Theres two different ways that our dependencies are initiated, either in createInternal or as a getter on a property. When compiled, Angular uses the direct path for each provider that it imports, so for instance when we write this code: The AoTd version will look a little like this: This is so when the code is then compiled and bundled together, we can take advantage of tree-shaking and only include the parts of each module we actually use. For instance, HttpModule would be imported as something like this: Which then is referred to using import6.HttpModule instead of HttpModule. This reduces the frequency with which the class changes. with any implementation of ProductService that is passed on to it. Lets take a look at what our code actually gets compiled into (Ive kept the ES6 imports in for clarity): From here, we can see the compiled code knows about http being equal to the Http service provided by @angular/http - its added as a decorator for our class here: So essentially, the @Component decorator is transformed into plain ES5, and some additional metadata is supplied through the __decorate assignment.
Angular will generate an injector for each of our modules, so in our case it will take AppModule (our decorated class) and create an injector named AppModuleInjector. You are {{ username }}! `, @angular/core/src/linker/ng_module_factory, @angular/core/src/testability/testability. Dependency injection pattern made our AppComponent testable, maintainable, etc. Reusing the Component: Reusing of the component is becomes easier. So whats the real code look like, as we know Angular is compiled? The reason we use @Inject is because we cannot use an OpaqueToken as the type of a parameter, for instance this will not work: Here, myToken is not a Type, its a value - which means TypeScript cannot compile it. In Angular we create components, which render a template. Premium Angular courses to skyrocket your skills to the top. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Why dont second unit directors tend to become full-fledged directors? how to Get All tokens against a specific Walllet Addresse? Angular uses the Dependency Injection design pattern, which makes it extremely efficient. Setter injection: The client uses a setter method into which the injector injects the dependency. rev2022.7.21.42635. Secondly, we can also override an existing provider with a different provider whilst keeping the token the same. Its important to note that all dependencies use a token to uniquely identify them, both when theyre registered and when they are looked up. How APIs can take the pain out of legacy system headaches (Ep. Heres what our AuthService might look like: Imagine we use this service heavily throughout our application. It just works with the ProductService passed onto it. implement an interface that exposes a setter method that accepts the dependency. and pass it to the AppComponent? This way, we can use AuthService everywhere in our application - without making further changes.
A simple question which I think might have a complex answer. Much like using wildcard imports, this is to avoid potential naming conflicts between providers. Which in turn tells Angular to lookup the Http token and supply it as a first parameter to the Components constructor - assigning it to this.http: This looks a little familiar to our old from $inject, however the class is being used as a token instead of a string. However, when we introduce @Inject alongside an OpaqueToken, things will work out nicely: We wont dive into OpaqueToken any further here, but this gives you an example of using @Inject for manually specifying tokens to be injected, as well as showing that the token can be anything. Making statements based on opinion; back them up with references or personal experience. Join 78,277 developers pushing their limits. But does it solve all our Problem ?. This constructor is not compatible with Angular Dependency Injection because its dependency at index 0 of the parameter list is invalid. Consider the following service, which can be generated using: As one can notice, we can create injectable dependencies by adding the @Injectable decorator to a class. the returnImportantValue function of the service. loosely coupled: Our Component is now loosely coupled to the ProductService. Our actual module is created too (AppModule) so it can be consumed by other modules. We inject the above dependency inside the following component: One can see we have imported our TestService at the top of the page. When we use a type definition in our constructor, Angular uses those types (which are classes) as the token for finding our dependencies. 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. So where does @Inject come into play? At this point, its pretty magical how it works.
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. The decorators you use for services, components, etc. You might have heard of the concept of a token (or even OpaqueToken). component not recognizing my Service passed by Dependency injection (Angular). Tips and tricks, motivation, courses and exclusive discounts. Ive changed all the imports to be named imports for readability as in the actual generated code, each module is imported using a wildcard to avoid naming conflicts.
As weve replaced it with a new class FacebookAuthService, all of our components will use that instead. This programming paradigm allows classes, components, and modules AppComponent does not know how to create the ProductService. A token is a key that is used to reference a provider (our Http import is a provider). For instance, our (streamlined) login form uses it to log the user in: Then we can bind our user information using the service to display the username: We then hook this all up into a module, such as AuthModule: There could also be various components that use the same AuthService. To learn more, see our tips on writing great answers. Properties are created on the AppModuleInjector for each provider/dependency: This is a snippet from the above compiled output - so were going to focus on the three properties defined on the class: Our module only declares the BrowserModule, so where have the CommonModule and ApplicationModule come from?
`, // do something if the user has logged in, ` The properties on the class, the module imports and how the dependency injection mechanism works. There is also a number appended on the end of every property in the module. Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Please refresh this page to activate it. 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. Our AppComponent is not dependent on a particular implementation of ProductService anymore. This could (and will) get very messy when a component or service requires a lot of dependencies. Easier to Test AppComponent is now easier to Test. Angular will produce VM (virtual machine) friendly code, to make it as performant as possible, which is fantastic.
This emits metadata about the type of the parameter into a decorator in our compiled JavaScript output. This is invoked as soon as the module is instantiated. That token is then passed into getInternal and the instance of the dependency is returned if it exists, source code extract again: So inside the getInternal method, you can see Angular is checking for our tokens using simple if statements, and will return the relevant property for the provider - if found. Typically wed create various modules when building our applications, as well as importing external modules, such as feature modules or libraries (such as ui-router). How is Angular able to know, just by type, what instance should be injected in? The only time wed need to use @Inject is alongside something like an OpaqueToken - which creates a unique blank token to be used as a dependency injection provider.