Register Factory Functions

In Parsley, you can register a factory function to provide more control and customization when creating service instances. A factory function differs from a constructor function because it allows you to pass additional parameters to configure the service creation process.

Instead of directly registering a constructor function that returns a service instance, you register a factory function that returns another function. This returned function is then used to create the actual service instances.

Here's a detailed look at how to register and use a factory function:

Example

Suppose you want to create a Greeter service where the salutation can be customized. You can use a factory function to achieve this. Here's how you would define and register a factory function:

package internal

import "fmt"

type Greeter interface {
	SayHello(name string, polite bool)
}

type greeter struct {
	salutation string
}

func (g *greeter) SayHello(name string, polite bool) {
	if polite {
		fmt.Printf("Good day, %s!\n", name)
	} else {
		fmt.Printf("%s, %s\n", g.salutation, name)
	}
}

func NewGreeterFactory(salutation string) func() Greeter {
	return func() Greeter {
		return &greeter{salutation: salutation}
	}
}

Supported Signatures

The function returned by the factory function supports various signatures, similar to constructor functions:

• func(...) T
• func(context.Context, ...) T
• func(...) (T, error)
• func(context.Context, ...) (T, error)

If the signature includes a context.Context, it must be the first parameter. Returning an error allows Parsley to handle and propagate errors during service resolution.

Example Usage

registry := registration.NewServiceRegistry()
_ = registration.RegisterTransient(registry, internal.NewGreeterFactory("Hi"))

resolver := resolving.NewResolver(registry)
scope := resolving.NewScopedContext(context.Background())

greeter, _ := resolving.ResolveRequiredService[internal.Greeter](scope, resolver)

const polite = false
greeter.SayHello("John", polite)