Verified against Semitexa Ultimate 2026.09.19.1020

Discovery Contributors

Attribute discovery is extensible. A package that defines its own attribute registers a contributor that says which attribute to watch and what to do with each hit — the same mechanism core uses for pipeline listeners, server lifecycle hooks and resource metadata.

#[AsDiscoveryContributor(priority: 50)]
final class SlotHandlerContributor implements DiscoveryContributor
{
    public function attribute(): string
    {
        return AsSlotHandler::class;
    }

    public function scopedToActiveModules(): bool
    {
        return true;
    }

    public function contribute(string $className, object $attribute, BootDiagnostics $diagnostics): void
    {
        // register $className against the registry this package owns
    }
}

That is Semitexa\Ssr\Application\Service\Discovery\SlotHandlerContributor. The class must implement DiscoveryContributor and be instantiable; discovery rejects an #[AsDiscoveryContributor] that is not, by name.

The three methods

attribute() returns the attribute class to watch. Returning a class that is not loadable is not an error — it means the owning package is not installed, and discovery skips the contributor. That is the one place class_exists() behaviour is intentional, and it lives here rather than being repeated at every call site.

scopedToActiveModules() decides visibility. Return true — the usual answer — and hits only count when they come from an active module or the project's own src/, so a slot declared by a module the current tenant has not enabled stays unregistered. Return false for framework-level contributions that apply regardless of tenant configuration.

contribute() runs once per attribute occurrence, so a class carrying the attribute repeatably is visited once per declaration. Throwing is safe and expected for a genuinely invalid declaration: discovery records it against BootDiagnostics and continues, so one malformed class cannot abort a boot. Validate loudly rather than skipping quietly.

Priority

priority orders contributors against each other, highest first. The four in semitexa/ssr show the reasoning:

The rule of thumb: whatever declares a thing must run before whatever attaches to it.

When you need one

Only when your package defines an attribute that needs a boot-time registry. Consuming existing attributes needs nothing — declare the attribute and let its owner's contributor do the work.