android - 关于ListView中ViewHolder模式实现优化

Question

所以众所周知的 ViewHolder 模式使用通常看起来像 (ListAdapter):

    ...

    @Override
    public View getView(final int position, View convertView, final ViewGroup parent) {

        final Album album = albums.get(position);

        ViewHolder viewHolder = null;
        if (convertView==null){
            convertView = inflater.inflate(R.layout.albums_list_item, null);

            final ImageView albumImage = (ImageView) convertView.findViewById(R.id.album_icon);

            final TextView txtTitle = (TextView) convertView.findViewById(R.id.album_title);

            final TextView txtDescription = (TextView) convertView.findViewById(R.id.album_copyright);

            viewHolder = new ViewHolder();
            viewHolder.albumImage = albumImage;
            viewHolder.txtTitle = txtTitle;
            viewHolder.txtDescription = txtDescription;
            convertView.setTag(viewHolder);
        }
        else
            viewHolder = (ViewHolder)convertView.getTag();

        viewHolder.txtTitle.setText(album.getTitle(locale));
        viewHolder.txtDescription.setText(album.getCopyrightInfo(locale));
        ...
        return convertView;
    }

而 ViewHolder 类通常是这样的:

static class ViewHolder{
    public ImageView previewImage;
    public TextView txtTitle;
    public TextView txtDescription;
}

我的问题是关于 ViewHolder 的实现。
1) 为什么不使用构造函数而不是初始化每个字段？
2)为什么它使用默认访问类型而不是 protected (实际上它必须是私有(private)的，但是由于 JIT 创建的静态访问器，这会影响性能)？好吧，我猜它只是关于继承。
那么为什么以下模式不是更好(不包括“ protected 与默认”访问类型):

protected static class ViewHolder{
    public final ImageView previewImage;
    public final TextView txtTitle;
    public final TextView txtDescription;

    public ViewHolder (final ImageView previewImage,  final TextView txtTitle, final TextView txtDescription){
        this.previewImage = previewImage;
        this.txtTitle = txtTitle;
        this.txtDescription = txtDescription;
    }
}

ListAdapter 的唯一变化是:

...
final TextView txtDescription = (TextView) convertView.findViewById(R.id.album_copyright);
viewHolder = new ViewHolder(albumImage, txtTitle, txtDescription);
convertView.setTag(viewHolder);
...

无论如何它必须调用构造函数。这只是品味问题吗？或者这个版本是否以某种方式变慢了，或者它是否以某种方式影响了性能？

Answer 1

我使用的方法与您的非常相似，但我更进一步，因为 ViewHolder 是适配器类私有(private)的，我通过将 View 传递给它的构造函数来将它与类紧密耦合并在那里设置值，例如

    private class ViewHolder
    {
      protected final ImageView image;
      protected final TextView  title;
      protected final TextView  status;

      public ViewHolder( final View root )
      {
         image = (ImageView) root.findViewById( R.id.artist_image );
         title = (TextView) root.findViewById( R.id.artist_title );
         status = (TextView) root.findViewById( R.id.artist_status );
      }
   }

并且在 getView(...)

 View row = convertView;

  if ( null == row || null == row.getTag() )
  {
     row = inflater.inflate( R.layout.adapter_artists, null );
     holder = new ViewHolder( row );
     row.setTag( holder );
  }
  else
  {
     holder = (ViewHolder) row.getTag();
  }

我喜欢这样做，因为它使我在 getView(...) 中的适配器代码更简单，并且具有 final 变量的优势。我可能会稍微提高速度使其受到保护，但我发现即使列表很大，性能也足够了。