Android Jetpack之利用Databinding打造通

个人原创，转载请注明出处：https://www.jianshu.com/p/14ab0ebea1a4

RecyclerView写多了，就对重复的adapter感到厌烦，尤其是用上databinding后重复代码相当多，于是干脆抽取出来写个供其他adapter继承的通用的BaseAdapter。废话不多说直接上代码：

BaseAdapter

...

//两个泛型，T代表item的model类型即itemList的bean类型，TBinding代表itemLayout对应生成的XXXBinding类，比如itemLayout是fruit_item.xml，对应的就是FruitItemBinding
//两个参数itemList对应具体要显示的arrayList，brId则对应itemLayout里相应variable的变量名，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
        }
        //因为TBinding的具体类型不确定，这里只能用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])
        //立即执行绑定，在对view变化时效敏感的地方常用，不加这句有可能出现itemView更新滞后、闪烁等问题
        holder.binding.executePendingBindings()
    }

    override fun getItemCount(): Int {
        return itemList.size
    }
    //获取具体子adapter对应的itemLayoutId
    abstract fun getLayoutId(): Int

}

代码注释说的很详细了，除了基本的显示功能外，只重写了一个常用的getItemCount()。下面我们来看看具体的子类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对应的layoutId
    }

}

是不是超级简单！省掉了不少重复的代码！当然这只包含了最基本的代码，通常我们还需要做一些扩展功能，比如点击事件：

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)
}

以及添加表头：

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) {
            //第一个item应该加载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) {
            //第一个item应该加载Header
            TYPE_HEADER
        } else {
            TYPE_NORMAL
        }
    }

    override fun getLayoutId(): Int {
        return R.layout.fruit_item //item对应的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)
    }

}

这个FruitAdapter只扩展了添加header和点击监听两个常用功能，有更多需求可继续扩展，这里就不再深入了。
至于具体的Fruit.class类和fruit_item.xml是什么样，已经不重要了。