What is the difference between elementinjector and rootmodule injector in angular?
The RootModule Injector becomes part of the ModuleInjector Tree. Angular Root Module loads the AppComponent, which is the root component of our app. The AppComponent gets its own Injector. We call this root Injector. This Injector becomes the root of the ElementInjector tree. The Root Component contains all other components.
What is the root component of an angular app?
We call this root Injector. This Injector becomes the root of the ElementInjector tree. The Root Component contains all other components. Angular App will create child components under the Root Component. All these child component can have their own child components creating a tree of components.
What is the injector instance of the root component?
The Injector instance of the Root Component becomes the root Injector for the Element Injector tree. It gets the Providers from the provider’s property of the Root Component.
What is elementinjector in angular?
Angular Root Module loads the AppComponent, which is the root component of our app. The AppComponent gets its own Injector. We call this root Injector. This Injector becomes the root of the ElementInjector tree. The Root Component contains all other components. Angular App will create child components under the Root Component.
What is Angular injectable?
The @Injectable() decorator specifies that Angular can use this class in the DI system. The metadata, providedIn: 'root' , means that the HeroService is visible throughout the application.
What is root in Angular service?
The service itself is a class that the CLI generated and that's decorated with @Injectable() . By default, this decorator has a providedIn property, which creates a provider for the service. In this case, providedIn: 'root' specifies that Angular should provide the service in the root injector.
What is provider and injector in Angular?
When you configure an injector with a provider, you are associating that provider with a dependency injection token, or DI token. The injector lets Angular create a map of any internal dependencies. The DI token acts as a key to that map. The dependency value is an instance, and the class type serves as a lookup key.
Why we use @inject in Angular?
To define a class as a service in Angular, use the @Injectable() decorator to provide the metadata that allows Angular to inject it into a component as a dependency. Similarly, use the @Injectable() decorator to indicate that a component or other class (such as another service, a pipe, or an NgModule) has a dependency.
What is forRoot and forChild in Angular?
forRoot creates a module that contains all the directives, the given routes, and the router service itself. forChild creates a module that contains all the directives and the given routes, but does not include the router service. It registers the routers and uses the router service created at the root level.
What is singleton in Angular?
A singleton service is a service for which only one instance exists in an application.
What is difference between @inject and injectable?
We use the @Inject parameter decorator to instruct Angular we want to resolve a token and inject a dependency into a constructor. We use the @Injectable class decorators to automatically resolve and inject all the parameters of class constructor.
What is decorators in Angular?
Decorators are a design pattern that is used to separate modification or decoration of a class without modifying the original source code. In AngularJS, decorators are functions that allow a service, directive or filter to be modified prior to its usage.
What is DI token in Angular?
The Dependency Injection system in Angular uses tokens to uniquely identify a Provider. There are three types of tokens that you can create in Angular. They are Type Token, String Token, and Injection Token. DI Tokens.
What is lazy loading in Angular?
Lazy loading is a technique in Angular that allows you to load JavaScript components asynchronously when a specific route is activated. It improves the speed of the application load time by splitting the application into several bundles. When the user navigates through the app, the bundles are loaded as required.
What is inject in TypeScript?
The Typed Inject project focuses on type safety and explicitness. It uses neither decorators nor decorator metadata, opting instead for manually declaring dependencies. It allows for multiple DI containers to exist, and dependencies are scoped either as singletons or as transient objects.
What is pipes in Angular?
Pipes are a useful feature in Angular. They are a simple way to transform values in an Angular template. There are some built in pipes, but you can also build your own pipes. A pipe takes in a value or values and then returns a value.
What are root components?
The root component is the very first component that is referenced and hosted in the main index. html web page. Everything else in Angular builds off of this root, so it helps to take a close look at how this root component works.
What are the services in Angular?
What Are Angular Services? Angular services are objects that get instantiated just once during the lifetime of an application. They contain methods that maintain data throughout the life of an application, i.e., data is available all the time.
What is useClass in Angular?
Use the Class Provider useClass , when you want to provide an instance of the provided class. The useClass expects us to provide a type. The Injector creates a new instance from the type and injects it. It is similar to calling the new operator and returning instance.
