src/Application/SimpleApplication.php
public function logger(LoggerInterface $logger = null) : LoggerInterface
    {
        if ($logger){
            $this->logger = $logger;
        }
        if (!$this->logger){
            $this->logger = new NullLogger;
        }
        return $this->logger;
    }
public function logger(LoggerInterface $logger = null) : LoggerInterface
    {
        if ($logger){
            $this->logger = $logger;
        }
        if (!$this->logger){
            $this->logger = new NullLogger;
        }
        return $this->logger;
    }
Set/get logger

@param LoggerInterface $logger

@return LoggerInterface