webiny-dependency-injection

---
name: webiny-dependency-injection
context: webiny-extensions
description: >
  The universal createImplementation DI pattern and all injectable services.
  Use this skill when the developer is writing any Webiny extension and needs to understand
  dependency injection, constructor injection, how to access Logger/BuildParams/IdentityContext,
  how to inject CMS use-cases (list/get/create/update/delete entries), or how the dependencies
  array works. This is the connective tissue across all extension types -- API, Admin, CLI,
  and Infrastructure.
---

# 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.

## The Universal Pattern

```typescript
import { SomeFactory } from "webiny/some/path";
import { Logger } from "webiny/api/logger";
import { BuildParams } from "webiny/api/build-params";

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

| Extension Type  | Factory                | Import Path              |
| --------------- | ---------------------- | ------------------------ |
| Content Models  | `ModelFactory`         | `"webiny/api/cms/model"` |
| GraphQL Schemas | `GraphQLSchemaFactory` | `"webiny/api/graphql"`   |
| API Keys        | `ApiKeyFactory`        | `"webiny/api/security"`  |
| CLI Commands    | `CliCommandFactory`    | `"webiny/cli/command"`   |
| Pulumi Handlers | `CorePulumi`           | `"webiny/infra/core"`    |

> **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.

```typescript
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

```typescript
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

```typescript
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]
});
```

## 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).

## 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