为什么食谱会在看起来不必要的时候推广覆盖getItemViewType和getViewTypeCount？

Question

我一直在使用Commonsware Android编程教程和教程5,额外的功劳2,挑战是使用多个布局在ListView中显示行,具体取决于对象的"类型名称"(餐厅的"类型"属性,这是一个字符串).因此,它表明压倒一切getItemViewType,并getViewTypeCount在自定义ArrayAdapter.此外,Android文档和其他在线食谱或博客文章也提出了相同的建议.

在这种情况下,遵循此配方并覆盖这两种方法可以正常工作,但会导致基于检查该Restaurant"type"属性值的冗余逻辑.例如(请注意,此适配器是一个内部类,并且restaurants是声明为外部Activity成员的Restaurant对象的ArrayList):

class RestaurantsAdapter extends ArrayAdapter<Restaurant> {

  private static final int ROW_TYPE_DELIVERY = 0;
  private static final int ROW_TYPE_TAKE_OUT = 1;
  private static final int ROW_TYPE_SIT_DOWN = 2;

  RestaurantsAdapter() {
    super(LunchListActivity.this, android.R.layout.simple_list_item_1, restaurants);
  }

  public int getViewTypeCount() {
    return 3;
  }

  public int getItemViewType(int position) {
    String type = restaurants.get(position).getType();
    if (type == "delivery") {
      return ROW_TYPE_DELIVERY;
    } else if (type == "take_out") {
      return ROW_TYPE_TAKE_OUT;
    } else {
      return ROW_TYPE_SIT_DOWN;
    }
  }

  // Sets the icon, name and address of the Restaurant for the view.
  public View getView(int position, View convertView, ViewGroup parent) {
    View row = convertView;
    RestaurantHolder viewHolder;

    if (row == null) {
      LayoutInflater inflater = getLayoutInflater();
      switch (getItemViewType(position)) {
        case ROW_TYPE_DELIVERY:
          row = inflater.inflate(R.layout.row_delivery, null);
          break;
        case ROW_TYPE_TAKE_OUT:
          row = inflater.inflate(R.layout.row_take_out, null);
          break;
        default:
          row = inflater.inflate(R.layout.row_sit_down, null);
          break;
      }

      viewHolder = new RestaurantHolder(row);
      row.setTag(viewHolder);
    } else {
      viewHolder = (RestaurantHolder)row.getTag();
    }

    viewHolder.populateFrom(restaurants.get(position));

    return row;
  }

}

错误的是重复逻辑(if/else in getItemViewType和switchin getView).所以,我将实现改为以下内容:

class RestaurantsAdapter extends ArrayAdapter<Restaurant> {

  RestaurantsAdapter() {
    super(LunchListActivity.this, android.R.layout.simple_list_item_1, restaurants);
  }

  // Sets the icon, name and address of the Restaurant for the view.
  public View getView(int position, View convertView, ViewGroup parent) {
    View row = convertView;
    RestaurantHolder viewHolder;

    if (row == null) {
      LayoutInflater inflater = getLayoutInflater();
      if (restaurants.get(position).getType() == "delivery") {
        row = inflater.inflate(R.layout.row_delivery, null);
      } else if (restaurants.get(position).getType() == "take_out") {
        row = inflater.inflate(R.layout.row_take_out, null);
      } else {
        row = inflater.inflate(R.layout.row_sit_down, null);
      }
      viewHolder = new RestaurantHolder(row);
      row.setTag(viewHolder);
    } else {
      viewHolder = (RestaurantHolder)row.getTag();
    }

    viewHolder.populateFrom(restaurants.get(position));

    return row;
  }

}

这实现了动态加载三个xml布局之一,删除冗余逻辑,略微减少代码与布局数量的耦合,并且不需要覆盖getViewTypeCount和的目标getItemViewType.

我的问题是:如果不必要,为什么要覆盖这两种方法呢？

Answer 1

如果一个人没有必要,为什么要覆盖这两个方法呢？

添加几十个不同类型的餐馆,并观察当您滚动时,您的行回收会变得混乱,如上所示.

getItemViewType()并getViewTypeCount()确保行回收工作.Android将维护单独的对象池,并且只返回一行以回收具有正确类型的对象.

在您的解决方案中,您可以膨胀R.layout.row_delivery一行,然后在您真正需要一R.layout.row_sit_down排时将其交还给您进行回收.

顺便说一句,不要用inflate(R.layout.row_take_out, null)在AdapterView.要使RelativeLayout规则正确处理,请使用inflate(R.layout.row_take_out, parent, false).