Question

我对泛型类型，子类型和它们之间的不匹配有疑问。我有特定的结构类和接口。我会告诉你，请解释我为什么会出现类型不匹配。

假设我正在准备我的MVP框架，并且我有以下接口和类：

这是最高的抽象

interface Presenter<in V : AbstractView> {
    fun attachView(view: V)

    fun detachView()

    fun onDestory() {

    }
}

抽象类包含Presenter的特定方法和实现

abstract class AbstractPresenter<V : AbstractView> : Presenter<V>, LifecycleObserver {

    private var viewReference: WeakReference<V?>? = null

    protected abstract fun onAttached(view: V)

    final override fun attachView(view: V) {
        viewReference = WeakReference(view)
        onAttached(view)
    }

    final override fun detachView() {
        viewReference?.clear()
        viewReference = null
        onDetached()
    }

    protected open fun onDetached() {
    }
}

合同

interface DashboardContract {

    interface View : AbstractView {

    }

    abstract class Presenter : AbstractPresenter<View>(){

    }
}

最后

class DashboardPresenter : DashboardContract.Presenter() {

    override fun onAttached(view: DashboardContract.View) {
    }
}

就AbstractView而言，它看起来更简单。只有界面AbstractView。在合同DashboardContract.View扩展AbstractView界面，我的DashboardActivity实现此DashboardContract.View界面。

class DashboardActivity : BaseActivity(), DashboardContract.View { ... }

因此，当我在DashboardPresenter创建DashboardActivity作为属性并创建方法fun getPresenter() : Presenter<AbstractView>时，我得到Type mismatch error为什么？不是Presenter<AbstractView>的子类型？

fun getPresenter() : AbstractPresenter<AbstractView> {
        return dashboardPresenter // The type is DashboardPresenter
    }

让我们来看看Java代码：

我正在看反编译Kotlin的Java代码。我把它放在下面。这就是Presenter的样子：

public interface Presenter {
   void attachView(@NotNull AbstractView var1);

   void detachView();

   void onDestory();

   @Metadata(...)
   public static final class DefaultImpls {
      public static void onDestory(Presenter $this) {
      }
   }
}

我认为如果我在Kotlin中使用泛型类，我也会在java中获得泛型类。我错了。

AbstractPresenter给出：

public abstract class AbstractPresenter implements Presenter, LifecycleObserver {
   private WeakReference viewReference;

   protected abstract void onAttached(@NotNull AbstractView var1);

   public final void attachView(@NotNull AbstractView view) {
      Intrinsics.checkParameterIsNotNull(view, "view");
      this.viewReference = new WeakReference(view);
      this.onAttached(view);
   }

   public final void detachView() {
      WeakReference var10000 = this.viewReference;
      if(this.viewReference != null) {
         var10000.clear();
      }

      this.viewReference = (WeakReference)null;
      this.onDetached();
   }

   protected void onDetached() {
   }

   public void onDestory() {
      DefaultImpls.onDestory(this);
   }
}

合同

public interface DashboardContract {
   @Metadata(...)
   public interface View extends AbstractView {
   }

   @Metadata(...)
   public abstract static class Presenter extends AbstractPresenter {
   }
}

DashboardPresetner：

public final class DashboardPresenter extends Presenter {

   protected void onAttached(@NotNull View view) {
      Intrinsics.checkParameterIsNotNull(view, "view");
   }

   // $FF: synthetic method
   // $FF: bridge method
   public void onAttached(AbstractView var1) {
      this.onAttached((View)var1);
   }
}

Answer 1

您必须更改Presenter中DashboardContract的父级，才能使用AbstractView代替View：

abstract class Presenter : AbstractPresenter<AbstractView>()

我不确定为什么不允许你使用View，这可能是一个缺陷在Kotlin的递归类型检查中。看看相应的java代码是什么并继续从中进行调查可能会很有趣。

Kotlin型不匹配

1 个答案: