Laravel：如何覆盖传输器实例

Question

我正在研究Laravel的服务提供商，并且开始尝试覆盖Illuminate\Mail\MailServiceProvider。

几天后，我到达了想要的地方：registerSwiftMailer方法。

真正引起我注意的是以下评论：

// ..., which allows us to
// override this transporter instances during app start-up if necessary.

要点是：如果需要，在应用程序启动过程中“真的”有可能“覆盖”传输器实例，又怎么可能从控制器中做到这一点？

---

registerSwiftMailer

    public function registerSwiftMailer()
    {
        $this->registerSwiftTransport();

        // Once we have the transporter registered, we will register the actual Swift
        // mailer instance, passing in the transport instances, which allows us to
        // override this transporter instances during app start-up if necessary.
        $this->app->singleton('swift.mailer', function ($app) {
            if ($domain = $app->make('config')->get('mail.domain')) {
                Swift_DependencyContainer::getInstance()
                                ->register('mime.idgenerator.idright')
                                ->asValue($domain);
            }

            return new Swift_Mailer($app['swift.transport']->driver());
        });
    }