Validation

MediatorK ships a lightweight validation API in the com.fajrbahr.mediatork.validator package — no annotation processing, no reflection, no external dependencies.

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RequestValidator

Implement RequestValidator<TRequest> and return a ValidationResult to signal pass or fail:

interface RequestValidator<TRequest : Any> {
    fun validate(request: TRequest): ValidationResult
}

Fail-fast — stop at first error

Use rulesFailFast { } or Kotlin's own require/check (thrown as IllegalArgumentException):

class CreateTodoValidator : RequestValidator<CreateTodoCommand> {
    override fun validate(request: CreateTodoCommand): ValidationResult = rulesFailFast {
        check(request.title.isNotBlank()) { "Title must not be blank" }
        check(request.title.length <= 200) { "Title must be 200 characters or fewer" }
        check(request.dueDate.isAfter(today())) { "Due date must be in the future" }
    }
}

Execution stops at the first failure. Only the first error is included in the result.

Collect all errors — rules { }

Use rules { } when you want every failure reported in one pass (e.g. form validation):

class CreateInvoiceValidator : RequestValidator<CreateInvoiceCommand> {
    override fun validate(request: CreateInvoiceCommand): ValidationResult = rules {
        check(request.id.isNotBlank()) { "Invoice ID is required" }
        check(request.id.startsWith("INV-")) { "Invoice ID must start with INV-" }
        check(request.amount > 0) { "Amount must be positive" }
    }
}

All checks run regardless of earlier failures. ValidationException.errors contains the full list.

Return directly

Return ValidationResult.Invalid when you have a single computed message:

class CreateInvoiceValidator : RequestValidator<CreateInvoiceCommand> {
    override fun validate(request: CreateInvoiceCommand): ValidationResult {
        if (request.id.isBlank()) return ValidationResult.Invalid("Invoice ID is required")
        return ValidationResult.Valid
    }
}

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Registering validators

Register validators via HandlerRegistry.registerValidator<TRequest>(validator):

class AppRegistrar(
    private val repo: InvoiceRepository,
) : MediatorRegistrar {
    override fun register(registry: HandlerRegistry) {
        registry register CreateInvoiceHandler(repo)
        registry.registerValidator(CreateInvoiceValidator())
    }
}

Or using the + shorthand inside a scope { } block:

registry.scope {
    +CreateInvoiceHandler(repo)
    +CreateInvoiceValidator()
}

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ValidationBehavior

MediatorK ships a ready-to-use ValidationBehavior that runs automatically when any validators are registered. When you call MediatorFactory.create, it detects registered validators and injects ValidationBehavior at order = -50 automatically — no manual setup needed.

If you need to customize the behavior order, you can construct and add it explicitly:

val mediator = MediatorFactory.create(
    registrars = listOf(AppRegistrar()),
    pipelineBehaviors = listOf(
        // ValidationBehavior is injected automatically from registered validators,
        // but you can override its order by passing it explicitly:
        ValidationBehavior(validators = registry.anyValidators(), order = -100),
    ),
)

ValidationBehavior runs before the handler for every request whose type has registered validators. It throws ValidationException if any validator returns ValidationResult.Invalid.

tip

ValidationBehavior runs at order = -50 by default so it executes before most behaviors.

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Handling ValidationException

Catch ValidationException in your ViewModel or exception handler:

try {
    mediator.send(CreateTodoCommand(title, dueDate))
} catch (e: ValidationException) {
    errorMessage = e.errors.joinToString(", ") { it.toString() }
}

ValidationException.errors is a List<*> — cast to your error type if you used a custom type in rules { }.

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ValidationResult API

sealed class ValidationResult {
    data object Valid : ValidationResult()
    data class Invalid(val errors: List<*>) : ValidationResult()
}

// Throw directly if invalid
result.throwIfInvalid()

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Next

→ Requests & Handlers
→ Kotlin JVM