WPF -当SelectedItem ComboBox ItemSource更改时恢复以前的ItemSource

Question

我正在实现一个可以由用户使用按钮刷新的ComboBox。我正在努力使以前选择的项在刷新后仍然存在于ComboBox中时自动重新选择。

MainWindow.xaml：

<ComboBox Canvas.Left="10" Canvas.Top="10" DisplayMemberPath="Name" IsEnabled="{Binding Path=Enabled}" ItemsSource="{Binding Path=Items}" SelectedItem="{Binding Mode=TwoWay, Path=SelectedItem}" Width="379"/>
<Button Content="{x:Static p:Resources.TextRefresh}" Canvas.Right="10" Canvas.Top="10" Click="OnClickButtonRefresh" Width="75"/>

MainWindow.xaml.cs：

public MainWindow()
{
    InitializeComponent();
    DataContext = m_BrowserInstances = new BrowserInstancesViewModel();
}

private void OnClickButtonRefresh(Object sender, RoutedEventArgs e)
{
    m_BrowserInstances.Populate();
}

编辑为当前版本的 BrowserInstancesViewModel.cs：

public sealed class BrowserInstancesViewModel : ViewModel
{
    private Boolean m_Enabled;
    public Boolean Enabled
    {
        get { return m_Enabled; }
    }

    private BrowserInstance m_SelectedItem;
    public BrowserInstance SelectedItem
    {
        get { return m_SelectedItem; }
        set
        {
            if (m_SelectedItem != value)
            {
                m_SelectedItem = value;
                NotifyPropertyChanged("SelectedItem");
            }
        }
    }

    private ObservableCollection<BrowserInstance> m_Items;
    public ObservableCollection<BrowserInstance> Items
    {
        get { return m_Items; }
    }

    public BrowserInstancesViewModel()
    {
        Populate();
    }

    private static Func<BrowserInstance, Boolean> Recover(BrowserInstance selectedItem)
    {
        return x =>
        {
            Process currentProcess = x.Process;
            Process selectedProcess = selectedItem.Process;

            if (currentProcess.Id != selectedProcess.Id)
                return false;

            if (currentProcess.MainModule.BaseAddress != selectedProcess.MainModule.BaseAddress)
                return false;

            if (currentProcess.MainWindowTitle != selectedProcess.MainWindowTitle)
                return false;

            return true;
        };
    }

    public void Populate()
    {
        BrowserInstance item = m_SelectedItem;
        List<BrowserInstance> items = new List<BrowserInstance>();

        foreach (Process process in Process.GetProcessesByName("chrome"))
            items.Add(new BrowserInstance(process));

        if (items.Count > 0)
        {
            m_Enabled = true;

            m_Items = new ObservableCollection<BrowserInstance>(items.OrderBy(x => x.Process.Id));

            if (item != null)
                m_SelectedItem = m_Items.SingleOrDefault(Recover(item));

            if (m_SelectedItem == null)
                m_SelectedItem = m_Items[0];
        }
        else
        {
            m_Enabled = false;

            m_Items = new ObservableCollection<BrowserInstance>();
            m_Items.Add(new BrowserInstance());

            m_SelectedItem = m_Items[0];
        }

        NotifyPropertyChanged("Enabled");
        NotifyPropertyChanged("Items");
        NotifyPropertyChanged("SelectedItem");
    }
}

我可以拿回以前选择的项目，但有时。当我需要选择一个默认值(索引0)时，如果无法恢复之前选择的项，代码看起来就不能正常工作了。

Answer 1

您需要将m_SelectedItem设置为SingleOrDefault(Recover(...))找到的项。

目前，您正在将其设置为旧实例。该实例不再存在于列表中，而且显然您的BrowserInstance类没有实现任何相等的成员。

基于当前代码的正确代码：

if(selectedItem != null)
    m_SelectedItem = m_Items.SingleOrDefault(Recover(selectedItem));
if(m_SelectedItem == null)
    m_SelectedItem = m_Items[0];

更新：

你上传的代码有两个问题。

1. 如果没有进程，添加的默认BrowserInstance对象的BrowserInstance属性的值是null。这将导致NullReferenceException在SingleOrDefault使用的比较代码中出现。 通过将前面的if更改为 if(selectedItem != null && selectedItem.Process != null)
2. 在Populate方法的末尾，您将引发用于Items的PropertyChanged事件--更新组合框中的值-以及为SelectedItem --将选定的项设置为用户先前选择的项。 这里的问题是，当为SelectedItem提出PropertyChanged时，WPF将使用null更新Items，因为它在新项列表中找不到以前选择的项。这有效地覆盖了在Populate方法中计算的新选定项。 通过不将新选定的项分配给m_SelectedItem，而将其赋值给selectedItem，并在引发Items的PropertyChanged事件之后将该值分配给SelectedItem来修正该问题： 公开无效填充(){ BrowserInstance selectedItem = m_SelectedItem；List items =新List()；foreach (Process process in Process.GetProcessesByName(“chrome”) items.Add(new BrowserInstance(process))；if (items.Count > 0) { m_Enabled = true；m_Items = new ObservableCollection(items.OrderBy(x => x.Process.Id))；if (selectedItem != null && selectedItem.Process != null) selectedItem = m_Items.SingleOrDefault(x => (x.Process.Id == selectedItem.Process.Id) &x.Process.MainModule.BaseAddress == == if (selectedItem == null) selectedItem = m_Items；M_Items.Add(新BrowserInstance())；selectedItem = m_Items；}NotifyPropertyChanged(“已启用”)；NotifyPropertyChanged(“项”)；SelectedItem = selectedItem；}

如果要正确地为BrowserInstance实现相等，则可以使用保留当前选定项的WPF功能。

Populate的代码可以简化如下：

public void Populate()
{
    BrowserInstance selectedItem = m_SelectedItem;
    List<BrowserInstance> items = new List<BrowserInstance>();

    foreach (Process process in Process.GetProcessesByName("chrome"))
        items.Add(new BrowserInstance(process));

    m_Enabled = items.Any();
    m_Items = new ObservableCollection<BrowserInstance>(items.OrderBy(x => x.Process.Id));
    if(!m_Enabled)
        m_Items.Add(new BrowserInstance());

    NotifyPropertyChanged("Enabled");
    NotifyPropertyChanged("Items");
    if (SelectedItem == null)
        SelectedItem = m_Items[0];
}

BrowserInstance的平等实现如下所示：

public sealed class BrowserInstance : IEquatable<BrowserInstance>
{

    // ...

    public bool Equals(BrowserInstance other)
    {
        if (ReferenceEquals(null, other))
            return false;
        if (ReferenceEquals(this, other))
            return true;
        if (m_Process == null)
        {
            if (other.m_Process == null)
                return true;
            return false;
        }

        if (other.m_Process == null)
            return false;

        return m_Process.Id == other.m_Process.Id && m_Process.MainModule.BaseAddress == other.m_Process.MainModule.BaseAddress;
    }

    public override bool Equals(object obj)
    {
        return Equals(obj as BrowserInstance);
    }

    public override int GetHashCode()
    {
        unchecked
        {
            return m_Process != null ? ((m_Process.Id.GetHashCode() * 397) ^ m_Process.MainModule.BaseAddress.GetHashCode()) : 0;
        }
    }
}