Question

一个小小的新手问题。我们为什么要初始化ViewHolder中的getView()？为什么我们不能在构造函数中初始化它？

Answer 1

您将拥有多个ViewHolder个对象。

ListView本质上不会为其每一行创建新的View个实例。这样，如果您拥有一百万个ListView个内容，则无需存储一百万个内容的布局信息。那么你需要存储什么？只是屏幕上的东西。然后，您可以反复重复使用这些视图。这样，您的ListView个百万个对象可能只有10个子视图。

在自定义数组适配器中，您将拥有一个名为getView()的函数，如下所示：

public View getView(int position, View convertView, ViewGroup parent) {
    //Here, position is the index in the list, the convertView is the view to be
    //recycled (or created), and parent is the ListView itself.

    //Grab the convertView as our row of the ListView
    View row = convertView;

    //If the row is null, it means that we aren't recycling anything - so we have
    //to inflate the layout ourselves.
    if(row == null) {
          LayoutInflater inflater = (LayoutInflater)getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
          row = inflater.inflate(R.layout.list_item, parent, false);
    }

    //Now either row is the recycled view, or one that we've inflated. All that's left
    //to do is set the data of the row. In this case, assume that the row is just a
    //simple TextView
    TextView textView = (TextView) row.findViewById(R.id.listItemTextView);

    //Grab the item to be rendered. In this case, I'm just using a string, but
    //you will use your underlying object type.
    final String item = getItem(position);

    textView.setText(item);

    //and return the row
    return row;
}

这会有效，但请花点时间看看你是否能发现这里的低效率。想想上面哪些代码将被冗余调用。

问题在于我们一遍又一遍地呼叫row.findViewById，即使在我们第一次查看它之后，它也永远不会改变。如果你的列表中只有一个简单的TextView，那可能不是那么糟糕，如果你有一个复杂的布局，或者你有多个想要设置数据的视图，你可能会失去一点时间查找你的观点一遍又一遍。

那么我们该如何解决这个问题呢？好吧，在我们查找之后将TextView存储在某个地方是有意义的。因此，我们引入了一个名为ViewHolder的类，它“保存”视图。所以在适配器内部引入一个内部类：

private static class ViewHolder {
    TextView textView;
}

这个类是私有的，因为它只是适配器的缓存机制，它是静态的，因此我们不需要引用适配器来使用它。

这将存储我们的视图，这样我们就不必多次调用row.findViewById。我们应该在哪里设置它？当我们第一次膨胀视图时。我们在哪里存放？视图有一个自定义的“标记”字段，可用于存储有关视图的元信息 - 正是我们想要的！然后，如果我们已经看过这个视图，我们只需要查找标记而不是查找行中的每个视图。

因此getView()内的if语句变为：

//If the row is null, it means that we aren't recycling anything - so we have
//to inflate the layout ourselves.
ViewHolder holder = null;
if(row == null) {
    LayoutInflater inflater = (LayoutInflater)getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    row = inflater.inflate(R.layout.list_item, parent, false);
    //Now create the ViewHolder
    holder = new ViewHolder();
    //and set its textView field to the proper value
    holder.textView =  (TextView) row.findViewById(R.id.listItemTextView);
    //and store it as the 'tag' of our view
    row.setTag(holder);
} else {
    //We've already seen this one before!
    holder = (ViewHolder) row.getTag();
}

现在，我们只需更新holder.textView的文本值，因为它已经是对回收视图的引用！所以我们的最终适配器代码变为：

public View getView(int position, View convertView, ViewGroup parent) {
    //Here, position is the index in the list, the convertView is the view to be
    //recycled (or created), and parent is the ListView itself.

    //Grab the convertView as our row of the ListView
    View row = convertView;

    //If the row is null, it means that we aren't recycling anything - so we have
    //to inflate the layout ourselves.
    ViewHolder holder = null;
    if(row == null) {
        LayoutInflater inflater = (LayoutInflater)getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        row = inflater.inflate(R.layout.list_item, parent, false);
        //Now create the ViewHolder
        holder = new ViewHolder();
        //and set its textView field to the proper value
        holder.textView =  (TextView) row.findViewById(R.id.listItemTextView);
        //and store it as the 'tag' of our view
        row.setTag(holder);
    } else {
        //We've already seen this one before!
        holder = (ViewHolder) row.getTag();
    }

    //Grab the item to be rendered. In this case, I'm just using a string, but
    //you will use your underlying object type.
    final String item = getItem(position);

    //And update the ViewHolder for this View's text to the correct text.
    holder.textView.setText(item);

    //and return the row
    return row;
}

我们已经完成了！

要考虑的一些事情：

如果您想要更改一行中有多个视图，这会如何变化？作为挑战，创建一个ListView，其中每行有两个TextView个对象和一个ImageView
在调试ListView时，请检查一些内容，以便您可以真正看到正在发生的事情：
1. 调用ViewHolder的构造函数的次数。
2. 在holder.textView.getText()

Answer 2

当我们每次填充行时滚动列表并为每一行创建新的行视图时，我们需要初始化视图持有者。就像我在行中有两个TextView一样，

  static class ViewHolder {
        protected TextView title;
        protected TextView type;

    }


     public View getView(int position, View convertView, ViewGroup parent) {
            View view = null;
            if (convertView == null) {
                LayoutInflater inflator = context.getLayoutInflater();
                view = inflator.inflate(R.layout.feeds_rowview, null);
                final ViewHolder viewHolder = new ViewHolder();
                view.setTag(viewHolder);
                viewHolder.title = (TextView) view.findViewById(R.id.Title);
                viewHolder.type = (TextView) view.findViewById(R.id.Type);

            } else {
                view = convertView;
            }

            ViewHolder holder = (ViewHolder) view.getTag();
            holder.title.setText(list.get(position).getTitle());
            holder.type.setText(list.get(position).getType());

            return view;
     }

ViewHolder - 良好的做法

2 个答案:

ViewHolder - 良好的做​​法

2 个答案:

ViewHolder - 良好的做法