Configuring Services With Immutable Setters

Description

A common need in some Symfony applications is to use immutable services while still using traits for composing their optional features. Although the Symfony service container supports setter injection, they have some drawbacks (e.g. setters can be called more than just at the time of construction so you cannot be sure the dependency is not replaced during the lifetime of the object).

A pattern to solve this problem are “wither methods”. These methods typically use the with word as the prefix of their names (e.g. withPropertyName()) and they return a copy of an instance of an immutable class with only that one property changed:

class MyService
{
    use LoggerAwareTrait;
 
    // ...
}
 
trait LoggerAwareTrait
{
    private $logger;
 
    public function withLogger(LoggerInterface $logger)
    {
        $new = clone $this;
        $new->logger = $logger;
 
        return $new;
    }
}
 
$service = new MyService();
$service = $service->withLogger($logger);

Syntax

# config/services.yaml
services:
    MyService:
        # ...
        calls:
            # the TRUE argument turns this into a wither method
            - ['withLogger', ['@logger'], true]

class MyService
{
    // ...
 
    /**
     * @required
     * @return static
     */
    public function withLogger(LoggerInterface $logger)
    {
        // ...
    }
}