Kotlin 中的抽象回收器片段答案

【问题标题】：Abstract recycler fragment in KotlinKotlin 中的抽象回收器片段
【发布时间】：2019-04-09 20:24:48
【问题描述】：

我正在尝试创建一个设置回收器视图的抽象片段类。这是我的适配器的相关部分：

class PeopleAdapter(context: Context?) : RecyclerView.Adapter<PeopleAdapter.PersonViewHolder>() {

    inner class PersonViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        // ...
    }

    // ...
}

这是抽象片段：

abstract class RecyclerFragment : Fragment() {

    protected abstract fun createAdapter(context: Context?): RecyclerView.Adapter<RecyclerView.ViewHolder>

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        return inflater.inflate(R.layout.recycler_view, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        val adapter = createAdapter(context)
        recycler_view.adapter = adapter

        // ...
    }
}

这就是我想使用它的方式：

class PeopleFragment : RecyclerFragment() {

    override fun createAdapter(context: Context?): RecyclerView.Adapter<RecyclerView.ViewHolder> {
        return PeopleAdapter(context)
    }
}

Android Studio 在return PeopleAdapter(context) 上抛出错误：

类型不匹配：推断的类型是 PeopleAdapter 但应有 RecyclerView.Adapter

我不明白为什么会出现问题。 PersonAdapter 扩展了RecyclerView.Adapter，而PersonViewHolder 扩展了RecyclerView.ViewHolder，那么是不是暗示PersonAdapter 扩展了RecyclerView.Adapater<RecyclerView.ViewHolder>？

我尝试改变createAdapter方法如下：

protected abstract fun <T : RecyclerView.ViewHolder?> createAdapter(context: Context?): RecyclerView.Adapter<T>

我也更改了实现：

override fun <T : RecyclerView.ViewHolder?> createAdapter(context: Context?): RecyclerView.Adapter<T> {
    return PeopleAdapter(context)
}

这一次，错误是：

类型不匹配：推断的类型是 PeopleAdapter 但应有 RecyclerView.Adapter

再次，我希望类型能够排列，但不确定我做错了什么。

【问题讨论】：

标签： android inheritance kotlin android-recyclerview abstract

【解决方案1】：

在抽象类中定义泛型类型，并在具体实现中提供它们：

class PeopleAdapter(context: Context?) : RecyclerView.Adapter<PeopleAdapter.PersonViewHolder>() {

        inner class PersonViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
            // ...
        }

        // ...
 }

abstract class RecyclerFragment<VH : RecyclerView.ViewHolder, out T :  RecyclerView.Adapter<VH>> : Fragment() {

    protected abstract fun createAdapter(context: Context?): T

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        return inflater.inflate(R.layout.recycler_view, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        val adapter = createAdapter(context)
        recycler_view.adapter = adapter

        // ...
    }
}

class PeopleFragment : RecyclerFragment<PeopleAdapter.PersonViewHolder, PeopleAdapter>() {

    override fun createAdapter(context: Context?): PeopleAdapter {
        return PeopleAdapter(context)
    }
}

您目前可以获得的最接近的是未经检查的演员表：

return PeopleAdapter(context) as RecyclerView.Adapter<RecyclerView.ViewHolder>

【讨论】：