Question

我正在实现一个ComboBox，用户可以使用按钮刷新它。我正在努力使其在刷新后仍然存在于ComboBox中时自动重新选择之前选择的项目。

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();
}

[编辑到当前版本] 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");
    }
}

我可以取回之前选择的项目，但有时只能。如果无法恢复上一个选定项目，我需要选择默认值（索引0）时，代码看起来不能正常工作。

Answer 1

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

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

根据您当前的代码更正代码：

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

更新：

您上传的代码有两个问题。

如果没有进程，则添加的默认Process对象的BrowserInstance属性的值为null。这导致NullReferenceException使用的比较代码中的SingleOrDefault 通过将前面的if更改为
来解决此问题
```
if(selectedItem != null && selectedItem.Process != null)
```

在Populate方法结束时，您为PropertyChanged举起Items事件 - 更新组合框中的值 - 以及SelectedItem - 来将所选项目设置为用户之前选择的项目这里的问题是，当SelectedItem引发null时，WPF将使用PropertyChanged更新Items，因为它在新项目列表中找不到先前选择的项目。这有效地覆盖了您在Populate方法中计算的新选定项目通过不将新选择的项目分配给m_SelectedItem但是selectedItem，并在SelectedItem引发PropertyChanged事件后将该值分配给Items来修复此问题：< / p>

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));

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

        m_Items = new ObservableCollection<BrowserInstance>(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 == selectedItem.Process.MainModule.BaseAddress));

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

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

        selectedItem = m_Items[0];
    }

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

如果要正确实现BrowserInstance的相等性，则可以使用保留当前所选项目的WPF功能。
Populate的代码可以简化为：

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：

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;
        }
    }
}

WPF - 当ComboBox ItemSsource更改时还原以前的SelectedItem

1 个答案: