ForChild is used when a module is lazy (loads when the module loaded on demand). As we've seen here, we can't just use an in-line factory function to provide feature-module configuration to our feature-module services. When we want to access our providers from any point in the application. If we were using the Just-in-Time (JIT) compiler, I could just create a factory function that translates the incoming options hash into an instance of the MyServiceOptions class: With the Ahead-of-Time (AoT) compiler, however, this approach breaks with the following error: Function calls are not supported in decorators. Im just really glad that these guys didnt write any Java framework that became popular and I need to write backend in this :). It is a "batteries-included" framework that comes with many powerful libraries and modules baked in so that you can start building your app quickly. The problem here is that the Dependency-Injection framework relies on "Types". Data Binding In Angular 11: one-way and two-way data binding. During build time of our applicatione we got an error like: The problem was the kind we imported the MyServiceModule from the library. In this post, we will cover the understanding of how to inject configurations with forRoot and forChild in Angular and finally, we going to use Angular forroot vs forchild patterns in a simple application. Switching to the old "import style" will arise again the mentioned error. With this setup, we can import this module in our root module. View this code in my JavaScript Demos project on GitHub. And, unfortunately, it goes a bit over my head. In this guide, you will learn how to use the Angular Router specifically the forRoot and forChild static methods. Let's see how you could use this in a real app! if wed implement something like this. Take the following for example. Problem: We needed to share a singleton language service across all lazy loaded modules. These instances are used in the context where they are created. This has the outcome that when the feature module is loaded, the module will use this theme instead of the root theme. To solve this Types problem, I ended up creating an intermediary injectable using the InjectionToken class. For example if you want to create just a form, prepare yourself to read almost 34 pages of text to start. In order to play nicely with the AoT compiler, you have to extract the factory function declaration out as a top-level exported value on the JavaScript module. For a real-world example, you can take a look at the implementation of the Angular Router. Pretty much every JavaScript framework has a router available for use in some form or another. The idea is that .forRoot() provides the Declarations + Services, and then the .forChild() provides just the Declarations (since the Services should already be available via the .forRoot() call). First thing is that it requires to learn a new language - TypeScript. This ensures that only a single instance of your service exists at the root level. Angular creates a factory for all the modules, except for the lazy modules, which when loaded on demand, have their factory. // implicitly consumed by the MyService constructor).

The Ahead-of-Time (AoT) compiler for Angular provides nice performance benefits; however, it does so with some caveats. Each time any component increments the value stored in the counter service, I want to share this with all components. Other recommended readings:, Other articles: Use a Robot to upgrade your node dependencies, Angular Developer, Angular and Observables: how to avoid the subscription with the help of Async Pipe, How to fix ng: command not found in angular, Streaming IAsyncEnumerable to RxJs front-end, import { NgModule,ModuleWithProviders } from '. At least in those two scenarios: forRoot() in Angular is something you can frequently see in the AppModule imports, for example in you use ngx-bootstrap: And this is just Angulars workaround for the problem described above: how to ensure that our SharedService will be a singleton service for every app? What Does A Digital Forensics Analyst Do? Instantly share code, notes, and snippets. This is the configuration we can face the problem with the secondary SharedService instance created by the LazyModule injector. But, the outcome is that we can still use proper Types to represent developer-provided .forRoot() arguments. Only one module will be registered and used, depending on how the modules are loaded. For more information, check out Angular's documentation. method to register the module and the service. This gets more complicated with lazy-loaded modules. If a feature module would dispatch an action it wouldn't reach the "real" root reducer, it will only reach the reducers configured by the lazy module. Unfortunately, I have no advice -- I'm just Googling for solutions when I run into this. By doing this we can guard the developers using the ThemeModule by not accidentally importing our module more than once via the forRoot() function. ThemeModule.forRoot() called twice. Why is it bad if a shared module provides a service to a lazy-loaded module? As such, I'm not sure that I understand what is going on. // I translate the given raw OPTIONS into an instance of the MyServiceOptions TYPE. We imported it through a my.service.module.ts file (initially we did not want do expose all components through the main entry point in our library): The fix for us was to add the module export also to the public_api.ts file of the library and imported it from there: Everythings works now. This MyServiceOptions instance is then implicitly available to the MyService class as part of the