备忘：Dingo 是如何使用 Fractal来转换的？

在\App\Http\Controllers\Api\UsersController::me中，如下代码与fractal官方文档代码是不同的，完全看不出有用到Fractal。
```
return $this->response->item($this->user(), new UserTransformer());
```
原因是：Fractal is the default transformation layer used by the Dingo package

查看配置文件可以看出

config/api.php

'transformer' => env('API_TRANSFORMER', Dingo\Api\Transformer\Adapter\Fractal::class),

注册serviceProvider也可以看出

\Dingo\Api\Provider\DingoServiceProvider::registerTransformer

//Register the transformer factory.
protected function registerTransformer()
{
    $this->app->singleton('api.transformer', function ($app) {
        return new TransformerFactory($app, $this->config('transformer'));
    });
}

而 new TransformerFactory 就设置了 adapter

//Create a new transformer factory instance.
public function __construct(Container $container, Adapter $adapter)
{
    $this->container = $container;
    $this->adapter = $adapter;
}

这个adapter会在下面用到，trace如下：

Dingo\Api\Transformer\Adapter\Fractal->createResource()
Dingo\Api\Transformer\Adapter\Fractal->transform()
Dingo\Api\Transformer\Factory->transform()
Dingo\Api\Http\Response->morph()

在\Dingo\Api\Transformer\Adapter\Fractal::createResource可以看到 new FractalItem($response, $transformer, $key)，这个就和 fractal官方文档代码吻合了。

//Create a Fractal resource instance.
protected function createResource($response, $transformer, array $parameters)
{
    $key = isset($parameters['key']) ? $parameters['key'] : null;

    if ($response instanceof IlluminatePaginator || $response instanceof IlluminateCollection) {
        return new FractalCollection($response, $transformer, $key);
    }

    return new FractalItem($response, $transformer, $key);
}