What is tree Shakable in Angular?
Tree-shakable dependencies are easier to reason about and compile to smaller bundles. Angular modules ( NgModule s) used to be the primary way to provide application-wide dependencies such as constants, configurations, functions, and class-based services.
How does dependency injection work in Angular?
Dependency injection (DI) is a paradigm. The way it works in Angular is through a hierarchy of injectors. A class receives its resources without having to create or know about them. Injectors receive instruction and instantiate a service depending on which one was requested.
What is an injector in a DI framework?
Injectors form the backbone of the DI framework. They store instructions to instantiate services so consumers do not have to. They receive service instances without needing to know anything about the source dependency! I should also note that other schematics without injectors can still utilize dependency injection.
What is providedin in Angular?
Using this method, and depending on if the service gets used, the service may or may not register with the injector. Angular calls this tree-shaking.
What is the paradigm of Angular?
The paradigm exists throughout Angular. It keeps code flexible, testable, and mutable. Classes can inherit external logic without knowing how to create it. Any consumers of those classes also do not need to know anything. DI saves classes and consumers alike from having to know more than necessary. Yet the code is as modular as it was ...
Why is DI important in Angular?
DI saves classes and consumers alike from having to know more than necessary. Yet the code is as modular as it was before thanks to the mechanisms supporting DI in Angular.
Can skeletons register with injectors?
Each skeleton can register services to an injector. In fact, TemplateService is a service. As of Angular 6, services can now register with injectors using @Injectable metadata.
Do injectors fall back on parents?
As previously mentioned, child injectors fallback on their parents. This fallback strategy ensures parents do not have to re- register for every injector. Refer to this article on Services and Injectors for an illustration of this concept.
What is injector in Angular?
Injector. The Angular creates an Injector instance for every Component, Directive, etc it loads. It also creates an injector instance for the Root Module and for every lazy loaded module. But eagerly loaded modules do not get their own injector but share the injector of the Root Module.
What is the Injector instance of the Root Component?
The Injector instance of the Root Component becomes the root Injector for the Element Injector tree. It gets the Providers from the provider’s property of the Root Component.
What is module injector tree?
Module Injector tree is for Modules (@NgModule). For Root Module & for every Lazy Loaded Module.
How does Angular dependency injection work?
In this tutorial, we will look at how Angular dependency injection works. The Angular creates a hierarchical dependency injection system. It creates a hierarchical tree of Injectors. Each Injector gets their own copy of Angular Providers. Together these two form the core of the Angular dependency injection framework. We will learn how Angular creates the injector tree. How injector resolves the dependency.
What happens when an injector fails to find the provider?
If the provider is found, the request returns the instance of the Provider. If not found then the request continues until the request reaches the topmost injector in the Element Injector tree.
Where is the topmost injector in an element?
The topmost Injector in the Element Injector tree belongs to the root component. If the dependency is not found, it does not throw the error but returns back.
When does Angular create the module injector tree?
Angular Creates the Module Injector tree when the Application starts.
What does @ injectable mean in Angular?
The @ Injectable () decorator specifies that Angular can use this class in the DI system . The metadata, providedIn: 'root', means that the HeroService is visible throughout the application.
What is Angular DI?
Angular's DI framework provides dependencies to a class upon instantiation. Use Angular DI to increase flexibility and modularity in your applications.
How to inject a dependency in a component?
To inject a dependency in a component's constructor (), supply a constructor argument with the dependency type. The following example specifies the HeroService in the HeroListComponent constructor. The type of heroService is HeroService.
What is dependency injection?
Dependency injection, or DI, is a design pattern in which a class requests dependencies from external sources rather than creating them.
When you create a class whose constructor () has parameters, what does it mean?
When you create a class whose constructor () has parameters, specify the type and metadata about those parameters so that Angular can inject the correct service.
What are injectors in Angular?
Injectors in Angular have rules that you can leverage to achieve the desired visibility of injectables in your applications. By understanding these rules, you can determine in which NgModule, Component or Directive you should declare a provider.
How many injectors are there above root?
There are two more injectors above root, an additional ModuleInjector and NullInjector ().
How to alter where the injector starts looking for flowerservice?
