Android ListView LayoutInflater

Question

我们目前正在大学里使用 ListView 。我的讲师给了我们一个简单的应用程序，它在列表中显示邮件消息，当用户选择一个时，它会在新 Activity 中显示消息的内容。我几乎了解所有发生的事情，但我想清除一些灰色区域!

基本上我想知道这部分代码的作用是什么？

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

    View v = convertView;
    if (v == null) {
        LayoutInflater vi = (LayoutInflater) getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        v = vi.inflate(R.layout.inbox_row, null);
    }

此方法位于扩展 ArrayAdapter 的类中。我认为这是某种形式的回收是对的吗？ View 何时进入和离开屏幕？....

非常感谢任何帮助。谢谢。

Answer 1

这正是你所说的，一种回收形式。

展开布局需要大量内存和大量时间，因此为了效率起见，系统将刚刚离开屏幕的内容传递给您，您可以简单地更新其文本和图像并将它们返回给 UI .

因此，例如，如果您的 ListView 在其列表中显示 6 个项目(由于它的高度)，它只会膨胀 6 个项目，并且在滚动期间它只会继续回收它们。

您应该使用一些额外的优化技巧，我相信评论者发布的视频链接会解释它们。

编辑

该示例是 Store 项目的 ArrayAdapter，但您可以根据需要制作它。适配器负责 UI 和数据之间的匹配和分离层。

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

    if (convertView == null)
    convertView = newView();

    // Store is the type of this ArrayAdapter
    Store store = getItem(position);
    Holder h = (Holder) convertView.getTag();

    // And here I get the data and address them to the UI
    // as you can see, if the convertView is not null,
    // I'm not creating a new one, I'm just changing text, images, etc
    h.storeName.setText(store.getStoreName());
    h.address.setText(store.getAddressLine1());
    h.postcode.setText(store.getPostCode());
    h.distance.setText(store.getDistance());

    return convertView;
}

// I like to separate in a different method when the convertView is null
// but that's me being organisation obsessive
// but it also makes easy to see which methods are only being called the 1st time
private View newView() {
    LayoutInflater inf = LayoutInflater.from(getContext());
    View v = inf.inflate(R.layout.map_result_list, null);
    Holder h = new Holder();
    // here we store that holder inside the view itself
    v.setTag(h);

    // and only call those findById on this first start
    h.storeName = (TextView) v.findViewById(R.id.txtLine1);
    h.address = (TextView) v.findViewById(R.id.txtLine2);
    h.postcode = (TextView) v.findViewById(R.id.txtLine3);
    h.distance = (TextView) v.findViewById(R.id.txtDistance);

    return v;
}

// this class is here just to hold reference to the UI elements
// findViewById is a lengthy operation so this is one of the optimisations
private class Holder {
    TextView storeName;
    TextView address;
    TextView postcode;
    TextView distance;
}