderConfig {
     factoryPathPrefix: string;
     factoryPathSuffix: string;

tools/public_api_guard/router/router.d.ts

@@ -101,6 +101,9 @@ export declare class DefaultUrlSerializer implements UrlSerializer {
     serialize(tree: UrlTree): string;
 }
 
+/** @deprecated */
+export declare type DeprecatedLoadChildren = string;
+
 export declare type DetachedRouteHandle = {};
 
 export declare type Event = RouterEvent | RouteConfigLoadStart | RouteConfigLoadEnd | ChildActivationStart | ChildActivationEnd | ActivationStart | ActivationEnd | Scroll;
@@ -145,7 +148,7 @@ export declare class GuardsCheckStart extends RouterEvent {
     toString(): string;
 }
 
-export declare type LoadChildren = string | LoadChildrenCallback;
+export declare type LoadChildren = LoadChildrenCallback | DeprecatedLoadChildren;
 
 export declare type LoadChildrenCallback = () => Type<any> | NgModuleFactory<any> | Observable<Type<any>> | Promise<NgModuleFactory<any> | Type<any> | any>;