For other dependencies I can use a #scope such as #Singleton to make it only have one instance.
For example:
#Component
#Singleton
interface ApplicationComponent {
fun getMySingleInstanceDependency(): MySingleInstanceDependency
}
#Singleton
class MySingleInstanceDependency #Inject constructor()
If I add a #subcomponent to the main component, it returns a new instance every time I require an instance. The only thing I can think of is to use the #Component.Builder to pass an instance of the subcomponent inside so it uses the same instance every time, but that doesn't sound right. Is there a better way to do this?
You are talking about different question. Lifecycle of object instance should nothing to do with subcomponent. Let me explain here your question.
By default all dependencies are created with your provide annotation. Which would be called each time we are requesting this item in different Dagger modules. Unless this particular dependency is annotated as singleton.
You subcomponent should nothing to do with above allocation.
Basically you would have one (or several) Dagger component which could include subcomponent. Their lifecycle is handled by Dagger scope.
Related
I am trying to implement a NotificationService in a correct way from the point of view of OOP. For this I have the next interface:
abstract class NotificationsService {
void initNotificationsHandlers();
int sendGeneralNotification({String? title, String? body});
//...
}
And his subclass:
class FirebaseNotificationService extends NotificationsService {
//All implementations...
}
The problem is when I implement it. I have to instance it on the main:
NotificationsService notificationsService = new FirebaseNotificationService();
But I have to use this service in more classes,and I don't want to instance the FirebaseNotificationService in every class because I would be violating the Dependency Inversion Principle. I want other classes just know the abstraction NotificationsService.
I have thought using something like this:
abstract class NotificationsService {
///Current notification service subclass used.
static final NotificationsService instance;
//...
}
And then implementing the class this way:
Main
NotificationsService.instance = new FirebaseNotificationService();
Other class
NotificationsService.instance.initNotificationsHandlers(); // For example, it could be any method
But it doesn't look very clean because I am using the NotificationService interface to "save" the current subclass. I think it shouldn't be his responsibility.
Maybe should I make another class which "saves" the current implementation? Or apply a singleton pattern? What is the OOP most correct way to do this?
Clarification: I am not asking for a personal opinion (otherwise this question should be close). I'm asking about the correct OOP solution.
In which language are you programming? Java probably, by reading your Code.
What you actually want is Dependency Injection and a Singleton (even though I think that Singleton is overkill for a NotificationService)
If we remain at the Java Standard, it works in this way:
The classes that need your NotificationService would have a constructor annotated with #Inject and an agument of type NotificationService (not your Implementation Class) - so your consumer classes rely on something abstract rather than something concrete, which makes it easier to change the implementation.
The Dependency Injection Container or Framework would take care that when your classes are being injected by them self somewhere, that their Dependencies are being satisfied in order to be able to construct this class.
How does it actually know which Implementation belongs to an Interface?
Well it depends on the Framework or Platform you are using but you either define your bindings of the interface to the concrete class or is is looking it up with reflection (if we are using Java)
If a class gets injected with a new Instance every time or always the same instance this depends on your annotations on the class itself. For example you could annotate it with #Singleton.
I hope it helps a bit.
I have a Kotlin application main class which provides a CoroutineScope. The main class creates various instances of other classes (composition), all of which should use the scope from the main class in order to implement proper structured concurrently patterns and enable a clean cancellation/shutdown..
NB: some of the child classes need to launch some background monitoring tasks in their initializer. So they need to have the coroutineScope available at class level.
What is the best and most idiomatic way to pass the scope to the component classes?
pass the scope to the child class as a constructor argument?
have each class inherit from CoroutineScope (having its own lifetime) and cancel that scope from the main class (risk of forgetting to cancel)
creating builder methods for the child classes, which would call the monitoring functions with the parent scope (not sure how this would be implemented)
....?
I have two different #Service classes that use the same methods.
I have extracted those methods in a separate class, that both will reference. To be able to access those methods, I added them to the companion object of the class, but the issue is that they use external services in their implementation, which I cannot wire in a companion object or pass it and access it there.
class CommonMethods {
companion object {
fun firstValidationField(input: User) {
// logic
input.timezone = userRepository.getTimezone(input.userId)
return input
}
etc
}
}
What is the best way to inject userRepository to this class so I can access it and have the common methods work for both external service classes?
I had these methods in the service class, so autowiring repositories there wasn't an issue. But extracting them as they are common, not sure how to approach this.
