Kotlin中RecyclerView适配器内声明双ViewHolder内部类报错求解

你在ChatAdapter中声明SentMessageViewHolder和ReceivedMessageViewHolder时出现的报错，翻译为：

期望成员声明

错误原因 #

问题出在ViewHolder类的次级构造函数写法上：

1. Kotlin中声明次级构造函数必须显式使用constructor关键字，你直接写函数名的写法不符合语法规范
2. 次级构造函数必须调用类的主构造函数（或其他次级构造函数），而你的次级构造函数没有调用继承自RecyclerView.ViewHolder的主构造（该构造要求传入View类型参数）

修改方案 #

给次级构造函数添加constructor关键字，并且在次级构造函数中通过this(binding.root)调用主构造函数（Binding对象的root属性就是对应的item布局View）

修改后的代码 #

class ChatAdapter(private val dataSet: ArrayList<ChatMessage>, private val receiverProfilImage: Bitmap, val senderId: String) :
    RecyclerView.Adapter<RecyclerView.ViewHolder>() {

    class SentMessageViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        lateinit var binding: ItemContainerSentMessageBinding

        // 添加constructor关键字，同时调用主构造函数
        constructor(binding: ItemContainerSentMessageBinding) : this(binding.root) {
            this.binding = binding
        }

        fun setData(chatMessage: ChatMessage) {
            binding.textMessage.text = chatMessage.message
            binding.textDateTime.text = chatMessage.dateTime
        }
    }

    class ReceivedMessageViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        lateinit var binding: ItemContainerReceivedMessageBinding

        // 添加constructor关键字，同时调用主构造函数
        constructor(binding: ItemContainerReceivedMessageBinding) : this(binding.root) {
            this.binding = binding
        }

        fun setData(chatMessage: ChatMessage, receiverProfilImage: Bitmap) {
            binding.textMessage.text = chatMessage.message
            binding.textDateTime.text = chatMessage.dateTime
            binding.imageProfil.setImageBitmap(receiverProfilImage)
        }
    }

    // 补充Adapter必须实现的方法（如getItemViewType、onCreateViewHolder、onBindViewHolder等）
    // ...
}

更简洁的写法（可选） #

你可以直接把Binding作为主构造参数，去掉次级构造函数，更符合Kotlin的简洁风格：

class SentMessageViewHolder(val binding: ItemContainerSentMessageBinding) : RecyclerView.ViewHolder(binding.root) {
    fun setData(chatMessage: ChatMessage) {
        binding.textMessage.text = chatMessage.message
        binding.textDateTime.text = chatMessage.dateTime
    }
}

class ReceivedMessageViewHolder(val binding: ItemContainerReceivedMessageBinding) : RecyclerView.ViewHolder(binding.root) {
    fun setData(chatMessage: ChatMessage, receiverProfilImage: Bitmap) {
        binding.textMessage.text = chatMessage.message
        binding.textDateTime.text = chatMessage.dateTime
        binding.imageProfil.setImageBitmap(receiverProfilImage)
    }
}

内容的提问来源于stack exchange，提问作者Captai-N