Mock generation

[cjnevin-foxtel]

Generate a different initializer for mock environment providing different default values.

This one extends on from my previous idea around respecting compiler directives.

It would be nice if we could also generate the static mock func for a type.

For example:

#if DEBUG
final class NetworkMock: NetworkInterface {}
#endif

@MemberwiseInit(.public, mock: true)
struct Dependencies {
   #if DEBUG
   @Init(Network(), mock: NetworkMock())
   #else
   @Init(Network())
   #endif
   let networkInterface: NetworkInterface
}

Also generates this static function:

#if DEBUG
extension Dependencies {
   static func mock(
      network: NetworkInterface = NetworkMock()
   ) -> Dependencies {
      Dependencies(
         network: network
      )
   }
}
#endif

Alternatively, we could generate a separate struct called DependenciesMock (name +Mock) that stores all of the properties and can be converted to a Dependencies instance. This would reduce test complexity since we wouldn't need to recreate the mocks we want to modify.

#if DEBUG
struct DependenciesMock {
    // Allows us to modify properties on NetworkMock before passing dependencies where they are needed
    // Also allows us to observe changes by keeping a reference to DependenciesMock
    let network = NetworkMock()

    var dependencies: Dependencies {
       Dependencies(network: network)
    }
}
#endif

This would greatly reduce the amount of code I need to write manually for each scenario I want to mock.