详解xamarin Android 实现ListView万能适配器

时间:2021-04-03 人气:0

详解xamarin Android 实现ListView万能适配器

早些时候接触xamarin Android 的列表，写了很多ListView的Adapter，建一个ListView就写一个Adapter,每一个Adapter里面还有去写一个ViewHolder的类来优化，自从看了hongyang博客的listview万能适配器的文章，学习良多，所以就写篇关于xamarin android ListView通用适配器的文章。

本章主要分为以下三点：

打造通用的ViewHolder优化ListView性能
使用泛型Adapter适应不同布局的ListView
C#委托和Java匿名内部类的比较

打造通用的ViewHolder优化ListView性能

 public class ViewHolder:Java.Lang.Object
  {
    private SparseArray<View> Views;
     View ConvertView;
    private Context context ;
     int mPosition;
    private ViewHolder(Context _context,ViewGroup parent ,int itemLayoutId,int position)
    {
      this.mPosition = position;
      Views = new SparseArray<View>();
      ConvertView = LayoutInflater.From(_context).Inflate(itemLayoutId,null);
      ConvertView.Tag = this;
    }
    public static ViewHolder Get(Context context , View convertView,ViewGroup parent ,int itemLayoutId,int position)
    {
      if (convertView == null)
      {
        return new ViewHolder(context, parent, itemLayoutId, position);
      }
      else
      {
        ViewHolder holder = (ViewHolder)convertView.Tag;
        holder.mPosition = position;
        return holder;
      }
    }
    public T GetView<T>(int viewId) where T :View
    {
      View view = Views.Get(viewId);
      if (view == null)
      {
        view = ConvertView.FindViewById<T>(viewId);
        Views.Put(viewId,view);
      }
      return (T)view;
    }
    public View GetConvertView()
    {
      return ConvertView;
    }
    /// <summary>
    /// 给TextView 设置文本
    /// </summary>
    /// <param name="viewId"></param>
    /// <param name="text"></param>
    /// <returns></returns>
    public ViewHolder SetText(int viewId ,string text)
    {
      TextView view = GetView<TextView>(viewId);
      view.Text = text;
      return this;
    }

    /// <summary>
    /// 给ImageView 设置图片
    /// </summary>
    public ViewHolder SetImageBitMap(int viewId , Bitmap bm)
    {
      ImageView view = GetView<ImageView>(viewId);
      view.SetImageBitmap(bm);
      return this;
    }
  }

稍微解释一下：

ViewHolder的构造方法中初始化变量，并将ListView的布局view的Tag设置成viewholder的实例
Get方法使用单例模式初始化ViewHolder
GetView泛型方法获取ListView布局中的空间
SetText、SetImageBitMap给一些常用的控件复值

使用泛型Adapter适应不同布局的ListView

CommonAdapter是一个泛型的基类，一些重复的重写方法都在这个基类，然后我们写一个子类继承这个CommonAdapter，就简化了很多代码

 public abstract class CommonAdapter<T> :BaseAdapter
  {
    Context mContext;
     List<T> mData;
     int mItemLayoutId;
    public CommonAdapter(Context context, List<T> data, int itemLayoutId):base()
    {
      this.mContext = context;
      mData = data;
      mItemLayoutId = itemLayoutId;
    }
    public override int Count
    {
      get
      {
        return mData.Count;
      }
    }
    public override Java.Lang.Object GetItem(int position)
    {
      return null;

    }
    public override long GetItemId(int position)
    {
      return position;
    }
    public override View GetView(int position, View convertView, ViewGroup parent)
    {
      var item = mData[position];
      ViewHolder viewHolder = ViewHolder.Get(mContext, convertView, parent, mItemLayoutId, position);
      convert(viewHolder,mData[position]);

      System.Diagnostics.Debug.Write(position);
      return viewHolder.GetConvertView();
    }
    public abstract void convert(ViewHolder helper, T item);
    public ViewHolder GetViewHolder(int position, View convertView, ViewGroup parent)
    {
      return ViewHolder.Get(mContext, convertView, parent, mItemLayoutId, position);
    }
  }

