泛型：抽象类和子类型

Question

我有一个名为 Presenter 的抽象类：

abstract class Presenter<V> {

    fun bind(view: V) {
        ...
    }

    ...
}

我有这些演示者的实现：

class FolderChooserPresenter : Presenter<FolderChooserView>() {
    ...
}

并查看在指定点调用绑定方法的类：

class FolderChooserActivity : BaseView(), FolderChooserView {

    @Inject lateinit var presenter: FolderChooserPresenter

    // method of baseview
    override fun onStart() {
        super.onStart()

        presenter.bind(this)
    }
}

我想要归档的是为像FolderChooserActivity 这样的类提供一个基类，它会自动调用绑定方法。
在所有实现中一遍又一遍地重复这些调用感觉很愚蠢。

我的方法是有一个抽象类，它扩展了调用绑定方法的BaseView。但这显然不起作用，因为绑定类需要实现而不是抽象类。

Answer 1

您可以向BaseView 类添加两个通用参数，并将this 转换为V：

open class Presenter<V> {
    fun bind(v: V) {}
}

open class BaseView<P, V> where P : Presenter<V> {
  lateinit var presenter: P

  fun onStart() {
    p.bind(this as V)
  }
}

你会像这样使用它

class FolderPresenter : Presenter<FolderChooserView>() {

}

class FolderChooserView : BaseView<FolderPresenter, FolderChooserView>()

不幸的是，如果您混淆了参数，您将无法从编译器获得任何帮助，因为未经检查的强制转换：

class SomeOtherView : BaseView<SomeOtherPresenter, FolderChooserView>()

Answer 2

@nhaarman 的答案很接近，但如果被绑定的类实际上不是视图的类型，则会留下漏洞。

这个演员阵容并不是一件坏事，如果有的话，也没有很多好的答案。你总是可以添加一个断言来给出一个非常直接的错误消息，因为这些会在创建活动时立即发生，你会在测试的早期看到错误

如果不创建将关系的所有部分作为一个整体来管理的东西，我认为您不会轻易获得更好的答案。我认为他的回答风险很小。

编译时检查以避免运行时错误

你可以编写一个函数，在编译时检查缺失的部分，人们可以像编译时断言一样使用它。

// a function that is used when people want to check validity at compile time,
// it does nothing but cause compiler error if wrong heirarchy

fun <A: V, P: Presenter<V>, V : View> checkValid() {
    // empty on purpose, used for compile time check
}

// successful:
checkValid<FolderChooserActivity, FolderPresenter, FolderChooserView>()

// error below: "Kotlin: Type argument is not within its bounds: should be subtype of 'test.so.FolderChooserView'"
checkValid<TryingToFoolItActivity, FolderPresenter, FolderChooserView>()

// this is blocked because SomeOtherActivity has its own compiler error so SomeOtherActivity type is not fully known
checkValid<SomeOtherActivity, FolderPresenter, FolderChooserView>()

使用函数创建类，并带有编译时检查以避免运行时错误

您还可以要求使用函数来构造*Activity 类。但是如果你不能强制人们确保他们拥有正确的视图基类，你就不能强制使用这个函数。无论如何，只是为了给这个问题更多的想法。

inline fun <reified A: V, P: Presenter<V>, V : View> makeActivity(): A {
    return A::class.java.newInstance() 
}

// successful:
val good1 = makeActivity<FolderChooserActivity, FolderPresenter, FolderChooserView>()

// error below: "Kotlin: Type argument is not within its bounds: should be subtype of 'test.so.FolderChooserView'"
val bad1 = makeActivity<TryingToFoolItActivity, FolderPresenter, FolderChooserView>()

完整代码：

我将完整的代码放在这里，以便我可以对此进行更多探索并尝试获得替代的完整答案。这实际上只是@nhaarman 答案的一种变体。

// sample classes

class FolderPresenter : Presenter<FolderChooserView>() { }

class BadPresenter : Presenter<RandomView>() { }

// successful declaration
class FolderChooserActivity : BaseActivity<FolderPresenter, FolderChooserView>(), FolderChooserView { }

// Error: "Kotlin: Type argument is not within its bounds: should be subtype of 'test.so.Presenter<test.renlearn.solrpref.FolderChooserView>'"
class SomeOtherActivity : BaseActivity<BadPresenter, FolderChooserView>(), FolderChooserView {}

// Runtime error, we are not a FolderChooserView
class TryingToFoolItActivity : BaseActivity<FolderPresenter, FolderChooserView>() {}

// now the version using a function to construct the activity, where
// this function adds the missing step of compiler time validation.

inline fun <reified A: V, P: Presenter<V>, V : View> makeActivity(): A {
    return A::class.java.newInstance()
}

// or a function that is used when people want to check validity at compile time,
// it does nothing but cause compiler error if wrong heirarchy

fun <A: V, P: Presenter<V>, V : View> checkValid() {
    // empty on purpose, used for compile time check
}

public fun foo() {
    // successful:
    val good1 = makeActivity<FolderChooserActivity, FolderPresenter, FolderChooserView>()

    // error below: "Kotlin: Type argument is not within its bounds: should be subtype of 'test.so.FolderChooserView'"
    val bad1 = makeActivity<TryingToFoolItActivity, FolderPresenter, FolderChooserView>()

    // this is blocked because SomeOtherActivity has its own compiler error so SomeOtherActivity type is not fully known
    val bad2 = makeActivity<SomeOtherActivity, FolderPresenter, FolderChooserView>()

    // successful:
    checkValid<FolderChooserActivity, FolderPresenter, FolderChooserView>()

    // error below: "Kotlin: Type argument is not within its bounds: should be subtype of 'test.so.FolderChooserView'"
    checkValid<TryingToFoolItActivity, FolderPresenter, FolderChooserView>()

    // this is blocked because SomeOtherActivity has its own compiler error so SomeOtherActivity type is not fully known
    checkValid<SomeOtherActivity, FolderPresenter, FolderChooserView>()
}