RecyclerView

RecyclerView可以用来代替ListView来展现大量的数据。Google在RecyclerView中提升了性能，和更多好用的API。

使用RecyclerView不仅需要设置adapter，还需要设计值layout manager。

1. 要创建adapter，需要继承并实现RecyclerView.Adapter和RecyclerView.ViewHolder。RecyclerView专门对view holder这部分的API做了一些优化。
   ```kotlin
   /**

• Created by uncle_charlie on 7/4/2016.
  */
  class CategoryRecyelerAdapter(var context: Context, var categoryList: ArrayList)
  : RecyclerView.Adapter() {

  override fun onBindViewHolder(holder: CategoryViewHolder?, position: Int) {
  val categoryInfo = categoryList.get(position)
  Picasso.with(context).load(categoryInfo.imageUrl)
  .resize(80, 80)
  .into(holder?.categoryImageView)
  }

  override fun onCreateViewHolder(parent: ViewGroup?, viewType: Int): CategoryViewHolder? {
  val layoutView = LayoutInflater.from(context).inflate(R.layout.category_grid_item, null)
  var viewHolder = CategoryViewHolder(layoutView)
  return viewHolder
  }

  override fun getItemCount(): Int {
  return categoryList.count()
  }

  inner class CategoryViewHolder(var categoryView: View) : RecyclerView.ViewHolder(categoryView) {
  var categoryImageView: ImageView? = null;
  var categoryTextView: TextView? = null;

  init {
      categoryImageView = categoryView.findViewById(R.id.product_imageview) as ImageView
      categoryTextView = categoryView.findViewById(R.id.product_textview) as TextView
  }

  }
  }
  ```
• CategoryViewHolder继承RecyclerView.ViewHolder。构造函数需要一个View类型的参数。使用这个view来获得view holder里hold的view，比如这里的image view和text view。
• onCreateViewHolder() 创建view holder。val layoutView = LayoutInflater.from(context).inflate(...)，解析RecyclerView的每一个item的layout。并把这个解析之后的view传入前面说的view holder。
• onBindViewHolder()，这里就是给view holder里的view赋值，比如这里用Picasso给image view设置图片：Picasso.with(context)...into(holder?.categoryImageView)。

• getItemCount()数据源里有多少数据。

Item的点击

但是RecyclerView里完全没有OnItemClickListener这个存在。不过，我们有这个：RecyclerView.OnItemTouchListener 但是RecyclerView.OnItemTouchListener只能响应一次tap事件。当然，有了这个tap，后面就可以扩展了。

/**
 * Created by uncle_charlie on 7/4/2016.
 */
class RecyclerItemClickListener(var context: Context, listener: OnItemClickListener)
: RecyclerView.OnItemTouchListener {
    private var mListener: OnItemClickListener? = null
    private var gestureListener: GestureDetector? = null

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

    init {
        this.mListener = listener
        gestureListener = GestureDetector(context, object : GestureDetector.SimpleOnGestureListener() {
            override fun onSingleTapUp(e: MotionEvent?): Boolean {
                return true
            }
        })
    }

    override fun onTouchEvent(rv: RecyclerView?, e: MotionEvent?) {
        throw UnsupportedOperationException()
    }

    override fun onInterceptTouchEvent(rv: RecyclerView?, e: MotionEvent?): Boolean {
        var childView = rv?.findChildViewUnder(e!!.x, e!!.y)
        if (childView != null && mListener != null && gestureListener!!.onTouchEvent(e)) {
            mListener?.onItemClick(childView, rv!!.getChildAdapterPosition(childView))
            return true
        }
        return false
    }

    override fun onRequestDisallowInterceptTouchEvent(disallowIntercept: Boolean) {
        throw UnsupportedOperationException()
    }
}

如何使用：

  recyclerView.addOnItemTouchListener(RecyclerItemClickListener(activity,
    object : RecyclerItemClickListener.OnItemClickListener {
        override fun onItemClick(view: View, position: Int) {
            categoryPresenter?.onItemClicked(position)
        }
    }))