public function addTransformer(DataTransformerInterface $transformer, $viewNamespace, $outputFormat)
    {
        if (!array_key_exists($viewNamespace, $this->viewsReferenceTransformers)) {
            $this->viewsReferenceTransformers[$viewNamespace] = [];
        }
        if (!array_key_exists($outputFormat, $this->viewsReferenceTransformers[$viewNamespace])) {
            $this->viewsReferenceTransformers[$viewNamespace][$outputFormat] = null;
        }
        $this->viewsReferenceTransformers[$viewNamespace][$outputFormat] = $transformer;
    }