The Angular Injector lives at the core of Angular Dependency injection system. In this article we will look at it in more detail. The Angular Injector is responsible instantiating the dependency and injecting into the component or service.
The Angular Providers array returns the Provider, which contains the information about how to create the instance of the dependency. The Injector creates the instance and injects it into Component or service. The Angular creates an injector when the application root module named as AppModule is bootstrapped. This injector is called as root injector. The root injector has the application wide scope. It acts as parent to all injectors.
Suggested Reading Angular Bootstrapping Process. Angular root module loads the AppComponent.
Angular Injector, @Injectable & @Inject
We call this as root component. The AppComponent gets its own injector. This injector created as the child of the root injector. 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. The Angular Injector is also created for all those components creating a Injector tree closely mimicking the component tree.
The Every Injector gets its own copy of Providers. The every injector has a Provider associated with it. Where you register your dependency defines the scope of the dependency. This Dependency is available to entire application. The dependency registered with the component is available to that component and any child component of that component. The Injectable is a decorator, which you need to add to the consumer of the dependency. This decorator tells angular that the constructor arguments must be injected using the Angular DI system.
We created an example application in the Angular Dependency injection tutorial. The ProductService has a dependency on the LoggerService. Hence it is decorated with the Injectable decorator. Remove Injectable from LoggerService will not result in any error as the LoggerService do not have any dependency. These decorators also tell Angular to use DI, hence you do not need to add the Injectable.
It is a manual way of injecting the dependency. We can manually inject the LoggerService by using the Inject decorator applied to the parameter loggerService as shown below.
The token is used to locate the dependency in the Providers. This site uses Akismet to reduce spam. Learn how your comment data is processed.RxJS Masterclass is here. Want to level up your Angular skills?Angular 6 Basics 19 - Creating and using multiple modules
Who doesn't. Here's what you need to know. Providers in Angular are key to how we develop our applications, and injecting dependencies can be done in various ways. With most things Angular, there is a lot of magic happening when it comes to dependency injection DI. With Angular 1. You can check out my old post on the DI annotation process for more on that if you like. This was a great approach - but it came with some limitations. For example:. The type definition here is Http note the capital Hand Angular automagically assigns this to http.
We could alternatively write our component like this:. At this point, Inject is a manual way of specifying this lookup token, followed by the lowercase http argument to tell Angular what to assign it against. This could and will get very messy when a component or service requires a lot of dependencies. The reason we use Inject is because we cannot use an OpaqueToken as the type of a parameter, for instance this will not work:.
However, when we introduce Inject alongside an OpaqueTokenthings will work out nicely:. This may change however, as there is a current issue to make Injectable mandatory however this is pretty fresh and may not land for a while, or ever. When using Angular decorators, the decorated class stores metadata about itself in a format that Angular can read - this includes the metadata about what dependencies it needs to fetch and inject.
If no Angular decorator has been used on a class there is no way for Angular to read what dependencies it requires. This is why we need to use Injectable. If our service injects providers we must add Injectablewhich providers no extra functionality, to tell Angular to store that metadata it needs.
This would break as the Http provider metadata would not be stored for Angular to compose it correctly. At this point, Angular is aware of the Http token and can supply it to http. Now that we know how Angular knows what to inject, we can learn how it resolves our dependencies and instantiates them.
Configuring Dependency Injection in Angular
This means that Angular can look up what is stored under the token for AuthService using the useClass value. This provides many benefits. The first, we can now have two providers with the exact same class name and Angular will not have any issues in resolving the correct service. Secondly, we can also override an existing provider with a different provider whilst keeping the token the same. Imagine we use this service heavily throughout our application. For instance, our streamlined login form uses it to log the user in:.
We then hook this all up into a module, such as AuthModule :.This lesson explains how we can inject simple service dependencies into Angular 2 components using providers. Access all courses and lessons, track your progress, gain confidence and expertise. When building applications, we often want to put business logic into isolated units so they can be reused across different parts of the application.
