個人原創，轉載請注明出處：http://www.lxweimin.com/p/14ab0ebea1a4

RecyclerView寫多了，就對重復的adapter感到厭煩，尤其是用上databinding后重復代碼相當多，于是干脆抽取出來寫個供其他adapter繼承的通用的BaseAdapter。廢話不多說直接上代碼：

BaseAdapter

...

//兩個泛型，T代表item的model類型即itemList的bean類型，TBinding代表itemLayout對應生成的XXXBinding類，比如itemLayout是fruit_item.xml，對應的就是FruitItemBinding
//兩個參數itemList對應具體要顯示的arrayList，brId則對應itemLayout里相應variable的變量名，比如variable的name是fruit，brId就是BR.fruit
abstract class BaseAdapter<T, TBinding: ViewDataBinding>(private val itemList: List<T>,
                                                                private val brId: Int) : RecyclerView.Adapter<BaseAdapter<T, TBinding>.ViewHolder>() {

    protected var mContext: Context? = null

    inner class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
        lateinit var binding: TBinding
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        if (mContext == null) {
            mContext = parent.context
        }
        //因為TBinding的具體類型不確定，這里只能用DataBindingUtil.inflate()，而不能用XXXBinding.inflate()
        val binding: TBinding = DataBindingUtil.inflate(LayoutInflater.from(mContext), getLayoutId(), parent, false)
        val holder = ViewHolder(binding.root)
        holder.binding = binding
        return holder
    }

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        holder.binding.setVariable(brId, itemList[position])
        //立即執行綁定，在對view變化時效敏感的地方常用，不加這句有可能出現itemView更新滯后、閃爍等問題
        holder.binding.executePendingBindings()
    }

    override fun getItemCount(): Int {
        return itemList.size
    }
    //獲取具體子adapter對應的itemLayoutId
    abstract fun getLayoutId(): Int

}

代碼注釋說的很詳細了，除了基本的顯示功能外，只重寫了一個常用的getItemCount()。下面我們來看看具體的子類adapter：

FruitAdapter

package com.zerlings.gabeisfaker.ui.main
...

class FruitAdapter (itemList: List<Fruit>, brId: Int) : BaseAdapter<Fruit, FruitItemBinding>(itemList, brId) {

    override fun getLayoutId(): Int {
        return R.layout.fruit_item //item對應的layoutId
    }

}

是不是超級簡單！省掉了不少重復的代碼！當然這只包含了最基本的代碼，通常我們還需要做一些擴展功能，比如點擊事件：

var onItemClickListener: OnItemClickListener? = null

override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        super.onBindViewHolder(holder, position)
        holder.itemView.setOnClickListener { v -> onItemClickListener?.onItemClick(v, position) }
}

interface OnItemClickListener {
        fun onItemClick(view: View, position: Int)
}

以及添加表頭：

companion object {

        private const val TYPE_HEADER = 1

        private const val TYPE_NORMAL = 0
}

var headerView: View? = null
        set(headerView) {
            field = headerView
            notifyItemInserted(0)
        }

override fun getItemViewType(position: Int): Int {

        return if (position == 0 && headerView != null) {
            //第一個item應該加載Header
            TYPE_HEADER
        } else {
            TYPE_NORMAL
        }
}

override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        return if (viewType == TYPE_HEADER) ViewHolder(headerView!!) else super.onCreateViewHolder(parent, viewType)
}

override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        if (getItemViewType(position) == TYPE_NORMAL) super.onBindViewHolder(holder, position)
}

最后完整的FruitAdapter：

...

class FruitAdapter (itemList: List<Fruit>, brId: Int) : BaseAdapter<Fruit, FruitItemBinding>(itemList, brId) {

    companion object {

        private const val TYPE_HEADER = 1

        private const val TYPE_NORMAL = 0
    }

    var onItemClickListener: OnItemClickListener? = null

    var headerView: View? = null
        set(headerView) {
            field = headerView
            notifyItemInserted(0)
        }

    override fun getItemViewType(position: Int): Int {

        return if (position == 0 && headerView != null) {
            //第一個item應該加載Header
            TYPE_HEADER
        } else {
            TYPE_NORMAL
        }
    }

    override fun getLayoutId(): Int {
        return R.layout.fruit_item //item對應的layoutId
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        return if (viewType == TYPE_HEADER) ViewHolder(headerView!!) else super.onCreateViewHolder(parent, viewType)
    }

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        if (getItemViewType(position) == TYPE_NORMAL)  {
            super.onBindViewHolder(holder, position)
            holder.itemView.setOnClickListener { v -> onItemClickListener?.onItemClick(v, position) }
        }
    }

    interface OnItemClickListener {
        fun onItemClick(view: View, position: Int)
    }

}

這個FruitAdapter只擴展了添加header和點擊監聽兩個常用功能，有更多需求可繼續擴展，這里就不再深入了。
至于具體的Fruit.class類和fruit_item.xml是什么樣，已經不重要了。