聚合（Aggregating）反向路由

在某些情况下，你可能想要在彼此不依赖的子项目之间共享反向路由。

例如, 你可能有一个web 子项目和一个api 子项目。这些子项目没有彼此依赖, 除了web 项目有链接指向api 项目 (为创建AJAX调用), 而api 项目想要链接到web (JSON资源的web链接)。在这种情况下, 使用反向路由是很方便的, 但因为这些项目不依赖对方，所以无法实现。

Play’s routes compiler offers a feature that allows a common dependency to generate the reverse routers for projects that depend on it so that the reverse routers can be shared between those projects. This is configured using the aggregateReverseRoutes sbt configuration item, like this:

lazy val common: Project = (project in file("common"))
  .enablePlugins(PlayScala)
  .settings(
    aggregateReverseRoutes := Seq(api, web)
  )

lazy val api = (project in file("api"))
  .enablePlugins(PlayScala)
  .dependsOn(common)

lazy val web = (project in file("web"))
  .enablePlugins(PlayScala)
  .dependsOn(common)

In this setup, the reverse routers for api and web will be generated as part of the common project. Meanwhile, the forwards routers for api and web will still generate forwards routers, but not reverse routers, because their reverse routers have already been generated in the common project which they depend on, so they don’t need to generate them.

Note that the common project has a type of Project explicitly declared. This is because there is a recursive reference between it and the api and web projects, through the dependsOn method and aggregateReverseRoutes setting, so the Scala type checker needs an explicit type somewhere in the chain of recursion.