In Angular, these units are called services, and they can be implemented in many different ways. If we take a look at this list component, we can see that it generates a list of items based on a collection called items, which is defined and initialized on the component. The browser renders that list.
If we go ahead and change one of the item's names, Angular reflects that change once we save the file. Items is now hard-coded in the component, but usually our data can come from many different places, so it's better to have an abstraction that takes care of returning the data, no matter where it comes from. What we want is a data service. As mentioned earlier, a service in Angular can be implemented in many different ways.
One way is to create a service using a class. Let's create a new file in source. We create a new class dataService and export it so it can be imported by other modules later on.
The next thing we do is we create a property items, which gets the list of items we defined in our list component. Let's copy them over. Next, we define a method on dataService called getItems, which simply returns the items of the class. Now, what we want to do is to inject an instance of dataService into our list component so we can use it to get our items.
Let's go back to our list component and import dataService. Next, we extend the component's constructor to ask for dependency of type data service by adding a constructor parameter with a type annotation for data service. The type annotation is where the magic happens. With this, we tell Angular's dependency injection to give us an instance of whatever it knows as data service. At this point, the dependency injection doesn't know what a data service is.
That's why we need to create a provider for data service. We create a provider by adding a providers property to our component, which is an array of provider definitions. The easiest way to create a provider for our data service is to simply add the data service type to that array.
Now Angular knows what data service is and how to create it when someone asks for it. Since we injected a dependency of data service using the private keyword, it's exposed as a property in the class. We can now call this. When we save the file, the browser reloads. As we can see, we still get our list of items, but this time, coming from our service that we've injected into the component.
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It can be used like so:. In the above we've asked for chatWidget to be the singleton Angular associates with the class symbol ChatWidget by calling Inject ChatWidget. It's important to note that we're using ChatWidget for its typings and as a reference to its singleton. We are not using ChatWidget to instantiate anything, Angular does that for us behind the scenes.
When using TypeScript, Inject is only needed for injecting primitives. TypeScript's types let Angular know what to do in most cases. The above example would be simplified in TypeScript to:.
Injectable lets Angular know that a class can be used with the dependency injector. Injectable is not strictly required if the class has other Angular decorators on it or does not have any dependencies. What is important is that any class that is going to be injected with Angular is decorated. However, best practice is to decorate injectables with Injectableas it makes more sense to the reader.
Here's an example of ChatWidget marked up with Injectable :. In the above example Angular's injector determines what to inject into ChatWidget 's constructor by using type information. This is possible because these particular dependencies are typed, and are not primitive types. In some cases Angular's DI needs more information than just types.
The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. I'm developping a little UI components framework for my personal needs and for fun.
I'm developping a Tab component and for testing purpose, I need to inject dynamically a component TabContainerComponent inside another component TabComponent. Below, the code of my two components:. I used a ComponentFactoryResolver and a ComponentFactory to create dynamically my new component TabContainerComponentand inject it in a placeholder inside my other component TabContainerin the addTab method:.
When I click on the "Add tab" button, I'm able to see the console. I tried also to check changes by implementing OnChanges interface in TabComponent class but without success.
In addTab used createEmbeddedView to add new tab container component. I thought that "ContentChildren" query in TabComponent should be updated by the newly added component but it doesn't. I have tried to subscribe to "changes" for the query list in TabCompoent but it doesn't get triggered. But I observed that "ViewChildren" query in AppComponent got updated every time a new component is added.
The working demo is available here. Learn more. Angular - Dynamically inject a component in another component Ask Question. Asked 1 year, 8 months ago.
Active 1 year, 8 months ago. Viewed 16k times. Below, the code of my two components: tab. Is anyone have an idea to solve my problem? Justin C. Given that you will inject the same component every time, why not work around this issue with a ngFor? I inject the same component only for testing purpose. Jul 24 '18 at Active Oldest Votes. Here is the way I could make it work - Tab. Thanks for your solution.You will do so by creating a very flexible dialog system, that demonstrates how dynamic components are used.
