scala – Cake Pattern可以用于非单例样式依赖吗？

trait HttpService { def get(query:String):String }
trait HttpServiceComponent {
  val httpService:HttpService
  class HttpServiceImpl(address:String) extends HttpService {
    def get(query:String):String = ...
  }
}

trait TradeService { def lastTrade(symbol:String):String }
trait TradeServiceComponent {
  this:HttpServiceComponent => // Depends on HttpService
  val tradeService:TradeService
  class TradeServiceImpl extends TradeService {
    def lastTrade(symbol:String):String =
      httpService.get("symbol=" + symbol)
  }
}

trait CompanyService { def getCompanySymbols(exchange:String):String }
trait CompanyServiceComponent {
  this:HttpServiceComponent =>  // Depends on different HttpService instance
  val companyService:CompanyService
  class CompanyServiceImpl extends CompanyService {
    def getCompanySymbols(exchange:String):String =
      httpService.get("exchange=" + exchange)
  }
}

trait App { def run(exchange:String):Unit }
trait AppComponent {
  this:CompanyServiceComponent with TradeServiceComponent =>
  val app:App
  class AppImpl extends App {
    def run(exchange:String) =
      companyService.getCompanySymbols(exchange).split(",").foreach(sym => {
        val lastTrade = tradeService.lastTrade(sym)
        printf("Last trade for %s: %s".format(sym,lastTrade))
      })
  }
}

正如你从答案中看到的那样(尤其是丹尼尔的,也是你自己的),这是可能的,但它看起来并不优雅.出现这种困难是因为当您使用Cake模式时,将所有必需的特征混合到一个对象中(使用“with”关键字),并且您不能将特征多次混合到一个实例中.这就是mixins的工作方式,而Cake也是基于它们的.

你可以强制Cake处理非单例依赖的事实并不意味着你应该这样做.我建议你在这种情况下简单地使用普通的构造函数,这就是自我类型注释不适合的地方：

trait HttpService { ... }

/* HttpServiceImpl has become a top-level class now,* as the Cake pattern adds no more value here.
 * In addition,trait HttpServiceComponent gets deleted */
class HttpServiceImpl(address:String) extends HttpService {
  ...
}

trait TradeService { def lastTrade(symbol:String):String }
trait TradeServiceComponent {
  // The dependency on HttpService is no longer declared as self-type
  val tradeService:TradeService
  // It is declared as a constructor parameter now
  class TradeServiceImpl(httpService: HttpService) extends TradeService {
    def lastTrade(symbol:String):String =
      httpService.get("symbol=" + symbol)
  }
}

trait CompanyService { def getCompanySymbols(exchange:String):String }
trait CompanyServiceComponent {
  // Again,self-type annotation deleted
  val companyService:CompanyService
  // Again,the dependency is declared as a constructor parameter
  class CompanyServiceImpl(httpService: HttpService) extends CompanyService {
    def getCompanySymbols(exchange:String):String =
      httpService.get("exchange=" + exchange)
  }
}

App和AppComponent特征保持其原始形式.现在,您可以通过以下方式使用所有组件：

object App {
  def main(args:Array[String]):Unit = {
    val appAssembly = new AppComponent 
        with TradeServiceComponent
        with CompanyServiceComponent {
      // Note,that HttpServiceComponent it neither needed nor mixed-in now
      val tradeService = new TradeServiceImpl(
        new HttpServiceImpl("http://trades-r-us.com"))
      val companyService = new CompanyServiceImpl(
        new HttpServiceImpl("http://exchange-services.com"))
      val app = new AppImpl
    }
    appAssembly.app.run(args(0))
  }
}

此外,您可能需要仔细检查Cake模式是否真的最适合您的需求,因为它实际上是一个复杂的模式,依赖注入只是其中的一部分.如果你只将它用于DI,我会建议你使用更简单的解决方案.我发表了关于here的博文.