Dependency Injection Patterns

TL;DR

Every Webiny extension type uses the same DI pattern: define a class implementing *.Interface, declare dependencies in the constructor, and export via *.createImplementation({ implementation, dependencies }). The DI container automatically provides the required services, ensures type safety, and validates at compile time. This pattern is the connective tissue across all extension types -- API, Admin, CLI, and Infrastructure.

Working Context

This skill applies to both extension developers (working in extensions/) and core developers (working in packages/). The DI pattern is universal — only the import paths differ.

Detect which context you're in by checking the file path: extensions/ → extension mode, packages/ → core mode. The dependencies array, constructor injection, scoping rules, and all other patterns are identical in both contexts.

The Universal Pattern

import { SomeFactory } from "webiny/some/path";
import { Logger, BuildParams } from "webiny/api";

class MyImplementation implements SomeFactory.Interface {
  constructor(
    private logger: Logger.Interface,
    private buildParams: BuildParams.Interface
  ) {}

  execute(/* factory-specific params */) {
    this.logger.info("Doing something...");
    // buildParams.get() returns T | null — always account for null.
    const value = this.buildParams.get<string>("MY_PARAM");
  }
}

export default SomeFactory.createImplementation({
  implementation: MyImplementation,
  dependencies: [Logger, BuildParams]
});

Key rules:

1. One class per file -- each extension file exports a single implementation.
2. Constructor injection -- dependencies are received as constructor parameters, in the same order as the dependencies array.
3. Dependencies array -- must exactly match the constructor parameter order and types.
4. Interface types -- always type constructor params as Feature.Interface.

Where This Pattern Appears

Event handlers use the same createImplementation pattern but are not injectable dependencies.

Examples Across Extension Types

API Extension (GraphQL Schema with DI)

GraphQL schemas use the builder pattern. The execute method receives a builder and uses addTypeDefs and addResolver to define the schema. Resolver-level DI is declared per-resolver via dependencies in addResolver, resolved at request time from the request-scoped container.

import { GraphQLSchemaFactory } from "webiny/api/graphql";
import { IdentityContext } from "webiny/api/security";

class WhoAmISchema implements GraphQLSchemaFactory.Interface {
  async execute(
    builder: GraphQLSchemaFactory.SchemaBuilder
  ): Promise<GraphQLSchemaFactory.SchemaBuilder> {
    builder.addTypeDefs(/* GraphQL */ `
      extend type Query {
        whoAmI: String
      }
    `);

    builder.addResolver({
      path: "Query.whoAmI",
      dependencies: [IdentityContext],
      resolver: (identityContext: IdentityContext.Interface) => {
        return () => {
          const identity = identityContext.getIdentity();
          return `Hello, ${identity.displayName}!`;
        };
      }
    });

    return builder;
  }
}

export default GraphQLSchemaFactory.createImplementation({
  implementation: WhoAmISchema,
  dependencies: []
});

Note: GraphQLSchemaFactory implementations typically have dependencies: [] because DI happens at the resolver level via addResolver({ dependencies }), not at the class constructor level.

CLI Command with DI

import { Ui } from "webiny/cli";
import { CliCommandFactory } from "webiny/cli/command";

class MyCommandImpl implements CliCommandFactory.Interface<{ name: string }> {
  constructor(private ui: Ui.Interface) {}

  execute(): CliCommandFactory.CommandDefinition<{ name: string }> {
    return {
      name: "greet",
      description: "Greet someone",
      params: [{ name: "name", description: "Name", type: "string" }],
      handler: async params => {
        this.ui.success(`Hello, ${params.name}!`);
      }
    };
  }
}

export default CliCommandFactory.createImplementation({
  implementation: MyCommandImpl,
  dependencies: [Ui]
});

Pulumi Handler with DI

import { Ui } from "webiny/infra";
import { CorePulumi } from "webiny/infra/core";

class MyPulumiImpl implements CorePulumi.Interface {
  constructor(private ui: Ui.Interface) {}

  execute(app: any) {
    this.ui.info("Deploying with environment:", app.env);
  }
}

export default CorePulumi.createImplementation({
  implementation: MyPulumiImpl,
  dependencies: [Ui]
});

Advanced Dependency Options

The dependencies array supports three forms per entry:

Multi-injection ({ multiple: true })

Use when a class needs all registered implementations of an abstraction. The container calls resolveAll() internally and injects the results as an array.

