.NET Framework WPF ListView懒加载首次点击异常排查

问题描述 #

开发基于.NET Framework的WPF应用时，为ListView实现懒加载：点击按钮从数据库分段获取数据并添加至ListView，直到垂直滚动条出现。但首次点击按钮时会加载数据库全部数据，后续点击则正常（仅加载至滚动条出现）。

现有代码 #

ListView的ItemsSource绑定至PeopleList（BindingList<Person>类型）：

private void DisplayAllPeople()
{
    _peopleTablePageNumber = 0;
    PeopleList = new BindingList<Person>();

    bool isScrollBarVisible = false;
    bool isEndOfTable = false;
    while (!isScrollBarVisible && !isEndOfTable)
    {
        IEnumerable<Person> peoplePage = _dataCollector.GetPersons()
            .Skip(_peopleTablePageNumber * _peoplePerPageCount)
            .Take(_peoplePerPageCount);

        PeopleList.AddRange(peoplePage);

        _peopleTablePageNumber++;

        ScrollViewer sv = FindVisualChild<ScrollViewer>(listViewPeople);
        Visibility VerticalScrollbarVisibility = sv.ComputedVerticalScrollBarVisibility;
        isScrollBarVisible = (VerticalScrollbarVisibility == Visibility.Visible);

        isEndOfTable = (peoplePage.Count() < _peoplePerPageCount);
    }
}

private childItem FindVisualChild<childItem>(DependencyObject obj) 
    where childItem : DependencyObject
{
    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
    {
        DependencyObject child = VisualTreeHelper.GetChild(obj, i);
        if (child != null && child is childItem)
            return (childItem)child;
        else
        {
            childItem childOfChild = FindVisualChild<childItem>(child);
            if (childOfChild != null)
                return childOfChild;
        }
    }
    return null;
}

DataCollector类的GetPersons方法：

public List<Person> GetPersons()
{
    return _dBContext.Persons.ToList();
}

原因分析 #

1. UI布局未及时更新
   首次执行循环时，向PeopleList添加数据后，WPF的UI渲染是异步的——ListView还没完成布局更新，此时获取的ComputedVerticalScrollBarVisibility始终为Collapsed，循环会持续执行直到加载完所有数据，直到数据量足够触发滚动条或数据库数据耗尽。

2. 未实现真正的数据库分页
   GetPersons()方法直接调用ToList()会一次性将所有数据库数据加载到内存，后续的Skip/Take只是在内存集合中做过滤，既浪费内存，也没实现真正的分段查询，违背了懒加载的初衷。

解决方案 #

1. 强制UI实时更新布局 #

在每次添加数据后调用UpdateLayout()，让ListView立即完成布局更新，确保滚动条状态判断准确：

private void DisplayAllPeople()
{
    _peopleTablePageNumber = 0;
    PeopleList = new BindingList<Person>();

    bool isScrollBarVisible = false;
    bool isEndOfTable = false;
    while (!isScrollBarVisible && !isEndOfTable)
    {
        // 改用分页查询方法（见下文）
        IEnumerable<Person> peoplePage = _dataCollector.GetPersonsPage(_peopleTablePageNumber, _peoplePerPageCount);

        PeopleList.AddRange(peoplePage);

        // 强制ListView更新布局，确保滚动条状态可被正确检测
        listViewPeople.UpdateLayout();

        _peopleTablePageNumber++;

        ScrollViewer sv = FindVisualChild<ScrollViewer>(listViewPeople);
        isScrollBarVisible = sv.ComputedVerticalScrollBarVisibility == Visibility.Visible;

        isEndOfTable = peoplePage.Count() < _peoplePerPageCount;
    }
}

2. 实现数据库层面的分页查询 #

修改DataCollector，直接在数据库执行Skip/Take，避免一次性加载全部数据：

// 替换原GetPersons方法，新增分页查询方法
public IEnumerable<Person> GetPersonsPage(int pageNumber, int pageSize)
{
    return _dBContext.Persons
        .Skip(pageNumber * pageSize)
        .Take(pageSize)
        .ToList();
}

额外优化：异步加载避免UI阻塞 #

将数据查询改为异步执行，防止长时间阻塞UI线程：

private async void DisplayAllPeople()
{
    _peopleTablePageNumber = 0;
    PeopleList = new BindingList<Person>();

    bool isScrollBarVisible = false;
    bool isEndOfTable = false;
    while (!isScrollBarVisible && !isEndOfTable)
    {
        // 异步从数据库获取分页数据
        var peoplePage = await Task.Run(() => 
            _dataCollector.GetPersonsPage(_peopleTablePageNumber, _peoplePerPageCount));

        // 回到UI线程更新数据和布局
        Application.Current.Dispatcher.Invoke(() => 
        {
            PeopleList.AddRange(peoplePage);
            listViewPeople.UpdateLayout();
        });

        _peopleTablePageNumber++;

        // 在UI线程检测滚动条状态
        bool scrollBarVisible = false;
        Application.Current.Dispatcher.Invoke(() => 
        {
            var sv = FindVisualChild<ScrollViewer>(listViewPeople);
            scrollBarVisible = sv.ComputedVerticalScrollBarVisibility == Visibility.Visible;
        });
        isScrollBarVisible = scrollBarVisible;

        isEndOfTable = peoplePage.Count() < _peoplePerPageCount;
    }
}

内容的提问来源于stack exchange，提问作者MR.Iraji