WPF中Button绑定的Command首次点击无法触发如何解决？

问题场景 #

我创建了一个为Button和ComboBox做了Command绑定的简单UI，希望在ComboBox的LostFocus事件触发时，根据下拉框选中值填充Button列表。

相关代码 #

MainWindow.xaml.cs

public partial class MainWindow : Window
{
    public MainWindow()
    {
        ViewModel ViewModel = new ViewModel();
        this.DataContext = ViewModel;
        //Test Data Feeded
        ViewModel.SelectedServer = ViewModel.ServerList.First();
        ViewModel.MyList = new ObservableCollection<Model>();
        for (int i = 0; i < Convert.ToInt16(ViewModel.SelectedServer); i++)
        {
            ViewModel.MyList.Add(new Model() { Name = (i + 1).ToString() });
        }
        InitializeComponent();
        txtServer.Focus();
    }
}

ViewModel.cs

public class ViewModel: BaseDataMdel
{
    ObservableCollection<Model> _MyList;
    List<string> serverList;
    string selectedServer;
    void Validated()
    {
        //Bsaed upon some validation we repopulate the collection
        if (MyList == null) MyList = new ObservableCollection<Model>(); else MyList.Clear();
        for (int i = 0; i < Convert.ToInt16(SelectedServer); i++)
        {
            MyList.Add(new Model() { Name = (i + 1).ToString() });
        }
    }
    private void ActionRequestedEvent(object param)
    {
        switch(param.ToString())
        {
            case "DataSetServerLostFocus":
                {
                    Validated();
                }
                break;
            default:
                MessageBox.Show(param.ToString()); break;
        }
    }
    private ICommand _ActionCommand1;
    public ICommand ActionCommand1
    {
        get
        {
            if (_ActionCommand1 == null)
                this._ActionCommand1 = new RelayCommand(param =>
                {
                    ActionRequestedEvent(param);
                });
            return _ActionCommand1;
        }
        set
        {
            _ActionCommand = value;
        }
    }
    private ICommand _ActionCommand;
    public ICommand ActionCommand
    {
        get
        {
            if (_ActionCommand == null)
                this._ActionCommand = new RelayCommand(param =>
                {
                    ActionRequestedEvent(param);
                });
            return _ActionCommand;
        }
        set
        {
            _ActionCommand = value;
        }
    }

    public ObservableCollection<Model> MyList
    {
        get
        {
            return _MyList;
        }

        set
        {
            _MyList = value;
            RaisePropertyChanged(nameof(MyList));
        }
    }

    public List<string> ServerList
    {
        get
        {
            return serverList;
        }

        set
        {
            serverList = value;
            RaisePropertyChanged(nameof(ServerList));
        }
    }

    public string SelectedServer
    {
        get
        {
            return selectedServer;
        }

        set
        {
            selectedServer = value;
            RaisePropertyChanged(nameof(SelectedServer));
        }
    }

    public ViewModel()
    {
        ServerList = new List<string>() { "1", "2", "3", "4", "5" };
    }
}

XAML代码

<Window x:Class="WpfApplication26.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
        xmlns:local="clr-namespace:WpfApplication26"
        mc:Ignorable="d" Name="Group"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel Orientation="Vertical" VerticalAlignment="Center" HorizontalAlignment="Center">
        <ComboBox Width="150" Name="txtServer" ItemsSource="{Binding ServerList}" SelectedItem="{Binding SelectedServer}" Height="25">
            <i:Interaction.Triggers>
                <i:EventTrigger EventName="LostFocus">
                    <i:InvokeCommandAction Command="{Binding ActionCommand}" CommandParameter="DataSetServerLostFocus"/>
                </i:EventTrigger>
            </i:Interaction.Triggers>
        </ComboBox>
        <GroupBox Header="Test Case" Width="175" Height="200">
            <ItemsControl ItemsSource="{Binding MyList}">
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Orientation="Horizontal" Margin="2">
                            <TextBox Width="120" Text="{Binding Name}"/>
                            <Button Content=".." Height="25" Width="25" Margin="3 0 0 0"  
                                    Command="{Binding DataContext.ActionCommand,ElementName=Group}"
                                    CommandParameter="{Binding Name}"/>
                        </StackPanel>
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>
        </GroupBox>
    </StackPanel>
</Window>

复现步骤 #

• 运行代码
• 点击列表内的Button
• 首次点击时Command不会被触发

已知原因 #

该问题是由集合重新遍历渲染导致：点击按钮时会先触发ComboBox的LostFocus事件，此时集合被清空重建，原有按钮元素被销毁，对应的点击命令无法完成触发流程。

已尝试的解决方法 #

• 在集合重新遍历前将点击的Button存入临时变量，待集合更新完成后编程触发按钮点击事件，该方法仅能部分解决问题，因为Window.GetWindow(button)对备份的按钮会返回null。

可行解决方案 #

方案1：低优先级调度集合刷新（最简便） #

点击按钮的输入事件优先级高于界面渲染调度，可将集合刷新逻辑用Dispatcher低优先级延迟执行，确保按钮点击命令先处理完成：
修改ViewModel中ActionRequestedEvent的DataSetServerLostFocus分支：

case "DataSetServerLostFocus":
{
    // 使用Input优先级，保证输入类事件先执行完成
    Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.Input, new Action(() =>
    {
        Validated();
    }));
}
break;

方案2：修改触发事件为SelectionChanged #

如果业务逻辑允许，将ComboBox的触发事件从LostFocus改为SelectionChanged，仅在选中值真实变化时才刷新集合，避免无意义的集合重建，也能规避该问题。
修改XAML中ComboBox的触发器配置：

<i:EventTrigger EventName="SelectionChanged">
    <i:InvokeCommandAction Command="{Binding ActionCommand}" CommandParameter="DataSetServerLostFocus"/>
</i:EventTrigger>

方案3：优化集合更新逻辑 #

不要每次刷新都全量清空重建集合，改为对比现有集合和目标集合的差异，仅做元素的新增、删除操作，保留已有列表元素，原有按钮不会被销毁，点击事件自然可以正常触发。

内容的提问来源于stack exchange，提问作者Bala