We will learn how to create dynamic components and attach them to the DOM or use them in other components. Also, you will discover how to provide objects to these dynamic components using dependency injection and custom angular injectors. At the end, we will come up with a dialog system that is quite similar to the one provided by angular material. Dynamic means, that the components location in the application is not defined at buildtime. That means, that it is not used in any angular template.
Because of the broad use of the word "dynamic", one could think that you could "dynamically" load just any component from the internet at runtime. That means that the component has to be defined in the same project or has to be imported into an angular module of the project. Before we get started, we need an angular project to work on. In this tutorial, we are going to generate a new angular-cli project. If you want to use an existing project, that should be fine, too.
To generate a new project, make sure you have the angular-cli installed and use this command at the desired project destination:.
To get started, let's create an empty dialog component. This component will consist of a white dialog area, while the rest of the screen is blurred. We want the dialog to live in its own module. To do that, we generate a new module using the anuglar-cli. The cli will add the component to the same folder as the module.
Also, make sure that the dialog component is declared in the dialog module instead of the app module. Because we are already assigning click-callbacks in our template, let's make sure they are defined in our component-class, as well. Notice, that the component does implement the AfterViewInit and OnDestroy interfaces, as we will the hooks later. While we will implement onOverlayClicked later, we make sure that the click event on the dialog itself is not propagated to the parent elements.
Otherwise, the onOverlayClicked callback would always fire, even if we hit the dialog itself and not the overlay the overlay is the gray area around the dialog.
We do so in the onDialogClicked method. Also, the component has to know which type of component it will has to create later. For that, we are adding the property childComponentType. Now that we have a working dialog component, we could just add it to the app-component and it would work just fine. Ok, maybe we would have to add an Input to toggle it on and off, but that would not be an issue, right? Because this is all about dynamic components.
Also, we want our component to live in the HTML-body to make sure that the dialog always fills the whole screen. We will use a service to instantiate the component and place it into the body. Doesn't that sound crazy? Actually, this approach is quite convenient to use. You maybe have used it already, as this is exactly the same approach as the guys from angular material are using for their dialog component.The Angular dependency injection is now core part of the Angular and allows dependencies to be injected into the component or class.
In this tutorial, we will learn what is Angular Dependency Injection is and how to inject dependency into a Component or class by using an example. Angular 9. We built a ProductService in the Angular Services. Dependency Injection DI is a technique in which we provide an instance of an object to another object, which depends on it. Let us look at the ProductServicewhich we created in our Angular Services tutorial.
You can download the source code from GitHub available under the folder Services. Our ProductService returns the hard-coded products when getProduct method invoked.
Configuring Dependency Injection in Angular
We instantiated the productService directly in our component as shown below. The ProductService Instance is local to the Component. The ProductService hardcoded in our AppComponent. What if we want to use BetterProductService. And then we decides to change the service to some other service. Again we need to search and replace the code manually. It is hard to test this Component as it is difficult to provide the Mock for the ProductService.
For Instance, what if we wanted to substitute out the implementation of ProductService with MockProductService during testing. Our Component Class has now tied one particular implementation of ProductService.
It will make it difficult to reuse our component. We would also like to make our ProductService singleton so that we can use it across our application. How to solve all these problems. Move the creation of ProductService to the constructor the AppComponent class as shown below. Now our AppComponent does not create the instance of the ProductService. It just asks for it in its Constructor. The AppComponent is now decoupled from the ProductService. The AppComponent does not know anything about the ProductService.
It just works with the ProductService passed onto it. The AppComponent does not care. Our Component is now loosely coupled to the ProductService. AppComponent does not know how to create the ProductService. AppComponent is now easier to Test. It will work with any implementation of ProductService that is passed on to it. You can just create a mockProductService Class and pass it while testing.
Reusing of the component is becomes easier. Our Component will now work with any ProductService as long as the interface is honored.