Dependency-Injection container. In the coming sections, you will learn about the forRoot and forChild static methods that are available on the RouterModule itself and how to use them to compose your routes. When you are using the forChild static method, you are basically telling Angular, "There is already a Router instance available in the app so please just register all of these routes with that instance." I tried to look into Angular Router module code, that uses forRoot(), but it is too hard to understand. In comparison to the first solution where we have the guard inside the createThemeRootGuard() function which is only provided in the forRoot() function, meaning that it will only be checked if we import the module with forRoot(). This is because this method tells Angular to instantiate an instance of the Router class under the hood, and there can be only one router in your app. In Angular 7 theres a new construction introduced, however the old syntax should be known as well by developers, because a plenty of external libs use it. The reason why this is a problem is that these modules must be treated as singletons. The source code is available on Stackblitz of the ForRoot and ForChild singleton pattern in Angular 11. Demo Application URL : URL or, is a pattern for singleton services and allows us to have control of the provider. However, since I was using Ahead-of-Time (AoT) compiling, I was running into a number of errors that wouldn't be relevant when using the Just-in-Time (JIT) compiler. After successfully running your application, the output of your application should be like the below screen. @Ben, I met a really good angular developer and we took a look to my code. We make it very clear we didn't expect this to happen, and we can save some time and frustrations for the developers using our module if our module is badly used. // NOTE: This value has to be exported otherwise the AoT compiler won't see it. I don't know much about the use of index.js files. I tried using your jit version but ModuleOptions gives me an error saying TS2304: Cannot find name 'ModuleOptions'. it has its own injector. specifically, when we want to deliver a provider that is visible only to the children modules of our module. In this library there are 2 lazy loaded modules. But, it is nice to provide a way for the consuming application to override these configuration values when importing the feature module. Instead of using a class for the options, use an object with an InjectionToken, static forRoot( options? But, since I don't really build libraries, I don't have good instincts for that stuff. As such, it seems odd that you'd get an error message telling you how to use these methods. Angular creates a factory for all the modules, except for the lazy modules, which when loaded on demand, have their factory. See Plunker: So in the application above we have two different injectors: root (at AppModule level; exists in every Angular app), and LazyModule injector. . In this article Im showing some fundamentals of Angular Hierarchical Dependency Injection feature and why and when to use forRoot() providers. Each injector can create its own service instances of the same service and you may think you have singleton services, while there are multiple distinct instances of them used by different modules. This is where the static module method, .forRoot(), comes into play. We can write such a simple guard to verify if the service is unique: And this simple shared service can be provided by some shared module: Lets now try to inject our shared service into AppComponent and LazyComponent: Remember, that the AppComponent belongs to the AppModule, while the LazyComponent belongs to the LazyModule. As such, our feature module has to translate this incoming raw data-bag into a fully-featured "Type" in order to play nice with the Dependency-Injector container. To test that the MyService feature module works with the optional overrides, I injected it into my AppComponent and logged it to the console: And, when we run this Angular application in the browser, we get the following output: As you can see, by translating the .forRoot() options object into an instance of the MyServiceOptions Type (using the factory function and the intermediary InjectionToken), the Dependency-Injection container was able to instantiate the MyService class, which we were then able to inject into the AppComponent. To solve the closure problem, you might try to just supply the "options" value using the "deps" option on your extracted factory function: However, since the "options" arguments is just a plain hash, TypeScript will fail with the following error: Error: Internal error: unknown identifier {"retryInterval":5000,"retryCount":3}. But, I have very little experience actually publishing anything. That's really interesting. We might not notice it when this happens but often it is the cause of unexpected behavior, which is sadly hard to debug. This will be used, under the hood, to instantiate the MyServiceOptions. In my libraries services, I directly Inject the InjectionToken. A lazy-loaded module will use the instances it has created instead of using the root instances. // When importing the MyServiceModule, we can provide optional configuration, // data. Using forRoot -> getting error, multiple forRoot is not allowed(Build compilation is success, but while importing the library in Application Root routing, it throws error). It still gives me the following error: ERROR in Error during template compile of 'ParentModule' Function calls are not supported in decorators but 'ChildModule' was called.

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