如何在 MVVM-WPF 中获取所选项目

Posted 2023-02-22

【中文标题】如何在 MVVM-WPF 中获取所选项目

【英文标题】：How to get selected item in MVVM-WPF

【发布时间】：2013-08-29 23:49:29

【问题描述】：

您好，我正在使用 WPF 和 MVVM，我的应用程序中有“编辑/更新”面板，它会更新 Grid 中的选定数据。它工作正常。我想添加一个按钮“关闭按钮”和“一个检查用户是否选择了项目。如果他没有选择任何项目并单击“编辑按钮”，它将向用户显示一个消息框以选择一个项目编辑。我​​不清楚的是如何通过“选定的项目来做这两件事”并在关闭面板之前检查文本框字段中是否有任何文本。 用户查看模型代码：

public class UserViewModel

    private IList<User> _UsersList;
    public UserViewModel()
    
        _UsersList = new List<User>
    
        new User  UserId = 1, FirstName = "Raj", LastName = "Beniwal", City = "Delhi", State = "DEL", Country = "INDIA" ,
        new User  UserId = 2, FirstName = "Mark", LastName = "henry", City = "New York", State = "NY", Country = "USA" ,
        new User  UserId = 3, FirstName = "Mahesh", LastName = "Chand", City = "Philadelphia", State = "PHL", Country = "USA" ,
        new User  UserId = 4, FirstName = "Vikash", LastName = "Nanda", City = "Noida", State = "UP", Country = "INDIA" ,
        new User  UserId = 5, FirstName = "Harsh", LastName = "Kumar", City = "Ghaziabad", State = "UP", Country = "INDIA" ,
        new User  UserId = 6, FirstName = "Reetesh", LastName = "Tomar", City = "Mumbai", State = "MP", Country = "INDIA" ,
        new User  UserId = 7, FirstName = "Deven", LastName = "Verma", City = "Palwal", State = "HP", Country = "INDIA" ,
        new User  UserId = 8, FirstName = "Ravi", LastName = "Taneja", City = "Delhi", State = "DEL", Country = "INDIA"            
    ;


public IList<User> Users

    get  return _UsersList; 
    set  _UsersList = value;            


private ICommand mUpdater;

public ICommand UpdateCommand

    get
    
        if (mUpdater == null)
        
            mUpdater = new Updater();
        
        return mUpdater;
    
    set
    
        mUpdater = value;
    


private class Updater : ICommand

    #region ICommand Members

    public bool CanExecute(object parameter)
                  
        return true;
    

    public event EventHandler CanExecuteChanged;

    public void Execute(object parameter)
    
    

    #endregion
    

用户视图窗口.Xaml 面板 1

<dxdo:LayoutPanel Caption="Panel1" x:Name="Panel1">
    <Grid Margin="0,0,0,20">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        <ListView Name="UserGrid" Grid.Row="1" Margin="4,178,12,13" ItemsSource="Binding Users">
            <ListView.View>
                <GridView x:Name="grdTest">
                    <GridViewColumn Header="UserId" DisplayMemberBinding="Binding UserId" Width="50" />
                    <GridViewColumn Header="First Name" DisplayMemberBinding="Binding FirstName" Width="80" />
                    <GridViewColumn Header="Last Name" DisplayMemberBinding="Binding LastName" Width="100" />
                    <GridViewColumn Header="City" DisplayMemberBinding="Binding City" Width="80" />
                    <GridViewColumn Header="State" DisplayMemberBinding="Binding State" Width="80" />
                    <GridViewColumn Header="Country" DisplayMemberBinding="Binding Country" Width="100" />
                </GridView>
             </ListView.View>
          </ListView>
       </Grid>
</dxdo:LayoutPanel>
<dxdo:LayoutPanel x:Name="Panel3">
    <Grid>
        <StackPanel>
            <Button Content="Edit" Height="23" HorizontalAlignment="Left" VerticalAlignment="Top" Width="141" Click="Button_Click_1" />
        </StackPanel>
    </Grid>
</dxdo:LayoutPanel>

