RecyclerView

RecyclerView是進階版的清單元件，它取代了基本的ListView與GridView，不只是因為RecyclerView擁有多元的呈現樣貌，而是它強制開發者實作ViewHolder類別，以實現畫面複用的機制。

指派給RecyclerView的Adapter必須繼承自RecyclerView.Adapter類別，以下說明如何使用RecyclerView

1. 客製化Adapter與ViewHolder，MyAdapter須繼承RecyclerView.Adapter類別，並建立ViewHolder繼承自 RecyclerView.ViewHolder類別。

   class MyAdapter(private val data: ArrayList<Item>, private val layout: Int):
   RecyclerView.Adapter<MyAdapter.ViewHolder>() {
   //實作RecyclerView.ViewHolder來儲存View
   class ViewHolder(V: View): RecyclerView.ViewHolder(V){
   	val img_photo = v.findViewById<ImageVlew>(R.id.img_photo)
   	val tv_msg = v.findViewById<TextView>(R.id.tv_msg)
   }
   //回傳資料數量
   override fun getItemcount() = data.size
   //建立ViewHolder與Layout並連彼此
   override fun onCreateViewHolder(viewGroup: ViewGroup, position: Int) :
   ViewHolder {
   	val v= LayoutInflater.from(viewGroup.context).inflate(layout, viewGroup, false)
   	return ViewHolder (v)
   }
   //將資科指派給元件呈現
   override fun onBindViewHolder (holder: ViewHolder, position: Int) {
   	holder.img_photo.setImageResource (data[position].photo)
   	holder.tv_msg.text = data[position].name
   }
   

2. 設定排列方式與方向。RecyclerView的排列方式由LayoutManager決定，最常見的是使用LinearLayoutManager與GridLayoutManager實現ListView與GridView的效果，並用LayoutManager的orientation屬性決定方向。

   //建立LipearLayoutManager物件
   val linearLayoutManager = LinearLayoutManager (this)
   //設定清單的排列方向
   linearLayoutManager.orientation = LinearLayoutManager.VERTICAL
   //連結LinearLayoutManager
   recyclerView.layoutManager = linearLayoutManager
   //連結adapter
   recyclerView.adapter = MyAdapter(item, R.layout,adapter_horizontal)
   //建立GridLayoutManager物件·並設定每欄的數量
   val gridLayoutManager = GridLayoutManager(this, 3)
   //設定清單的排列方向
   gridLayoutManager.orientation = GridLayoutManager.VERTICAL
   //連結GridLayoutManager
   recyclerView.layoutManager  = gridLayoutManaget
   //連結adapter
   recyclerView.adapter = MyAdapter(item, R.layout,adapter_vertical)