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                <?php  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 * @copyright Copyright (C) eZ Systems AS. All rights reserved.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 * @license For full copyright and license information view LICENSE file distributed with this source code.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                declare(strict_types=1);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                namespace eZ\Publish\Core\Repository\Strategy\ContentThumbnail;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                use eZ\Publish\API\Repository\FieldTypeService;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                use eZ\Publish\API\Repository\Values\Content\Field;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                use eZ\Publish\API\Repository\Values\Content\Thumbnail;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                use eZ\Publish\API\Repository\Values\ContentType\ContentType;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                use eZ\Publish\SPI\Repository\Strategy\ContentThumbnail\Field\ThumbnailStrategy as ContentFieldThumbnailStrategy;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                use eZ\Publish\SPI\Repository\Strategy\ContentThumbnail\ThumbnailStrategy;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                final class FirstMatchingFieldStrategy implements ThumbnailStrategy  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    /** @var \eZ\Publish\API\Repository\FieldTypeService */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    private $fieldTypeService;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    /** @var \eZ\Publish\SPI\Repository\Strategy\ContentThumbnail\Field\ThumbnailStrategy */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    private $contentFieldStrategy;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    public function __construct(  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        ContentFieldThumbnailStrategy $contentFieldStrategy,  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        FieldTypeService $fieldTypeService  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    ) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        $this->contentFieldStrategy = $contentFieldStrategy;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        $this->fieldTypeService = $fieldTypeService;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    }  | 
            
            
                                                                                                            
                                                                
            
                                    
            
            
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                    public function getThumbnail(ContentType $contentType, array $fields): ?Thumbnail  | 
            
            
                                                                        
                            
            
                                    
            
            
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                    { | 
            
            
                                                                        
                            
            
                                    
            
            
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                        $fieldDefinitions = $contentType->getFieldDefinitions();  | 
            
            
                                                                        
                            
            
                                    
            
            
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                        foreach ($fieldDefinitions as $fieldDefinition) { | 
            
            
                                                                        
                            
            
                                    
            
            
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                            $field = $this->getFieldByIdentifier($fieldDefinition->identifier, $fields);  | 
            
            
                                                                        
                            
            
                                    
            
            
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                            if ($field === null) { | 
            
            
                                                                        
                            
            
                                    
            
            
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                                continue;  | 
            
            
                                                                        
                            
            
                                    
            
            
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                            }  | 
            
            
                                                                        
                            
            
                                    
            
            
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                            $fieldType = $this->fieldTypeService->getFieldType($fieldDefinition->fieldTypeIdentifier);  | 
            
            
                                                                        
                            
            
                                    
            
            
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                            if (  | 
            
            
                                                                        
                            
            
                                    
            
            
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                                $fieldDefinition->isThumbnail  | 
            
            
                                                                        
                            
            
                                    
            
            
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                                && $this->contentFieldStrategy->hasStrategy($field->fieldTypeIdentifier)  | 
            
                            
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                                && !$fieldType->isEmptyValue($field->value)  | 
            
            
                                                                        
                            
            
                                    
            
            
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                            ) { | 
            
            
                                                                        
                            
            
                                    
            
            
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                                return $this->contentFieldStrategy->getThumbnail($field);  | 
            
            
                                                                        
                            
            
                                    
            
            
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                            }  | 
            
            
                                                                        
                            
            
                                    
            
            
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                        }  | 
            
            
                                                                        
                            
            
                                    
            
            
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                        return null;  | 
            
            
                                                                        
                            
            
                                    
            
            
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                    }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    private function getFieldByIdentifier(string $identifier, array $fields): ?Field  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        /** @var \eZ\Publish\API\Repository\Values\Content\Field $field */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        foreach ($fields as $field) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                            if ($field->fieldDefIdentifier === $identifier) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                                return $field;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                            }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        return null;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    }  | 
            
            
                                                                                                            
                                                                
            
                                    
            
            
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                }  | 
            
            
                                                        
            
                                    
            
            
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Let’s take a look at an example:
In the above example, the authenticate() method works fine as long as you just pass instances of MyUser. However, if you now also want to pass a different implementation of User which does not have a getDisplayName() method, the code will break.
Available Fixes
Change the type-hint for the parameter:
Add an additional type-check:
Add the method to the interface: