#Service Container

The ARK Core service container is responsible for managing bindings, dependencies and dependency injection. Our container of choice is InversifyJS which describes itself as A powerful and lightweight inversion of control container for JavaScript & Node.js apps powered by TypeScript.

An IoC container uses a class constructor to identify and inject its dependencies. InversifyJS has a friendly API and encourages the usage of the best OOP and IoC practices. The wide variety of features it provides combined with its strictness make it a perfect choice for Core where type safety is important.

#Binding

The service container provides various methods of binding values to the container, we’ll only look at the most common ones. Check out the InversifyJS Features and API to get a full overview of what it is capable of.

#Binding Constant Values

A constant value is any value that does not change after its creation. It should be bound once to the container and not be changed.

1import { app, Container, Contracts } from "@arkecosystem/core-kernel";
2 
3app
4    .bind<Contracts.Kernel.Log.Logger>(Container.Identifiers.LogService)
5    .toConstantValue(new ConsoleLogger());

#Binding Dynamic Values

A dynamic value is any value that is the result of meeting a condition to be created or calling another function and returning its value.

1import { app, Container, Contracts } from "@arkecosystem/core-kernel";
2 
3app
4    .bind<Contracts.Kernel.Log.Logger>(Container.Identifiers.LogService)
5    .toDynamicValue((context: interfaces.Context) => { return new ConsoleLogger(); });

Dynamic value bindings are a good fit for the builder or any other creational patterns.

#Binding Contracts to Implementations

An implementation bound to a contract is useful in cases where you wish to make use of dependency injection. You’ll bind your ConsoleLogger implementation to the ILogger contract and InversifyJS will take care of returning an instance of the ConsoleLogger implementation when the ILogger contract is resolved.

1import { app, Container, Contracts } from "@arkecosystem/core-kernel";
2 
3app
4    .bind<Contracts.Kernel.Log.Logger>(Container.Identifiers.LogService)
5    .to(ConsoleLogger);

#Resolving

Now that we know how to create bindings we also need a way of resolving them from the container, lets take a look at how we can do that.

#Resolving any bindings

The get method can be used to resolve any previously created binding from the container. If the binding with the given name doesn’t exist an exception will be thrown.

1import { app, Container, Contracts } from "@arkecosystem/core-kernel";
2 
3app.get<Contracts.Kernel.Log.Logger>(Container.Identifiers.LogService);

#Resolving Contracts to Implementations

The resolve method can be used if you want to get an instance of a class that depends on dependency injection.

1import { app, Contracts } from "@arkecosystem/core-kernel";
2 
3app.resolve<Contracts.Kernel.Log.Logger>(ConsoleLogger);

You should rarely find yourself calling this, if you do end up in a situation where you call it for basically everything you should rethink the architecture of you plugin and how you interact with the container.

#Full Documentation

You can find the full documentation in the InversifyJS repository with dozens of guides and best practices for how to work with the container and how to avoid common pitfalls.