列表视图

为实现各种排列组合类的视图（包括但不限于Spinner、ListView、GridView等等），Android提供了五花八门的适配器用于组装某个规格的数据，常见的适配器有：数组适配器ArrayAdapter、简单适配器SimpleAdapter、基本适配器BaseAdapter、翻页适配器PagerAdapter。适配器的种类虽多，却个个都不好用，以数组适配器为例，它与Spinner配合实现下拉框效果，其实现代码纷复繁杂，一直为人所诟病。故而在下拉框一小节之中，干脆把ArrayAdapter连同Spinner一股脑都摒弃了，取而代之的是Kotlin扩展函数selector。
到了列表视图ListView这里，与之搭档的一般是基本适配器BaseAdapter，这个BaseAdapter更不简单，基于它的列表适配器得重写好几个方法，还有那个想让初学者撞墙的ViewHolder。总之，每当要实现类似新闻列表、商品列表之类的页面，一想到这个难缠的BaseAdapter，心里便发怵。譬如下图所示的六大行星的说明列表，左侧是图标，右边为文字说明，很普通的一个页面。

可是这个行星列表页面，倘若使用Java编码，就得书写下面一大段长长的代码：

public class PlanetJavaAdapter extends BaseAdapter  {
    private Context mContext;
    private ArrayList<Planet> mPlanetList;
    private int mBackground;

    public PlanetJavaAdapter(Context context, ArrayList<Planet> planet_list, int background) {
        mContext = context;
        mPlanetList = planet_list;
        mBackground = background;
    }

    @Override
    public int getCount() {
        return mPlanetList.size();
    }

    @Override
    public Object getItem(int arg0) {
        return mPlanetList.get(arg0);
    }

    @Override
    public long getItemId(int arg0) {
        return arg0;
    }

    @Override
    public View getView(final int position, View convertView, ViewGroup parent) {
        ViewHolder holder = null;
        if (convertView == null) {
            holder = new ViewHolder();
            convertView = LayoutInflater.from(mContext).inflate(R.layout.item_list_view, null);
            holder.ll_item = (LinearLayout) convertView.findViewById(R.id.ll_item);
            holder.iv_icon = (ImageView) convertView.findViewById(R.id.iv_icon);
            holder.tv_name = (TextView) convertView.findViewById(R.id.tv_name);
            holder.tv_desc = (TextView) convertView.findViewById(R.id.tv_desc);
            convertView.setTag(holder);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }
        Planet planet = mPlanetList.get(position);
        holder.ll_item.setBackgroundColor(mBackground);
        holder.iv_icon.setImageResource(planet.image);
        holder.tv_name.setText(planet.name);
        holder.tv_desc.setText(planet.desc);
        return convertView;
    }

    public final class ViewHolder {
        public LinearLayout ll_item;
        public ImageView iv_icon;
        public TextView tv_name;
        public TextView tv_desc;
    }
}

上面Java实现的适配器类PlanetJavaAdapter，果真又冗长又晦涩，然而这段代码模版基本上是列表视图的标配，只要用Java编码，就必须依样画瓢。如果用Kotlin实现这个适配器类会是怎样的呢？马上利用Android Studio把上述Java代码转换为Kotlin编码，转换后的Kotlin代码类似以下片段：

class PlanetKotlinAdapter(private val mContext: Context, private val mPlanetList: ArrayList<Planet>, private val mBackground: Int) : BaseAdapter() {

    override fun getCount(): Int {
        return mPlanetList.size
    }

    override fun getItem(arg0: Int): Any {
        return mPlanetList[arg0]
    }

    override fun getItemId(arg0: Int): Long {
        return arg0.toLong()
    }

    override fun getView(position: Int, convertView: View?, parent: ViewGroup): View {
        var view = convertView
        var holder: ViewHolder?
        if (view == null) {
            holder = ViewHolder()
            view = LayoutInflater.from(mContext).inflate(R.layout.item_list_view, null)
            holder.ll_item = view.findViewById(R.id.ll_item) as LinearLayout
            holder.iv_icon = view.findViewById(R.id.iv_icon) as ImageView
            holder.tv_name = view.findViewById(R.id.tv_name) as TextView
            holder.tv_desc = view.findViewById(R.id.tv_desc) as TextView
            view.tag = holder
        } else {
            holder = view