Abstraction:

interface IPageType {
  name: string;
  label: string;
  modify(form: IFormModel): void;
}

export const PageType = createAbstraction<IPageType>("PageType");

export namespace PageType {
  export type Interface = IPageType;
}

Multiple implementations registered separately:

// StaticPageType.ts
class StaticPageTypeImpl implements PageType.Interface {
  name = "static";
  label = "Static Page";
  modify(form: IFormModel) {
    /* no-op — base form is sufficient */
  }
}

export const StaticPageType = PageType.createImplementation({
  implementation: StaticPageTypeImpl,
  dependencies: []
});

// ProductPageType.ts (in another package/extension)
class ProductPageTypeImpl implements PageType.Interface {
  name = "product";
  label = "Product Page";
  modify(form: IFormModel) {
    form.fields(fields => ({
      product: fields.select().label("Product").required("Product is required")
    }));
    form.field("title").disabled(true);
    form.field("path").disabled(true);
  }
}

export const ProductPageType = PageType.createImplementation({
  implementation: ProductPageTypeImpl,
  dependencies: []
});

Consumer injects the array:

class CreatePagePresenterImpl implements CreatePagePresenter.Interface {
  constructor(
    private factory: FormModelFactory.Interface,
    private pageTypes: PageType.Interface[],
    private modifiers: CreatePageFormModifier.Interface[]
  ) {}
}

export const CreatePagePresenter = PresenterAbstraction.createImplementation({
  implementation: CreatePagePresenterImpl,
  dependencies: [
    FormModelFactory,
    [PageType, { multiple: true }],
    [CreatePageFormModifier, { multiple: true }]
  ]
});

Registration — each implementation is a separate container.register() call:

export const CreatePageFeature = createFeature({
  name: "CreatePage",
  register(container) {
    container.register(StaticPageType); // first PageType impl
    container.register(ProductPageType); // second PageType impl
    container.register(CreatePagePresenter);
  }
});

When CreatePagePresenter is resolved, pageTypes receives [StaticPageTypeImpl, ProductPageTypeImpl] in registration order.

Optional dependency ({ optional: true })

Use when a dependency may not be registered. The container injects undefined instead of throwing.

class MyPresenterImpl {
  constructor(
    private required: RequiredService.Interface,
    private analytics: AnalyticsService.Interface | undefined
  ) {}
}

export const MyPresenter = Abstraction.createImplementation({
  implementation: MyPresenterImpl,
  dependencies: [RequiredService, [AnalyticsService, { optional: true }]]
});

Container API Reference

Registration

Resolution

Lifetime Scopes

• Transient (default): New instance on every resolve().
• Singleton (.inSingletonScope()): Cached after first resolution, one instance per container.

Convention: Use cases = transient. Repositories, gateways, services, registries = singleton.

Decorators

Decorators wrap resolved instances. The decoratee is always the last constructor parameter. The dependencies array does NOT include the decoratee.

class LoggingServiceDecorator implements MyService.Interface {
  constructor(
    private logger: Logger.Interface,
    private decoratee: MyService.Interface // LAST param — injected automatically
  ) {}

  execute() {
    this.logger.info("Before");
    this.decoratee.execute();
  }
}

export const MyServiceLoggingDecorator = MyService.createDecorator({
  decorator: LoggingServiceDecorator,
  dependencies: [Logger] // decoratee is NOT listed
});

// Registration:
container.registerDecorator(MyServiceLoggingDecorator);

Composites

Composites aggregate multiple implementations behind a single resolve() call. Created via Abstraction.createComposite():

class AllValidatorsComposite implements Validator.Interface {
  constructor(private validators: Validator.Interface[]) {}

  validate(input: unknown) {
    for (const v of this.validators) v.validate(input);
  }
}

export const ValidatorComposite = Validator.createComposite({
  implementation: AllValidatorsComposite,
  dependencies: [[Validator, { multiple: true }]]
});

// Registration:
container.registerComposite(ValidatorComposite);

Key Rules

1. Always import from the feature path, not the package root.
2. Use Feature.Interface for constructor parameter types.
3. The dependencies array order must match the constructor parameter order.
4. Read the abstractions.ts file in the feature folder to see available methods.
5. Extensions with no dependencies use dependencies: [].
6. BuildParams.get<T>(name) returns T | null — always type the receiving property/variable as nullable (e.g. string | null) and handle the null case.
7. BuildParam declarations belong inside the extension's Extension.tsx, not in webiny.config.tsx. Expose required params as React props on the extension component so the consumer decides where values come from (see webiny-full-stack-architect skill for the full pattern).
8. For multi-injection, type the constructor param as T[] and use [Abstraction, { multiple: true }] in the dependencies array.
9. Each implementation of a multi-bound abstraction is a separate container.register() call — they accumulate.

Related Skills

• webiny-custom-graphql-api -- DI in GraphQL schema extensions
• webiny-cli-extensions -- DI in CLI command extensions
• webiny-full-stack-architect -- Full-stack extension skeleton and registration pattern
• webiny-api-architect -- API-side architecture using DI
• webiny-admin-architect -- Admin-side architecture using DI