给定一个特质 MyTrait：
trait MyTrait {
def doSomething = println("boo")
}
它可以与extends或with一同混合到类中：
class MyClass extends MyTrait
在创建新实例时，也可以混合使用：
var o = new MyOtherClass with MyTrait
o.doSomething
但是...特质（或者其他任何有区别的）能够被添加到现有实例中吗？
我正在使用Java中的JPA加载对象，并且我想使用特质为它们添加一些功能。这是否有可能？
我想按照以下方式混入一个特质：
var o = DBHelper.loadMyEntityFromDB(primaryKey);
o = o with MyTrait //adding trait here, rather than during construction
o.doSomething
//if I had a class like this
final class Test {
def f = println("foo")
}
trait MyTrait {
def doSomething = {
println("boo")
}
}
object MyTrait {
implicit def innerObj(o:MixTest) = o.obj
def ::(o:Test) = new MixTest(o)
final class MixTest private[MyTrait](val obj:Test) extends MyTrait
}
您可以按以下方式使用此特性：
import MyTrait._
val a = new Test
val b = a :: MyTrait
b.doSomething
b.f
对于你的示例代码：
val o = DBHelper.loadMyEntityFromDB(primaryKey) :: MyTrait
o.doSomething
object AnyTrait {
implicit def innerObj[T](o: MixTest[T]):T = o.obj
def ::[T](o: T) = new MixTest(o)
final class MixTest[T] private[AnyTrait](val obj: T) extends MyTrait
}
val a = new Test
a.f
val b = a :: AnyTrait
b.f1
b.f
val c = "say hello to %s" :: AnyTrait
println(c.intern) // you can invoke String's method
println(c.format("MyTrait")) //WRONG. you can't invoke StringLike's method, though there defined a implicit method in Predef can transform String to StringLike, but implicit restrict one level transform, you can't transform MixTest to String then to StringLike.
c.f1
val d = 1 :: AnyTrait
println(d.toLong)
d.toHexString // WRONG, the same as above
d.f1
DynamicMixinCompanion trait中的通用Mixin功能。客户端应该为他想要动态Mixin功能的每个trait创建一个扩展DynamicMixinCompanion的伴随对象。这个伴随对象需要定义匿名trait对象(::)。trait DynamicMixinCompanion[TT] {
implicit def baseObject[OT](o: Mixin[OT]): OT = o.obj
def ::[OT](o: OT): Mixin[OT] with TT
class Mixin[OT] protected[DynamicMixinCompanion](val obj: OT)
}
trait OtherTrait {
def traitOperation = println("any trait")
}
object OtherTrait extends DynamicMixinCompanion[OtherTrait] {
def ::[T](o: T) = new Mixin(o) with OtherTrait
}
object Main {
def main(args: Array[String]) {
val a = "some string"
val m = a :: OtherTrait
m.traitOperation
println(m.length)
}
}
m是OtherTrait的一个实例，但不是String的实例。（它是“隐式”的，每当需要时在编译时将其“转换”回字符串。）您可以通过在main函数的末尾添加println（“m是String / OtherTrait的实例：”+ m.isInstanceOf [String] +“/”+ m.isInstanceOf [OtherTrait]）来清楚地看到这一点。 - Hbf我通常使用 implicit 将新方法混合到现有对象中。
比如，如果我有以下代码：
final class Test {
def f = "Just a Test"
...some other method
}
trait MyTrait {
def doSomething = {
println("boo")
}
}
object HelperObject {
implicit def innerObj(o:MixTest) = o.obj
def mixWith(o:Test) = new MixTest(o)
final class MixTest private[HelperObject](obj:Test) extends MyTrait
}
然后，您可以使用已经存在的对象Test来调用MyTrait方法。
val a = new Test
import HelperObject._
val b = HelperObject.mixWith(a)
println(b.f)
b.doSomething
import HelperObject._
val o = mixWith(DBHelper.loadMyEntityFromDB(primaryKey));
o.doSomething
我正在思考一种完美的语法来定义这个HelperObject：
trait MyTrait {
..some method
}
object MyTrait {
implicit def innerObj(o:MixTest) = o.obj
def ::(o:Test) = new MixTest(o)
final class MixTest private[MyTrait](obj:Test) extends MyTrait
}
//then you can use it
val a = new Test
val b = a :: MyTrait
b.doSomething
b.f
// for your example
val o = DBHelper.loadMyEntityFromDB(primaryKey) :: MyTrait
o.doSomething
那么隐式类呢？与其他答案中的最终内部类和“mixin”函数相比，它对我来说似乎更容易。
trait MyTrait {
def traitFunction = println("trait function executed")
}
class MyClass {
/**
* This inner class must be in scope wherever an instance of MyClass
* should be used as an instance of MyTrait. Depending on where you place
* and use the implicit class you must import it into scope with
* "import mypackacke.MyImplictClassLocation" or
* "import mypackage.MyImplicitClassLocation._" or no import at all if
* the implicit class is already in scope.
*
* Depending on the visibility and location of use this implicit class an
* be placed inside the trait to mixin, inside the instances class,
* inside the instances class' companion object or somewhere where you
* use or call the class' instance with as the trait. Probably the
* implicit class can even reside inside a package object. It also can be
* declared private to reduce visibility. It all depends on the structure
* of your API.
*/
implicit class MyImplicitClass(instance: MyClass) extends MyTrait
/**
* Usage
*/
new MyClass().traitFunction
}
https://alvinalexander.com/scala/scala-2.10-implicit-class-example
我不确定以下代码的返回值是什么：
var o = DBHelper.loadMyEntityFromDB(primaryKey);
但假设它是DBEntity。你可以将类DBEntity转换为扩展你的特质MyTrait的类。
像这样：
trait MyTrait {
def doSomething = {
println("boo")
}
}
class MyClass() extends MyTrait
// Have an implicit conversion to MyClass
implicit def dbEntityToMyClass(in: DBEntity): MyClass =
new MyClass()
我相信你也可以通过使用隐式类来简化这个问题。
implicit class ConvertDBEntity(in: DBEntity) extends MyTrait
::运算符来混合一个trait。::运算符用于序列，例如：val x = 1 :: 2 :: 3 :: Nil
在我看来，将其用作继承的手段有点别扭。
implicit定义一个方法，并将该方法导入到你的作用域中时，该方法可以帮助你将由方法参数指定的对象转换为由方法返回指定的另一个对象，以便在需要调用后者的未定义于前者的方法时使用。 - Googol ShanMyTrait对象中添加一个通用参数到::中就可以让它适用于任何类型。它是否也可以与我们想要混合的任意特性一起使用呢...？ - axel22