问题描述
Firebase 的 snapshot.getValue()
期望按如下方式调用:
Firebase's snapshot.getValue()
expects to be called as follows:
snapshot?.getValue(Person::class.java)
但是我想用通过类声明传递到类中的泛型参数替换 Person
,即
However I would like to substitute Person
with a generic parameter that is passed into the class via the class declaration i.e.
class DataQuery<T : IModel>
并使用该通用参数执行以下操作:
and use that generic parameter to do something like this:
snapshot?.getValue(T::class.java)
但是当我尝试这样做时,我收到一个错误提示
but when I attempt that I get an error stating that
只有类可以用在类文字的左侧
only classes can be used on the left-hand side of a class literal
是否可以像在 C# 中那样对泛型参数提供类约束,或者是否可以使用其他一些语法来获取泛型参数的类型信息?
Is it possible to provide a class constraint on the generic parameter like in C# or is there some other syntax I can use to get the type info for the generic param?
推荐答案
对于具有泛型参数 T 的类,您不能这样做,因为您没有 T 的类型信息,因为 JVM 擦除了类型信息.因此这样的代码不能工作:
For a class with generic parameter T, you cannot do this because you have no type information for T since the JVM erases the type information. Therefore code like this cannot work:
class Storage<T: Any> {
val snapshot: Snapshot? = ...
fun retrieveSomething(): T? {
return snapshot?.getValue(T::class.java) // ERROR "only classes can be used..."
}
}
但是,如果 T 的类型被具体化并在一个内联函数中使用,你就可以完成这项工作:
But, you can make this work if the type of T is reified and used within an inline function:
class Storage {
val snapshot: Snapshot? = ...
inline fun <reified T: Any> retrieveSomething(): T? {
return snapshot?.getValue(T::class.java)
}
}
注意内联函数 if public 只能访问类的公共成员.但是您可以有两个函数变体,一个接收非内联的类参数并访问私有内部结构,另一个内联辅助函数从推断的类型参数中进行具体化:
Note that the inline function if public can only access public members of the class. But you can have two variants of the function, one that receives a class parameter which is not inline and accesses private internals, and another inline helper function that does the reification from the inferred type parameter:
class Storage {
private val snapshot: Snapshot? = ...
fun <T: Any> retrieveSomething(ofClass: Class<T>): T? {
return snapshot?.getValue(ofClass)
}
inline fun <reified T: Any> retrieveSomething(): T? {
return retrieveSomething(T::class.java)
}
}
您也可以使用 KClass
代替 Class
以便仅使用 Kotlin 的调用者可以使用 MyClass::class
而不是 <代码>MyClass::class.java
You can also use KClass
instead of Class
so that callers that are Kotlin-only can just use MyClass::class
instead of MyClass::class.java
如果你想让类与泛型上的内联方法合作(意味着类Storage
只存储T
类型的对象):
If you want the class to cooperate with the inline method on the generics (meaning that class Storage
only stores objects of type T
):
class Storage <T: Any> {
val snapshot: Snapshot? = ...
inline fun <reified R: T> retrieveSomething(): R? {
return snapshot?.getValue(R::class.java)
}
}
内联函数中具体化类型的链接:https://kotlinlang.org/docs/reference/inline-functions.html#reified-type-parameters
The link to reified types in inline functions: https://kotlinlang.org/docs/reference/inline-functions.html#reified-type-parameters
这篇关于如何在 Kotlin 中获取泛型参数类的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,WP2