面板 2：

<dxdo:LayoutPanel Caption="Panel2" x:Name="Panel2">
    <Grid>
        <StackPanel Margin="0,0,0,10">
            <Grid Margin="0,0,0,20">
                <Grid.RowDefinitions>
                    <RowDefinition Height="Auto" />
                    <RowDefinition Height="*" />
                    <RowDefinition Height="Auto" />
                </Grid.RowDefinitions>
                <TextBox Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="80,7,0,0" Name="txtUserId" VerticalAlignment="Top" Width="178" Text="Binding ElementName=UserGrid, Path=SelectedItem.UserId" />
                <TextBox Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="80,35,0,0" Name="txtFirstName" VerticalAlignment="Top" Width="178" Text="Binding ElementName=UserGrid, Path=SelectedItem.FirstName" />
                <TextBox Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="80,62,0,0" Name="txtLastName" VerticalAlignment="Top" Width="178" Text="Binding ElementName=UserGrid, Path=SelectedItem.LastName" />
                <Label Content="UserId" Grid.Row="1" HorizontalAlignment="Left" Margin="12,12,0,274" Name="label1" />
                <Label Content="Last Name" Grid.Row="1" Height="28" HorizontalAlignment="Left" Margin="12,60,0,0" Name="label2" VerticalAlignment="Top" />
                <Label Content="First Name" Grid.Row="1" Height="28" HorizontalAlignment="Left" Margin="12,35,0,0" Name="label3" VerticalAlignment="Top" />
                <TextBox Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="80,143,0,0" x:Name="txtCity" VerticalAlignment="Top" Width="178" Text="Binding SelectedItem.City, ElementName=UserGrid" />
                <Label Content="Country" Grid.Row="1" Height="28" HorizontalAlignment="Left" Margin="12,141,0,0" x:Name="label2_Copy" VerticalAlignment="Top" />
                <TextBox Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="80,88,0,0" x:Name="txtCountry" VerticalAlignment="Top" Width="178" Text="Binding SelectedItem.Country, ElementName=UserGrid" />
                <Label Content="City" Grid.Row="1" Height="28" HorizontalAlignment="Left" Margin="12,86,0,0" x:Name="label2_Copy1" VerticalAlignment="Top" />
                <TextBox Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="80,115,0,0" x:Name="txtSTate" VerticalAlignment="Top" Width="178" Text="Binding SelectedItem.State, ElementName=UserGrid" />
                <Label Content="State" Grid.Row="1" Height="28" HorizontalAlignment="Left" Margin="12,113,0,0" x:Name="label2_Copy2" VerticalAlignment="Top" />
            </Grid>
            <Button Content="Update" Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="310,40,0,0" Name="btnUpdate" VerticalAlignment="Top" Width="141" Command="Binding Path=UpdateCommad" />
            <Button Content="Close" Grid.Row="1" Height="23" HorizontalAlignment="Right" VerticalAlignment="Top" Width="141" Click="Button_Click_2" />
            <TextBox Width="166" Background="White" Height="33"  HorizontalAlignment="Right" VerticalAlignment="Bottom" Text="Binding Path=SelectedCustomer.LastName,Mode=OneWay,UpdateSourceTrigger=PropertyChanged" />    
        </StackPanel>
    </Grid>
</dxdo:LayoutPanel>    

（如果我的问题你不清楚，请问我。谢谢）

【问题讨论】：

在用户选择要执行此操作的项目之前，您的编辑按钮不应该是 aktiv，您可以使用 RelayCommands

Button_Click_1, Button_Click_2 - 这不是 MVVM。

也看看this question

【参考方案1】：

在 ViewModel 中创建一个属性来保存选定的用户：

public User SelectedUser  get; set; 

将 ListView 的 SelectedItem 绑定到该属性：

<ListView Name="UserGrid" ItemsSource="Binding Users" SelectedItem="Binding SelectedUser">

现在您只需检查SelectedUser 属性是否为空。

【讨论】：

如何判断是否为空？在哪里定义这个？

If(SelectedUser == null) ShowNoUserSelectedWarning();

编译器说 SelectedUser 它是属性但使用类似变量。

【参考方案2】：

如何通过“选中的项目来做这两件事”

这通常可以通过视图模型中的SelectedSomething 属性来实现。此属性应绑定到控件的SelectedItem。

并在关闭面板之前检查文本框字段中是否有任何文本

这称为“验证”。 WPF 以多种方式支持验证，其中之一是在您的视图模型中实现IDataErrorInfo。我推荐这种方式，因为它是 .NET 的事实上的标准（它也用于 WinForms 和 ASP .NET）。

检查用户是否选择了项目。如果他没有选择任何项目并单击“编辑按钮”，它将向用户显示一个消息框以选择要编辑的项目

同样，这通常是通过绑定到按钮的ICommand 实例来解决的。如果ICommand.CanExecute 返回 false，绑定按钮将被禁用。

例如，您应该检查CanExecute for command，绑定到Close 按钮的验证错误，如果有任何错误，则返回false。或者您应该检查CanExecute 中的SelectedSomething 属性以获取命令，绑定到Edit 按钮，如果SelectedSomething == null 则返回false。

在 MVVM 世界中，如果您使用 RelayCommand/DelegateCommand 作为默认的 ICommand 实现，事情会变得更容易。

更新。

命令的代码示例。 查看模型：

public class UserViewModel

    public UserViewModel()
    
        EditCommand = new RelayCommand(EditSelectedUser, () => SelectedUser != null);
    

    private void EditSelectedUser()
    
        // some edit code here
    

    public User SelectedUser  get; set; 
    public ICommand EditCommand  get; private set; 

XAML：

<Button Content="Edit" Command="Binding EditCommand"/>

【讨论】：

如果您不想在视图模型中保留 SelectedItem 属性，您也可以将所选项目作为参数传递给命令。您也可以使用相同的命令发出警告，如果未选择任何内容，即传递的参数为空，并且如果参数不为空，则进一步处理。

@Dennis 感谢您的回复。如何实现这个Again, usually, this being solved with ICommand instance, bound to the button. If ICommand.CanExecute returns false, bound button becomes disabled你能通过代码描述一下吗？