写一个newsAdapter继承CommonAdapter，只需要将赋值的代码写在重写方法convert里面极客，这样就简化了很多的代码

 public class NewsAdapter<T> : CommonAdapter<T>
  {
    public NewsAdapter(Context context, List<T> data, int resId) : base(context, data, resId)
    {

    }

    public override void convert(ViewHolder helper, T item)
    {
      NewsViewModel model = (NewsViewModel)Convert.ChangeType(item, typeof(NewsViewModel));
      helper.SetText(Resource.Id.tv_news_title, model.Title);
      helper.SetText(Resource.Id.tv_news_id, model.NewsID.ToString());
      helper.SetText(Resource.Id.tv_news_desc, model.Desc);
    }
  }

C#委托和Java匿名内部类的比较

看来上面的例子，虽然简化了很多代码，但是每个ListView还是得新建一个Adapter，看来hongyang大神的博客在java中用匿名内部类实现一个Adapter万能通用，但是c#中没有匿名内部内的概念，这就尴尬了。。。。。

c#中虽然没有这种概念，但语言是相通的，但是委托可以实现。万能通用的adapter代码如下：

 public class Common1Adapter<T> : BaseAdapter
  {
    Context mContext;
    List<T> mData;
    int mItemLayoutId;
    public delegate View GetViewEvent(int position, View convertView, ViewGroup parent, T item, ViewHolder viewHolder);
    public event GetViewEvent OnGetView;
    public Common1Adapter(Context context, List<T> data, int itemLayoutId) : base()
    {
      this.mContext = context;
      mData = data;
      mItemLayoutId = itemLayoutId;
    }
    public override int Count
    {
      get
      {
        return mData.Count;
      }
    }
    public override Java.Lang.Object GetItem(int position)
    {
      return null;

    }
    public override long GetItemId(int position)
    {
      return position;
    }
    public override View GetView(int position, View convertView, ViewGroup parent)
    {
      var item = mData[position];
      ViewHolder viewHolder = ViewHolder.Get(mContext, convertView, parent, mItemLayoutId, position);
      if (OnGetView != null)
        return this.OnGetView(position,convertView,parent,item,viewHolder);
      return convertView;
    }
  }
}

在使用时，只需要在Activity中注册OnGetView 事件，这样多个ListView的Adapter就只需要一个通用的Adapter就行了

View OnGetView(int position,View convertView,ViewGroup parent,NewsViewModel model,ViewHolder viewHolder)
    {
      viewHolder.SetText(Resource.Id.tv_news_title, model.Title);
      viewHolder.SetText(Resource.Id.tv_news_id, model.NewsID.ToString());
      viewHolder.SetText(Resource.Id.tv_news_desc, model.Desc);
      return viewHolder.GetConvertView();
    }
  List<NewsViewModel> list_news = new List<Adaptes.NewsViewModel>() {
        new Adaptes.NewsViewModel () { NewsID =1,Title="测试标题124578cdascdas",Desc ="测acdsdas试内内容10c2da4s内容10c2da4s内容10c2da4s容10c2da4sc5das4cdas"},
        new Adaptes.NewsViewModel () { NewsID =2,Title="cascascda24578cdascdas",Desc ="测dasc试内容10c2da4s内容10c2da4s内容10c2da4s内容10c2da4sc5das4cdas"},
      };
      Common1Adapter<NewsViewModel> adapter = new Adaptes.Common1Adapter<Adaptes.NewsViewModel>(this, list_news, Resource.Layout.item_listview_news);
      adapter.OnGetView += OnGetView;
      lv_news.Adapter = adapter;

以上就是Android listview 万能适配器的实例，如有疑问请留言或者到本站社区交流讨论，感谢阅读，希望能帮助到大家，谢谢大家对本站的支持！

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