Defining factories for instantiating services

Now, let's suppose that we want to create a complex object, for example, one that represents a Transport Layer Security (TLS) connection. A few of the properties of such an object are a socket, a set of crypto protocols, and a certificate. In the context of this problem, the features of the DI mechanism of Angular we have looked at so far might seem a bit limited.

For example, we might need to configure some of the properties of the TLSConnection class without coupling the process of its instantiation with all the configuration details (choose appropriate crypto algorithms, open the TCP socket over which we will establish the secure connection, and so on).

In this case, we can take advantage of the useFactory property of the provider's configuration object:

let injector = ReflectiveInjector.resolveAndCreate([ 
  { 
    provide: TLSConnection, 
    useFactory: (socket: Socket, certificate: Certificate, crypto: Crypto) 
      ...