在 Kotlin 中将 class 转换为它的子 class

Question

我有一个名为 Opportunity 的 class 列表，其中充满了扩展 Opportunity 的对象。该列表可以包含 CommunityOpportunites 或 SponsorshipOpportunities。

我覆盖了 getItemViewType 并根据相关位置中的任何子对象为每个项目分配一个值class，并且每个项目都有不同的 ViewHolder：

override fun getItemViewType(position: Int): Int {

    return if (opportunityList[position] is SponsorshipOpportunity){

        Log.i(TAG,"Item type is sponsorshipId")

        SPONSORSHIP

    } else{
        Log.i(TAG,"Item type is community")
        COMMUNITY

    }

}

    inner class CommunityViewHolder(var view: CommunityTileBinding):RecyclerView.ViewHolder(view.root)
    inner class SponsorshipViewHolder(var view: SponsorshipTileBinding):RecyclerView.ViewHolder(view.root)



companion object{
    private const val COMMUNITY = 0
    private const val SPONSORSHIP = 1
}

在 onCreateViewHolder() 中，我为项目的 class 创建了正确的 ViewHolder，在 onBindViewHolder() 中，我尝试将列表中的项目（构造函数中的机会类型）转换为子class 视图中的项目：

override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {


    val inflater = LayoutInflater.from(parent.context)

    return when (viewType){

        COMMUNITY->CommunityViewHolder(DataBindingUtil.inflate(inflater, R.layout.community_tile, parent, false))
        SPONSORSHIP-> SponsorshipViewHolder(DataBindingUtil.inflate(inflater, R.layout.sponsorship_tile, parent, false))

        else-> throw IllegalArgumentException()


    }
}

override fun getItemCount(): Int = opportunityList.size


override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {

  when(holder.itemViewType){
      COMMUNITY-> {

          (holder as CommunityViewHolder).view.communityOpportunity = opportunityList[position] as CommunityOpportunity
      }
      SPONSORSHIP->{
          (holder as SponsorshipViewHolder).view.sponsorship = opportunityList[position] as SponsorshipOpportunity
          holder.view.postActionText.text = context.resources.getString(R.string.watch_respond)

      }
  }

}

但是，我得到以下 class 转换异常

java.lang.ClassCastException: com.example.model.Opportunity cannot be cast to com.example.model.CommunityOpportunity

当我尝试在 onBindViewHolder 中的相关行时，即使打印了确认该项目是 getItemViewType() 中的 CommunityOpportunity 的日志语句。

有没有更好的方法来询问，或者有没有更好的方法让我在 RecyclerView 中显示多个 ViewHolder/Object 类型？

编辑：以下是相关的 xml 布局：

xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools">

<data>
    <variable
        name="sponsorship"
        type="com.example.weare8sample.model.SponsorshipOpportunity"/>
</data>

<LinearLayout

    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="10dp"
    android:elevation="2dp"
    android:orientation="vertical"
    android:background="@drawable/tile_background">

    <ImageView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:foreground="@drawable/tile_background"
        android:imageUrl="@{sponsorship.coverTileUri}">

    </ImageView>



    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:gravity="center_vertical"
        android:padding="10dp">

        <TextView
            android:id="@+id/postActionText"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:fontFamily="@font/lato_bold"
            tools:text="@string/watch_respond">

        </TextView>

        <ImageView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentRight="true"
            android:src="@drawable/ic_chevron_right_black_36dp">

        </ImageView>



    </RelativeLayout>

</LinearLayout>

xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools">

<data>

    <variable
        name="communityOpportunity"
        type="com.example.weare8sample.model.CommunityOpportunity" />
</data>


<ImageView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="10dp"
    android:background="@drawable/tile_background"
    android:imageUrl="@{communityOpportunity.mediaImageUri}">

</ImageView>

Answer 1

为 getItemViewType 中的所有类型添加显式类型检查，如果是未知类型则抛出，以正确处理所有情况。

现在，如果有第三种类型的机会，它将被假定为 COMMUNITY 类型。

Answer 2

在您的模型中使用密封 class 是这种情况下的最佳选择

sealed class Opportunity {
  data class CommunityOpportunites( 
                 // class fields
             ):Opportunity()
  data class SponsorshipOpportunities(
                                   // class fields
                                      ):Opportunity()
}

并且在您的 getItemViewType 方法中应该是这样的

override fun getItemViewType(position: Int) = when (list[position]) {
    is Opportunity.CommunityOpportunites-> 0
    is Opportunity.SponsorshipOpportunities-> 1
}

和 onCreateViwHolder 方法

override fun onCreateViewHolder(parent: ViewGroup, viewType: Int) = 
    when (viewType) {
    0 -> ViewHolder(
        LayoutInflater.from(parent.context).inflate(R.layout.item1, parent, 
        false)
    )
    else-> 
     LayoutInflater.from(parent.context).inflate(R.layout.item2, parent, 
           false)
    )
}

你不必使用多个 ViwHolders 编写 ViewHolder inner class 像这样

inner class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
    fun bind() {
        val d = list[adapterPosition]
        when (d) {
            is Opportunity.CommunityOpportunites -> {
                itemView.apply {
                    //bind the data in the list to view you needn't cast d to 
                    //CommunityOpportunites Kotlin smart cast does it for you
                }
            }
            is Opportunity.SponsorshipOpportunities -> {
                itemView.apply {
                    //bind the data in the list to view you needn't cast d to 
                    //SponsorshipOpportunities Kotlin smart cast does it for you
                }
            }
        }
    }
}

我认为这应该有效:)

在 Kotlin 中将 class 转换为它的子 class

Cast class to its subclass in Kotlin

android

kotlin

android-recyclerview

android-architecture-components