Setter Injection

Dependencies can also be injected by setters.

interface Logger {
    public function log($message);
}

class Component {
    private $logger;
    private $databaseConnection;

    public function __construct(DatabaseConnection $databaseConnection) {
        $this->databaseConnection = $databaseConnection;
    }

    public function setLogger(Logger $logger) {
        $this->logger = $logger;
    }

    public function core() {
        $this->logSave();    
        return $this->databaseConnection->save($this);
    }

    public function logSave() {
         if ($this->logger) {
            $this->logger->log('saving');
        }
    }
}

This is especially interesting when the core functionality of the class does not rely on the dependency to work.

Here, the only needed dependency is the DatabaseConnection so it’s in the constructor. The Logger dependency is optional and thus does not need to be part of the constructor, making the class easier to use.

Note that when using setter injection, it’s better to extend the functionality rather than replacing it. When setting a dependency, there’s nothing confirming that the dependency won’t change at some point, which could lead in unexpected results. For example, a FileLogger could be set at first, and then a MailLogger could be set. This breaks encapsulation and makes logs hard to find, because we’re replacing the dependency.

To prevent this, we should add a dependency with setter injection, like so :

interface Logger {
    public function log($message);
}

class Component {
    private $loggers = array();
    private $databaseConnection;

    public function __construct(DatabaseConnection $databaseConnection) {
        $this->databaseConnection = $databaseConnection;
    }

    public function addLogger(Logger $logger) {
        $this->loggers[] = $logger;
    }

    public function core() {
        $this->logSave();
        return $this->databaseConnection->save($this);
    }

    public function logSave() {
        foreach ($this->loggers as $logger) {
            $logger->log('saving');
        }
    }
}

Like this, whenever we’ll use the core functionality, it won’t break even if there is no logger dependency added, and any logger added will be used even though another logger could’ve been added. We’re extending functionality instead of replacing it.

Found a mistake? Have a question or improvement idea? Let me know.

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