Custom Parameter Resolvers

If the built-in handler injection surface is not enough, VEF lets you extend it.

Two extension groups

You can add:

• request-time parameter resolvers
• startup-time factory parameter resolvers

The relevant DI groups are:

• vef:api:handler_param_resolvers
• vef:api:factory_param_resolvers

What a resolver does

A handler parameter resolver tells the framework:

• which Go type it handles
• how to resolve a value of that type

The same idea applies to factory parameter resolvers, except those run at startup time rather than per request.

When you need one

Custom resolvers are useful when:

• you want a domain-specific request-scoped object injected directly
• you want to inject a service wrapper derived from context
• you need a reusable handler contract across many resources

Minimal example

package tenantresolver

import (
  "reflect"

  "github.com/gofiber/fiber/v3"

  "github.com/coldsmirk/vef-framework-go/api"
)

type TenantContext struct {
  ID string
}

type TenantResolver struct{}

func (*TenantResolver) Type() reflect.Type {
  return reflect.TypeFor[TenantContext]()
}

func (*TenantResolver) Resolve(ctx fiber.Ctx) (reflect.Value, error) {
  tenant := TenantContext{ID: ctx.Get("X-Tenant-ID")}
  return reflect.ValueOf(tenant), nil
}

Registration example:

fx.Provide(
  fx.Annotate(
    func() api.HandlerParamResolver { return &TenantResolver{} },
    fx.ResultTags(`group:"vef:api:handler_param_resolvers"`),
  ),
)

Recommendation

Use custom resolvers for cross-cutting conventions, not for one-off shortcuts. If only one handler needs a value, plain function calls are usually simpler.

Next step

Read Parameters And Metadata if you first need to understand what the built-in request decoding layer already injects for you.