Kotlin中通用类类型的用法 [英] Generic class type usage in Kotlin

问题描述

我正在尝试编写一个通用基本活动，该活动指定其ViewModel类型为通用参数:

I am trying to write a generic base activity, that specifies it's ViewModel type a generic parameter:

abstract class BaseActivity<T : ViewModel> : AppCompatActivity()

现在，我正在尝试为ViewModel编写一个惰性的初始化属性:

Now I am trying to write a lazy initialized property for my ViewModel:

val viewModel by lazy { ViewModelProviders.of(this, getFactory()).get(T) }

显示的错误是Type Parameter T is not an expression

也使用::class或::class.java并没有帮助.谁能解释这个问题?

Also using ::class or ::class.java did not help. Can anyone explain the problem?

编辑:我试图使用这样的标准化内联函数:

I tried to use a reified inline function like this:

inline fun <reified T : ViewModel?> AppCompatActivity.obtainViewModel(factory: ViewModelProvider.Factory): T {
    return ViewModelProviders.of(this, factory).get(T::class.java)
}

并像这样使用它:

abstract class BaseActivity<T> : AppCompatActivity() {
    val viewModel by lazy { obtainViewModel<T>(getFactory()) 
}

现在我得到了一个错误，即T不能用作化参数.

Now I get the error, that T cannot be used as a reified parameter.

到目前为止，最好的解决方案似乎是:

The best solution so far seems to be this: Link, but its overhead to implement the abstract token in every extending class.

推荐答案

您的类具有类型参数T，不幸的是，该参数在运行时被擦除.结果，调用get(T)不起作用(我猜该方法实际上期望使用Class吗?).

Your class has a type parameter T, which unfortunately gets erased at runtime. As a result, the call get(T) does not work (I guess the method expects a Class actually?).

您似乎已经注意到了这一点，因此尝试通过将处理封装到使用reified类型的方法中来对其进行修复.但是，您不能将T作为固定参数传递，因为在调用reified类型的方法时，该类型已经被擦除.结果，obtainViewModel<T>也将不起作用.您可以做的是对常规泛型方法使用T，下面将进行演示:

You seem to have already noticed that and thus tried to fix it with encapsulating the handling into a method using reified type. Yet, you cannot pass T as a reified parameter since this type will already be erased when the reified-typed method is called. As a result, obtainViewModel<T> will not work either. What you can do, is using T for ordinary generic methods, which the following demonstrates:

class Typed<T> {
    val lazyProp by lazy {
        listOf<T>()
    }
}

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