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                <?php  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                namespace IrishDan\NotificationBundle\Formatter;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                use IrishDan\NotificationBundle\Emailable;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                use IrishDan\NotificationBundle\Exception\MessageFormatException;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                use IrishDan\NotificationBundle\Message\Message;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                use IrishDan\NotificationBundle\Notification\NotificationInterface;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                class MailDataFormatter extends BaseFormatter implements MessageFormatterInterface  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    const CHANNEL = 'mail';  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    private $mailConfiguration;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    public function __construct(array $mailConfiguration)  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        $this->mailConfiguration = $mailConfiguration;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    }  | 
            
            
                                                                                                            
                                                                
            
                                    
            
            
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                    public function format(NotificationInterface $notification)  | 
            
            
                                                        
            
                                    
            
            
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                    { | 
            
            
                                                        
            
                                    
            
            
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                        $notification->setChannel(self::CHANNEL);  | 
            
            
                                                        
            
                                    
            
            
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                        parent::format($notification);  | 
            
            
                                                        
            
                                    
            
            
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                        // Build the message.  | 
            
            
                                                        
            
                                    
            
            
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                        $message          = new Message();  | 
            
            
                                                        
            
                                    
            
            
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                        $notificationData = $notification->getDataArray();  | 
            
            
                                                        
            
                                    
            
            
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                        /** @var Emailable $notifiable */  | 
            
            
                                                        
            
                                    
            
            
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                        $notifiable = $notification->getNotifiable();  | 
            
            
                                                        
            
                                    
            
            
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                        if (!$notifiable instanceof Emailable) { | 
            
            
                                                        
            
                                    
            
            
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                            throw new MessageFormatException(  | 
            
            
                                                        
            
                                    
            
            
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                                'Notifiable must implement Emailable interface in order to format email message'  | 
            
            
                                                        
            
                                    
            
            
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                            );  | 
            
            
                                                        
            
                                    
            
            
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                        }  | 
            
            
                                                        
            
                                    
            
            
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                        // Set the channel key  | 
            
            
                                                        
            
                                    
            
            
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                        $message->setChannel(self::CHANNEL);  | 
            
            
                                                        
            
                                    
            
            
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                        // Build the dispatch data array.  | 
            
            
                                                        
            
                                    
            
            
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                        $dispatchData = [  | 
            
            
                                                        
            
                                    
            
            
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                            'to'   => $notifiable->getEmail(),  | 
            
            
                                                        
            
                                    
            
            
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                            'from' => empty($this->mailConfiguration['default_sender']) ? '' : $this->mailConfiguration['default_sender'],  | 
            
            
                                                        
            
                                    
            
            
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                        ];  | 
            
            
                                                        
            
                                    
            
            
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                        // Build the message data array.  | 
            
            
                                                        
            
                                    
            
            
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                        $messageData          = [];  | 
            
            
                                                        
            
                                    
            
            
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                        $messageData['body']  = empty($notificationData['body']) ? '' : $notificationData['body'];  | 
            
            
                                                        
            
                                    
            
            
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                        $messageData['title'] = empty($notificationData['title']) ? '' : $notificationData['title'];  | 
            
            
                                                        
            
                                    
            
            
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                        $message->setDispatchData($dispatchData);  | 
            
            
                                                        
            
                                    
            
            
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                        $message->setMessageData($messageData);  | 
            
            
                                                        
            
                                    
            
            
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                        return $message;  | 
            
                            
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                    }  | 
            
            
                                                        
            
                                    
            
            
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                }  | 
            
            
                        
If you return a value from a function or method, it should be a sub-type of the type that is given by the parent type f.e. an interface, or abstract method. This is more formally defined by the Lizkov substitution principle, and guarantees that classes that depend on the parent type can use any instance of a child type interchangably. This principle also belongs to the SOLID principles for object oriented design.
Let’s take a look at an example:
Our function
my_functionexpects aPostobject, and outputs the author of the post. The base classPostreturns a simple string and outputting a simple string will work just fine. However, the child classBlogPostwhich is a sub-type ofPostinstead decided to return anobject, and is therefore violating the SOLID principles. If aBlogPostwere passed tomy_function, PHP would not complain, but ultimately fail when executing thestrtouppercall in its body.