Slim Dependency Injection

When to use

• The user needs to register service definitions in the Slim container so classes get their dependencies.
• The user wants Slim to resolve an invokable controller class from the container instead of a closure.
• The user wants to configure a PSR-11 container for a Slim app via AppFactory::setContainer.

When NOT to use

• Do not reach for this skill to pull services out of the container inside handlers on demand; that is a service-locator anti-pattern, not injection.
• Do not introduce a container for a trivial Slim app with a couple of routes and no shared services.

Core guidance

• Set the container before creating the app — Build the PSR-11 container, pass it with AppFactory::setContainer($container) before AppFactory::create(), so Slim uses it to resolve handlers.

Quick recipe

1. Create a PSR-11 container (e.g. PHP-DI), add definitions, and call AppFactory::setContainer($container) before AppFactory::create(). (verify: The app boots and resolves class-name handlers.)
2. Add definitions binding interfaces to implementations and factory closures for configured services. (verify: Requesting an interface from the container returns the bound implementation.)
3. Register the route with Controller::class and declare its dependencies as constructor parameters. (verify: The controller runs with its dependencies injected.)

Best practices

• Let the container autowire classes whose dependencies are other typed classes, and write a factory definition (closure receiving the container) only when a service needs configuration or a chosen implementation
• Declare each dependency as a constructor parameter and let the container supply it, rather than storing the container and calling $c->get(...) inside methods
• Register a definition mapping each interface to a concrete class so controllers can type-hint the interface and receive the bound implementation
• Register routes with a Controller::class (invokable, or [Controller::class, 'method']) so the container instantiates the controller with its dependencies

Anti-patterns

See references/anti-patterns.md.

Common mistakes

1. Container ignored because it was set too late — Class-name route handlers fail to resolve because the container was set after AppFactory::create(). Fix: Call AppFactory::setContainer($container) before AppFactory::create().
2. NotFoundException for an unregistered service — Resolving an interface or non-autowirable service throws a container NotFoundExceptionInterface. Fix: Add a definition mapping the interface or service id to a concrete factory or class.
3. Shared service accumulates request-specific state — A container service is shared (built once), so storing per-request state on it leaks across requests in long-running servers. Fix: Keep shared services stateless; pass per-request data as method arguments or request attributes.

Version notes

• Slim 4 requires an external PSR-11 container (PHP >=7.4) — Slim 4 ships without a built-in container and expects any PSR-11 container (PHP-DI, League Container) via AppFactory::setContainer; Slim 3 bundled Pimple with a different API.

Security

• Not applicable — no I/O, auth, or untrusted input.

Testing

• Activation and decision behavior are verified in eval/scenarios/slim/dependency-injection.yaml.

Additional resources

• references/anti-patterns.md
• references/core.md
• references/examples.md
• references/pitfalls.md
• references/recipes.md
• references/version-differences.md

Knowledge graph

Related: slim/routing