vapor-docs/3.0/docs/concepts/services.md

Services

Services is a framework for managing types and implementations in a type-safe fashion with protocol and environment support.

Let's break that statement up.

Usage

Types and implementations

For any given type or protocol you can implement multiple factories and implementations. An example where this is useful is configuration files.

services.register(HTTPServerConfig.self) { container in
  return HTTPServerConfig(port: 8081)
}

The above lines of code register a new HTTPServerConfig to the HTTPServerConfig.self Type. This means that whenever I, or any code requests an HTTPServerConfig, the above function will be executed which will return me a config on port 8081.

let serverConfig = try services.make(HTTPServerConfig.self)

print(serverConfig.port) // prints `8081`

Protocols and Environments

In Swift, we prefer working with protocols. This is useful for testing and adds flexibility. Assuming the simple logging protocol and implementation below we'll try to set up a protocol oriented service.

enum Level {
  case verbose, error
}

protocol Logger {
  func log(_ message: String, level: Level)
}

struct VerboseLogger: Logger {
  init() {}

  func log(_ message: String, level: Level) {
    print(message)
  }
}

struct ErrorLogger: Logger {
  init() {}

  func log(_ message: String, level: Level) {
    if level == .error {
      print(message)
    }
  }
}

Using the above code we could use the VerboseLogger for development, and error logger on production.

services.register(Logger.self) { container in
  if container == .development {
    return VerboseLogger()
  } else {
    return ErrorLogger()
  }
}

If we set Vapor's environment to development we'll get an instance of VerboseLogger and otherwise we'll get an ErrorLogger.

let logger = container.make(Logger.self)
logger.log("hello", level: .verbose) // only logs on development