我有这个 Play Framework 2 代码(简化):
import formatters.json.IdeaTypeFormatter._
object IdeaTypes extends Controller {
def list = Action { request =>
Ok(toJson(IdeaType.find(request.queryString)))
}
def show(id: Long) = Action {
IdeaType.findById(id).map { ideatype =>
Ok(toJson(ideatype))
}.getOrElse(JsonNotFound("Type of idea with id %s not found".format(id)))
}
}
IdeaType
类扩展 Entity
,它是伴随对象,IdeaType
, 扩展 EntityCompanion
.正如您所料,我在每个 Controller 中都有这种代码,所以我想将基本行为提取到特征中,如下所示:
abstract class EntityController[A<:Entity] extends Controller {
val companion: EntityCompanion
val name = "entity"
def list = Action { request =>
Ok(toJson(companion.find(request.queryString)))
}
def show(id: Long) = Action {
companion.findById(id).map { entity =>
Ok(toJson(entity))
}.getOrElse(JsonNotFound("%s with id %s not found".format(name, id)))
}
}
但我收到以下错误:
[error] EntityController.scala:25: No Json deserializer found for type List[A].
[error] Try to implement an implicit Writes or Format for this type.
[error] Ok(toJson(companion.find(request.queryString)))
[error] ^
[error] EntityController.scala:34: No Json deserializer found for type A.
[error] Try to implement an implicit Writes or Format for this type.
[error] Ok(toJson(entity))
[error] ^
我不知道如何分辨隐含的
Writes
将由实现 EntityController
的类实现trait(或继承抽象类 EntityController
)- 编辑
到目前为止,我正在这样做:
abstract class CrudController[A <: Entity](
val model: EntityCompanion[A],
val name: String,
implicit val formatter: Format[A]
) extends Controller {
并像这样使用它
object CrudIdeaTypes extends CrudController[IdeaType](
model = IdeaType,
name = "type of idea",
formatter = JsonIdeaTypeFormatter
)
我无法让 Scala 使用隐式自动选择它。我试过这个导入,但没有用
import formatters.json.IdeaTypeFormatter._
最佳答案
如果您希望 Controller 类本身定义隐式,则只需声明抽象的隐式值,并在派生类中定义它们。
abstract class EntityController[A<:Entity] extends Controller {
protected implicit def entityWriter: Writes[A]
protected implicit def entityListWriter: Writes[List[A]]
...
}
class MyEntity extends Entity {
...
}
class MyEntityController extends EntityController[MyEntity] {
protected def entityWriter: Writes[MyEntity] = ...
protected def entityListWriter: Writes[List[MyEntity]] = ...
}
但是,在 Controller 外部定义这些隐式更实用,通常在实体的伴随对象中,以便编译器无需导入即可自动找到它们。
然后,将隐式值传递给
EntityController
的构造函数:abstract class EntityController[A<:Entity](implicit entityWriter: Writes[A], entityListWriter: Writes[List[A]] ) extends Controller {
...
}
class MyEntity extends Entity {
...
}
object MyEntity {
protected implicit def entityWriter: Writes[A]
protected implicit def entityListWriter: Writes[List[A]]
}
class MyEntityController extends EntityController[MyEntity] {
...
}
最后一点,可能不需要 List[MyEntity] 的隐式(因此只有 MyEntity 的隐式需要显式定义)。我没有检查过,但通常在使用这种“类型类模式”时,
框架已经为每个 List[T] 定义了一个隐式,前提是 T 有一个隐式。这可能是这种情况,虽然我没有检查过。
关于scala - 定义依赖于隐式的抽象类或特征,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/12946359/