Zend\Code\Generator\MethodGenerator::setReturnType PHP Method

    public function setReturnType($returnType = null)
    {
        $this->returnType = null === $returnType ? null : TypeGenerator::fromTypeString($returnType);
        return $this;
    }

    private function buildProxyClass(string $entityInterfaceName, string $proxyNamespace, string $proxyClassName) : string
    {
        $reflectionClass = new ReflectionClass($entityInterfaceName);
        if (!$reflectionClass->isInterface()) {
            throw InvalidInterfaceException::fromInvalidInterface($entityInterfaceName);
        }
        $classGenerator = new ClassGenerator();
        $classGenerator->setNamespaceName($proxyNamespace);
        $classGenerator->setName($proxyClassName);
        $classGenerator->setImplementedInterfaces([$entityInterfaceName, ProxyInterface::class]);
        $classGenerator->addProperty('initializer', null, PropertyGenerator::FLAG_PRIVATE);
        $classGenerator->addProperty('relationId', null, PropertyGenerator::FLAG_PRIVATE);
        $classGenerator->addProperty('realEntity', null, PropertyGenerator::FLAG_PRIVATE);
        $constructorGenerator = new MethodGenerator('__construct', [['name' => 'initializer', 'type' => 'callable'], ['name' => 'relationId']]);
        $constructorGenerator->setBody('
            $this->initializer = $initializer;
            $this->relationId = $relationId;
        ');
        $classGenerator->addMethodFromGenerator($constructorGenerator);
        $getRelationIdGenerator = new MethodGenerator('__getRelationId');
        $getRelationIdGenerator->setBody('
            return $this->relationId;
        ');
        $classGenerator->addMethodFromGenerator($getRelationIdGenerator);
        $getRealEntityGenerator = new MethodGenerator('__getRealEntity');
        $getRealEntityGenerator->setBody('
            if (null === $this->realEntity) {
                $this->realEntity = ($this->initializer)();
                \\Assert\\Assertion::isInstanceOf($this->realEntity, \\' . $entityInterfaceName . '::class);
            };

            return $this->realEntity;
        ');
        $classGenerator->addMethodFromGenerator($getRealEntityGenerator);
        foreach ($reflectionClass->getMethods(ReflectionMethod::IS_PUBLIC) as $reflectionMethod) {
            $parameters = [];
            $parameterGenerators = [];
            $returnType = $reflectionMethod->getReturnType();
            foreach ($reflectionMethod->getParameters() as $reflectionParameter) {
                $parameterGenerator = new ParameterGenerator($reflectionParameter->getName(), $reflectionParameter->getType(), $reflectionParameter->isDefaultValueAvailable() ? $reflectionParameter->getDefaultValue() : null);
                $parameterGenerator->setVariadic($reflectionParameter->isVariadic());
                $parameterGenerators[] = $parameterGenerator;
                if ($reflectionParameter->isVariadic()) {
                    $parameters[] = '...$' . $reflectionParameter->getName();
                } else {
                    $parameters[] = '$' . $reflectionParameter->getName();
                }
            }
            $methodGenerator = new MethodGenerator();
            $methodGenerator->setName($reflectionMethod->getName());
            $methodGenerator->setVisibility(MethodGenerator::VISIBILITY_PUBLIC);
            $methodGenerator->setParameters($parameterGenerators);
            $methodGenerator->setReturnType($returnType);
            $body = '
                if (null === $this->realEntity) {
                    $this->realEntity = ($this->initializer)();
                    \\Assert\\Assertion::isInstanceOf($this->realEntity, \\' . $entityInterfaceName . '::class);
                };
            ';
            if ('void' !== $returnType) {
                $body .= 'return ';
            }
            $body .= '$this->realEntity->' . $reflectionMethod->getName() . '(' . implode(', ', $parameters) . ');';
            $methodGenerator->setBody($body);
            $classGenerator->addMethodFromGenerator($methodGenerator);
        }
        $fileGenerator = new FileGenerator();
        $fileGenerator->setClass($classGenerator);
        $filename = null === $this->proxyFolder ? tempnam(sys_get_temp_dir(), $proxyClassName) : sprintf('%s/%s.php', $this->proxyFolder, $proxyClassName);
        $fileGenerator->setFilename($filename);
        $fileGenerator->write();
        return $filename;
    }