I use #Autowired to inject it
If firstValidationField() is really so simple as you wrote in the question, you can inject the userRepository in your both services, and userRepo.getTimeZone(..) that's it.
However, if the method impl. is more complex than that, or there are other commonly used methods, I would suggest wrapping those methods in an additional #service, say UserService, and inject the UserService into your other #services.
Usually, Util classes are not designed to be instantiated, it contains mainly static method calls. IMO, "Injecting" objects to a Util class isn't the right direction to go.
I've read what the annotation says but I'm kinda dumb, so couldn't understand propertly
Identifies injectable constructors, methods, and fields. May apply to static as well as instance members. An injectable member may have any access modifier (private, package-private, protected, public). Constructors are injected first, followed by fields, and then methods. Fields and methods in superclasses are injected before those in subclasses. Ordering of injection among fields and among methods in the same class is not specified.
Can you explain me what is #Inject for? If it is possible with a real life analogy with something less abstract
#Inject is a Java annotation for describing the dependencies of a class that is part of Java EE (now called Jakarta EE). It is part of CDI (Contexts and Dependency Injection) which is a standard dependency injection framework included in Java EE 6 and higher.
The most notorious feature of CDI is that it allows you to inject dependencies in client classes. What do I mean by dependencies? It is basically what your class needs to do whatever it needs to do.
Let me give you an example so that it is easier to understand. Imagine that you have a class NotificationService that is supposed to send notifications to people in different formats (in this case, email and sms). For this, you would most probably like to delegate the actual act of sending the notifications to specialized classes capable of handling each format (let's assume EmailSender and SmsSender). What #Inject allows you to do is to define injection points in the NotificationService class. In the example below, #Inject instructs CDI to inject an EmailSender and SmsSender implementation objects via the constructor.
public class NotificationService {
private EmailSender emailSender;
private SmsSender smsSender;
#Inject
public NotificationService(EmailSender emailSender, SmsSender smsSender) {
this.emailSender = emailSender;
this.smsSender = smsSender;
}
}
It is also possible to inject an instance of a class in fields (field injection) and setters (setter injection), not only as depicted above in constructors.
One of the most famous JVM frameworks taking advantage of this dependency injection concept is Spring.
I don't understand.. if I can use import in Aurelia, why do I have to wire up the constructor with #autoinject() and all that? I'm sure I'm missing something, but, from what I can tell, I can just use my imported module whenever I want.
import something from "whatever"
export class SomeViewModel {
activate() {
// use something
}
}
Typically, in an Aurelia application, the thing you are importing isn't an instance of Something it's the class Something. To actually use whatever has been imported, you need an instance of it.
import Something from 'whatever';
let something = new Something();
When you use Aurelia's Dependency Injection system, you are utilizing a design pattern called "Inversion of Control." Instead of your class (or you) being in charge of instantiating its dependencies, it lists what dependencies it has and then has instances of the dependencies injected in to its constructor function.
This helps with testability, as now you can pass mocked instances of the dependencies to a class in your test fixtures (note that in your tests, your tests will pass the mocks to the constructor, and not rely on Aurelia's DI container).This also allows you to tap in to the Dependency Injection container's ability to be configured to create dependencies using different object lifestyles such as singletons and transient.
--- Edits to answer OP's questions from comments ---
If I import a module defined as export default class Something into an
aurelia view model using constructor injection, it does not need to be
instantiated. It is an instance of the class Something.
This is because Aurelia's Dependency Injection container is instantiating an instance for you. This is why your code looks like this:
import {inject} from 'aurelia-framework';
import Something from 'somewhere';
#inject(Something)
export class Foo {
constructor(something) {
this.something = something;
}
//...
}
and not
import Something from 'somewhere';
export class Foo {
constructor(Something) {
this.something = something;
}
//...
}
You are telling Aurelia "I need one of these, please give it to me," and Aurelia says "Sure thing, I've created one or I already had one lying around, here it is."
In other words, it appears that aurelia's constructor DI only works
with class exports, and it does instantiate the class. It looks like
if I want to import something like moment js into my aurelia view
model, I should just continue doing things the way I've always done
them (not using aurelia's DI). Does that sound correct?
This is correct. Libraries like moment give you a function to use, and not a class that can be instantiated by Aurelia. For these you would continue to use them as in the past.
Well technically you can use the imported modules without Aurelia's DI, but in most situations that would be a bad thing. The Dependency Injection layer gives you so much versatility and flexibility. It handles caching, it supports singleton and transient dependencies, handles lifetime and makes thing neater from an architectural perspective.