To alter where the injector starts looking for FlowerService, add @ SkipSelf () to the <app-child> @ Inject declaration for the FlowerService. This declaration is in the <app-child> constructor as shown in child.component.ts:
What is element injector hierarchy?
ElementInjector hierarchy—created implicitly at each DOM element. An ElementInjector is empty by default unless you configure it in the providers property on @ Directive () or @ Component ().
What does @ NgModule do?
If you configure an app-wide provider in the @ NgModule () of AppModule, it overrides one configured for root in the @ Injectable () metadata. You can do this to configure a non-default provider of a service that is shared with multiple applications.
Why does injecting the service with @self and @optional return null?
In this example, there is a parent provider and injecting the service will return the value, however, injecting the service with @ Self () and @ Optional () will return null because @ Self () tells the injector to stop searching in the current host element.
Why use @self in Angular?
Use @ Self () so that Angular will only look at the ElementInjector for the current component or directive.
Can injectable decorator be removed?
Before, a class with Injectable decorator didn’t indicate that it could have dependency, it was not related to how it would be used in other parts. So, if a service does not have any dependency , @Injectable () can be removed without causing any issue.
Is dependency injection angular?
Dependency injection model is quite complex topic in angular. Knowing how it works internally makes you confident in what you do. So I strongly suggest you looking into angular source code from time to time…
Question
I'm looking for the best approach for injecting a known/defined component into the root of an application and projecting @Input () options onto that component.
Requirement
This is necessary for creating things like modals/tooltips in the body of the application so that overflow:hidden /etc will not distort the position or cut it off completely.
Research
I've found that I can get the ApplicationRef 's and then hackily traverse upwards and find the ViewContainerRef.
Do you need to import @injectable decorated?
The only other action required in order to use the @Injectable decorated service is to import it and constructor inject it and that's it. No need to import it in the AppModule.
Can you inject someservice in Angular 6?
Now we can inject the SomeService that the module provides because we have imported the module. With Angular 6 the need to import the SomeNPModule into the AppModule is removed because we are using @Injectable (providedIn: 'root) on the service itself, and when the annotation runs it automatically makes the service available in the root injection container?
Injectors
- Injectors are data structures that store instructions detailing where and how services form. They act as intermediaries within the Angular DI system. Module, directive, and component classes contain metadata specific to injectors. A new injector instance accompanies every one of these classes. In this way, the application tree mirrors its hierarchy...
Dependency Injection
- Take a look at the skeletons for each class: service, module, directive, and component. Each skeleton can register services to an injector. In fact, TemplateService is a service. As of Angular 6, services can now register with injectors using @Injectablemetadata. Notice the providedIn: string (@Injectable) and providers: [] (@Directive, @Componet and @Module) metadata. They tell injec…
Service
- The providedIn: string metadata of @Injectable specifies which injector to register with. Using this method, and depending on if the service gets used, the service may or may not register with the injector. Angular calls this tree-shaking. By default the value is set to ‘root’. This translates to the root injector of the application. Basically, setting the field to ‘root’makes the service available an…
Module, Directive, and Component
- Modules and components each have their own injector instance. This is evident given the providers: [] metadata field. This field takes an array of services and registers them with the injector of the module or component class. This approach happens in the @NgModule, @Directive, or @Componentdecorators. This strategy omits tree-shaking, or the optional remova…
Instantiating References
- References to the DOM can instantiate from any class. Keep in mind that references are still services. They differ from traditional services in representing the state of something else. These services include functions to interact with their reference. Directives are in constant need of DOM references. Directives perform mutations on their host elements through these references. See t…
Instantiating Services
- The Services and Injectorsarticle explains this section to an extent. Though, this section rehashes the previous section or the most part. Services will often provide references to something else. They may just as well provide an interface extending a class’ capabilities. The next example will define a logging service that gets added to a component’s injector via its providers: []metadata. …
Conclusion
- Dependency injection (DI) is a paradigm. The way it works in Angular is through a hierarchy of injectors. A class receives its resources without having to create or know about them. Injectors receive instruction and instantiate a service depending on which one was requested. DI shows up a lot in Angular. The official Angular documentation explains why the paradigm is so prevalent. …
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