[英]Get TypeTag[A] from Class[A]
I have createOld
method that I need to override and I cannot change it. 我有需要重写的
createOld
方法,我无法更改它。 I would like to use TypeTag
to pattern match provided type in createNew
. 我想使用
TypeTag
模式匹配createNew
中提供的类型。 The goal is to find out how to call createNew
from createOld
. 目的是找出如何从
createOld
调用createNew
。 My current understanding is that compiler doesn't have enough type information about A
in createOld
method if it doesn't already come with TypeTag[A]
. 我目前的理解是,如果
TypeTag[A]
尚未createOld
则createOld
方法中的A
类型信息不足。
object TypeTagFromClass {
class C1
class C2
// How to get TypeTag[A] needed by createNew?
def createOld[A](c: Class[A]): A = createNew ???
def createNew[A : TypeTag]: A = {
val result = typeOf[A] match {
case a if a =:= typeOf[C1] => new C1()
case a if a =:= typeOf[C2] => new C2()
}
result.asInstanceOf[A]
}
}
It is possible to create a TypeTag
from a Class
using Scala reflection, though I'm not sure if this implementation of TypeCreator
is absolutely correct: 可以使用Scala反射从
Class
创建TypeTag
,尽管我不确定TypeCreator
这种实现是否绝对正确:
import scala.reflect.runtime.universe._
def createOld[A](c: Class[A]): A = createNew {
val mirror = runtimeMirror(c.getClassLoader) // obtain runtime mirror
val sym = mirror.staticClass(c.getName) // obtain class symbol for `c`
val tpe = sym.selfType // obtain type object for `c`
// create a type tag which contains above type object
TypeTag(mirror, new TypeCreator {
def apply[U <: Universe with Singleton](m: api.Mirror[U]) =
if (m eq mirror) tpe.asInstanceOf[U # Type]
else throw new IllegalArgumentException(s"Type tag defined in $mirror cannot be migrated to other mirrors.")
})
}
However, you don't really need full TypeTag
if you don't need to inspect generic parameters and full Scala type information. 但是,如果您不需要检查通用参数和完整的Scala类型信息,则实际上并不需要完整的
TypeTag
。 You can use ClassTag
s for that: 您可以使用
ClassTag
:
def createNew[A: ClassTag]: A = {
val result = classTag[A].runtimeClass match {
case a if a.isAssignableFrom(classOf[C1]) => new C1()
case a if a.isAssignableFrom(classOf[C2]) => new C2()
}
result.asInstanceOf[A]
}
Or with some implicit sugar: 或加一些隐性糖:
implicit class ClassTagOps[T](val classTag: ClassTag[T]) extends AnyVal {
def <<:(other: ClassTag[_]) = classTag.runtimeClass.isAssignableFrom(other.runtimeClass)
}
def createNew[A: ClassTag]: A = {
val result = classTag[A] match {
case a if a <<: classTag[C1] => new C1()
case a if a <<: classTag[C2] => new C2()
}
result.asInstanceOf[A]
}
You can simplify that even further by using plain old Java newInstance()
method: 您可以使用普通的旧Java
newInstance()
方法进一步简化该操作:
def createNew[A: ClassTag]: A = classTag[A].runtimeClass.newInstance().asInstanceOf[A]
This, of course, would only work if you don't need different constructor parameters for different classes. 当然,只有在不需要为不同的类使用不同的构造函数参数时,这才起作用。
Calling this createNew
from createOld
is much simpler than the one with TypeTag
s: 从
createOld
调用此createNew
比使用TypeTag
的调用要简单得多:
def createOld[A](c: Class[A]): A = createNew(ClassTag[A](c))
So it is not very safe and correct (cause you don't use the power of scala type system), but you can make (using your logic) to do the following: 因此它不是很安全正确(因为您没有使用scala类型系统的功能),但是您可以(使用逻辑)进行以下操作:
def createNew[A](implicit t: TypeTag[A]): A = {
val result: Any = t.tpe.toString match {
case "C1" => new C1
case "C2" => new C2
}
result.asInstanceOf[A]
}
createNew[C1] //> its all ok
createNew[C2] //> its all ok
createNew[C3] //> crashes here; lets pretend we got C3 class
To use it with createOld
, just pass implicit argument: 要将其与
createOld
一起createOld
,只需传递隐式参数:
def createOld[A](c: Class[A])(implicit t: TypeTag[A]): A = createNew[A]
createOld[C1] //> its all ok
createOld[C2] //> its all ok
createOld[C3] //> crashes here; lets pretend we got C3 class
I think I should not tell you twice that it is not very good. 我想我不应该再告诉你这不是很好。 We can improve this code by using
shapeless
: 我们可以通过使用
shapeless
来改进此代码:
Lets create a poly function, which has a TypeTag
as an argument: 让我们创建一个以
TypeTag
作为参数的多边形函数:
import shapeless._; import scala.reflect.runtime.universe._;
def getTypeTag[T](implicit t: TypeTag[T]) = t //> to get TypeTag of a class
// here is low prority implicit
trait createPolyNewErr extends Poly1 {
implicit def newErr[T] = at[T](_ => "Error can not create object of this class")
}
object createPolyBew extends createPolyNewError {
implicit def newC1 = at[TypeTag[C1]](_ => new C1)
implicit def newC2 = at[TypeTag[C2]](_ => new C2)
}
createPolyNew(getTypeTag[C1]) //> success
createPolyNew(getTypeTag[C2]) //> success
createPolyNew(getTypeTag[C3]) //> String: Error can not create object of this class no crash!
We also can write a function, in order not to use function getTypeTag[T]
every time: 我们还可以编写一个函数,以免每次都不使用函数
getTypeTag[T]
:
def createPoly[T]
(implicit t: TypeTag[T],
cse: poly.Case[createPolyNew.type, TypeTag[T] :: HNil]) = cse(t)
createPoly[C1] //> its all ok
createPoly[C2] //> its all ok
createPoly[C3] //> String: Error can not create object of this class no crash!
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