ViewHolder - 良好做法

Question

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

Answer 1

您将有多个ViewHolder对象存在。

A ListView其性质不会为其每行创建新的View实例。这是如此，如果你有一百万件事情的ListView，你不需要存储一百万件布局信息。那么你需要存储什么？只是在屏幕上的东西。然后，您可以重复使用这些视图。这样，您的ListView百万个对象可能只有10个子视图。

在您的自定义阵列适配器，你将有一个名为getView()功能，看起来是这样的：

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; 
} 

这是可行的，但需要一些时间，看看你是否可以在这里发现的低效率。考虑一下上面哪段代码会被重复调用。

问题是，我们一次又一次地打电话给row.findViewById，即使第一次查找它后，它永远不会改变。如果你的列表中只有一个简单的TextView，那么它可能并不是那么糟糕，如果你有一个复杂的布局，或者你有多个你想要设置数据的视图，你可能会失去一些时间来查找你的视图，再次。

那么我们如何解决这个问题呢？那么，在我们查看之后将TextView存储在某个地方是有意义的。所以我们引入一个名为ViewHolder的类，它可以“保留”视图。所以适配器的内部，引入一个内部类，像这样：

private static class ViewHolder { 
    TextView textView; 
} 

这个类是私有的，因为它只是一个适配器一个缓存机制，所以我们并不需要的参考它是静态的适配器来使用它。

这将存储我们的视图，以便我们不必多次拨打row.findViewById。我们应该在哪里设置它？当我们第一次夸大观点时。我们在哪里存储它？视图有一个自定义的“标签”字段，可用于存储关于视图的元信息 - 正是我们想要的！然后，如果我们已经看到了这种观点，我们只需要查找的标签，而不是查找每个行内观点..

所以getView()内部的if语句变成：

//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了！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; 
} 

我们完成了！

有些事情要考虑：

1. 如何做到这一点的变化，如果你有要更改行多个视图？作为一个挑战，让一个ListView每行有两个TextView对象和ImageView
2. 当调试的ListView，查了几件事情，让你可以真正看到发生了什么事情：
   1. 多少次ViewHolder的构造函数被调用。
   2. 的holder.textView.getText()价值是什么，你在getView()

Answer 2

正如我们每次滚动列表时填充行并为每行创建新行视图，我们都需要初始化视图持有者。就像我有两行TextView然后，

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; 
    }