Introduction to Angular Dependency Injection

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

Applies to: Angular 2 to the latest edition of i.e. Angular 8. Angular 9, Angular 10

What is Dependency

We built a ProductService in the Angular Services. The AppComponent is depends on the ProductService to provide the list of Products to display.

In short, the AppComponent has a Dependency on ProductService.

What is Angular Dependency Injection

Dependency Injection (DI) is a technique in which we provide an instance of an object to another object, which depends on it. This is technique is also known as “Inversion of Control” (IoC)

Let us look at the ProductService, which 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.

We instantiated the productService directly in our component as shown below.

The ProductService Instance is local to the Component. The AppComponent is now tightly coupled to the ProductService, This tight coupling brings a lot of Issues.

The ProductService hardcoded in our AppComponent. What if we want to use BetterProductService. We need change wherever the ProductService is used and rename it to BetterProductService. What if we wanted to use either ProductService or BetterProductService based on users preference.

What if ProductService depends on  another Service. 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. You can pass ProductService, BetterProductService or MockProductService. The AppComponent does not care.

Now the responsibility of Creating the ProductService falls on the creator of the AppComponent.

The above pattern is known as Dependency Injection Pattern.

Why use Dependency Injection

Our Component is now loosely coupled to the ProductService. AppComponent does not know how to create the ProductService. 

AppComponent is now easier to Test. Our AppComponent is not dependent on a Particular implementation of ProductService anymore. 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.

Dependency injection pattern made our AppComponent testable, maintainable etc.

But does it solve all our Problem ?. No, we just moved the Problem out of Component to the Creator of the Component.

How do we create an instance of ProductService and pass it to the AppComponent? That is what Angular Dependency Injection does.

Angular Dependency Injection Framework

Angular Dependency Injection framework implements the Dependency injection Pattern in Angular. It creates & maintains the Dependencies and injects them into the Components or Services which requests for it.

Parts of Angular Dependency Injection Framework 

There are five main parts of the Angular Dependency injection Framework.

Consumer

The Component that needs the Dependency. In the above example, the AppComponent is the Consumer

Dependency

The Service that is being injected. In the above example the ProductService is the Dependency

DI Token

The DI Token uniquely identifies a Dependency. We use DI Token when we register dependency

Provider

The  Providers Maintains the list of Dependencies along with their Tokens. The DI Token is used to identify the Dependency.  

Injector

Injector holds the Providers and is responsible for resolving the dependencies and injecting the instance of the Dependency to the Consumer

How Dependency Injection works in Angular

The dependencies are registered with the Provider. This is done in the Providers metadata of the Injector.

Angular Provides an instance of Injector & Provider to every Consumer.

Consumer when instantiated, It declares the Dependencies it needs in its constructor.

Injector reads the Dependencies from the constructor of the Consumer and looks for the dependency in the provider. The Provider provides the instance and injector, then injects it into the consumer. If the instance of the Dependency is already exists, then it is reused making the dependency singleton.

How to Use Dependency Injection

We had created a simple ProductService in our last tutorial. Let us now update it to use Dependency Injection. 

First, we need to register the dependencies with the provider. This is done in the providers metadata array of @Component decorator.

Next, we need to tell angular that our component needs dependency injection. This is done by using the @Injectable() decorator.

@Injectable() decorator is not needed, if the class already has other Angular decorators like @Component, @pipe or @directive etc. Because all these are a subtype of Injectible.

Since our AppComponent is already decorated with @Component, we do not need to decorate with the @Injectable

@Injectible is also not needed if the class does not have any dependencies to be injected. However it is best practice is to decorate every service class with @Injectable(), even those that don’t have dependencies.

Next, our AppComponent needs to ask for the dependencies. This is done in the constructor as shown below

That’s it.

The Services are usually not added to Providers array of the Component, but to the Providers array of the @NgModule. Then they will be available to be used in all the components in the application

When AppComponent is instantiated it gets its own Injector instance. The Injector knows that AppComponent requires ProductService by looking at its constructor. It then looks at the Providers for a match and Provides an instance of ProductService to the AppComponent

The Complete AppComponent is as follows

Injecting Service into Service

We looked at how to inject ProductService to a component. Now let us look at how to inject service into another service.

Let us build loggerService, which logs every operation into a console window and inject it into our ProductService.

Logger Service

Create the logger.service.ts and add the following code

The LoggerService has just one method log, which takes a message and writes it to the console.

We are also using @Injectible metadata to decorate our logger class. Technically, this is not required here as the logger class does not have any external dependencies.

Make it a practice to add @Injectible metadata for the following reasons

Future proofing: No need to remember @Injectable() when we add a dependency later.

Consistency: All services follow the same rules, and we don’t have to wonder why a decorator is missing.

Product Service

Now we want to inject this into our ProductService class

The ProductService needs loggerService to be injected. Hence the class requires @Injectible metadata

Next, In the constructor of the ProductService ask for the loggerService.

And update the GetProducts method to use the Logger Service.

Finally, we need to register LoggerService with the Providers metadata.

Angular  does not have any options add providers in the Service Class. The Providers must be added to the Component/Directive/Pipe or to the Module.

Open the AppComponent Update the Providers array to include LoggerService

That’s it. As you click on the Get Products button, you will see the Console window updated with the Log messages

Using NgModule to Provide Dependencies

In the above example, we registered the dependencies in the Providers array of the component class. The dependencies are only available to the component where it is registered and to its child components.

To Make the dependencies available to the entire application, we need to register it in the root module.

Remove the providers: [ProductService,LoggerService], from the AppComponent and move it to the  AppModule as shown below

Providing the service in the root module will create a single, shared instance of service and injects into any class that asks for it.

The above code works because, we Angular creates the Tree of Injectors with parent child relationship similar to the Component Tree. We will cover this next tutorials.

The services injected at the module level are app-scoped, which means that they can be accessed from every component/service within the app.Any service provided in the Child Module is available in the entire application.

The services is provided in a lazy module are module scoped and available only to the lazy loaded module.

The services provided in the Component level are available only to the Component & and to the child components.

Suggested Reading

Angular Services
Angular Injector
Angular Providers
Angular Hierarchical Dependency Injection

Summary

We learned what is Angular Dependency Injection. We created two services and registered it in the Providers array of Component. We then injected one of the service into Component and the other one into another service.

Later, we removed the Providers from the Component and moved it to the Root Module, with the same result.

You can download the source code from Github. In the next tutorial’s we look at the Providers and injector in more detail.