Question

我跟随一些在线教程来在ListView的动态视图之间进行切换。

在我的MainWindow中，左窗格中有一个ListView，其中包含子ViewModel列表的ItemsSource。（每个子ViewModel都实现一个接口）
每个ViewModel都有自己的View作为DataTemplate。
我正在从MainWindow调用所选视图的GenerateReport（）方法。但是所选视图的值将为空。

从Github下载我的源代码。

重现此问题的步骤：

运行该应用程序，然后在“学生报告”的ID和名称中键入文本。（StudentReportViewModels属性中的断点已正确命中，并且值已更新。）
然后单击“生成报告”按钮。 StudentReportViewModels的属性值将为null。

如何解决此问题？请帮忙。

来源： MainWindow.xaml

<Window.Resources>
    <DataTemplate DataType="{x:Type vm:StudentsReportViewModel}">
        <view:StudentsReport/>
    </DataTemplate>
    <DataTemplate DataType="{x:Type vm:MarksReportViewModel}">
        <view:MarksReport />
    </DataTemplate>
</Window.Resources>
<Window.DataContext>
    <local:MainWindowViewModel/>
</Window.DataContext>
<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="200"/>
        <ColumnDefinition Width="Auto"/>
        <ColumnDefinition Width="*"/>
    </Grid.ColumnDefinitions>

    <ListView ItemsSource="{Binding Reports}" SelectedItem="{Binding SelectedReport, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
        <ListView.ItemTemplate>
            <DataTemplate>
                <TextBlock Text="{Binding Name}"/>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>

    <GridSplitter Grid.Row="1" Grid.Column="1" Width="5" ResizeBehavior="PreviousAndNext" HorizontalAlignment="Stretch"/>
    <Grid Grid.Column="2">
        <Grid.RowDefinitions>
            <RowDefinition Height="2*"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <ScrollViewer Grid.Row="0" VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Auto">
            <ContentControl Content="{Binding SelectedReport.ViewModel, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
        </ScrollViewer>
        <Button Content="Generate Report" Grid.Row="2" Margin="5" HorizontalAlignment="Right" Command="{Binding GenerateReportCommand}"/>
    </Grid>
</Grid>

MainWindowViewModel：

public class MainWindowViewModel : INotifyPropertyChanged
{
    private ICommand _generateReportCommand;

    private Report selectedReport;
    public Report SelectedReport
    {
        get
        {
            return this.selectedReport;
        }

        set
        {
            if (value != this.selectedReport)
            {
                this.selectedReport = value;
                NotifyPropertyChanged();
            }
        }
    }

    public List<Report> Reports { get; set; }

    public ICommand GenerateReportCommand
    {
        get
        {
            if (_generateReportCommand == null)
            {
                _generateReportCommand = new RelayCommand(
                    p => GenerateReport()
                    );
            }

            return _generateReportCommand;
        }
    }
    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
    public MainWindowViewModel()
    {
        Reports = new List<Report>
        {
            new Report{ Name = "Students Report", ViewModel = new StudentsReportViewModel()},
            new Report{ Name = "Marks Report", ViewModel = new MarksReportViewModel()}
        };

        SelectedReport = Reports[0];
    }

    public void GenerateReport()
    {
        SelectedReport.ViewModel.GenerateReport();
    }
}

StudentsReport.xaml

<TextBox Height="25" Width="100" Margin="5" Text="{Binding Id, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
        <TextBox Height="25" Width="100" Margin="5" Text="{Binding Name, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>

StudentsReportViewModel：

public class StudentsReportViewModel : INotifyPropertyChanged, IReportViewModel
{
    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    private string id;
    public string Id
    {
        get
        {
            return this.id;
        }

        set
        {
            if (value != this.id)
            {
                this.id = value;
                NotifyPropertyChanged();
            }
        }
    }

    private string name;
    public string Name
    {
        get
        {
            return this.name;
        }

        set
        {
            if (value != this.name)
            {
                this.name = value;
                NotifyPropertyChanged();
            }
        }
    }

    public StudentsReportViewModel()
    {

    }

    public void GenerateReport()
    {
        System.Windows.MessageBox.Show($"Id = {Id}, Name = {Name}");
    }
}

接口：

public interface IReportViewModel
{
    void GenerateReport();
}

型号：

public class Report
{
    public string Name { get; set; }

    public IReportViewModel ViewModel { get; set; }
}

Answer 1

您的StudentsReport.xaml UserControl绑定到在XAML中创建的StudentsReportViewModel实例：

<UserControl.DataContext>
    <vm:StudentsReportViewModel/>
</UserControl.DataContext>

但是，“生成报告”按钮正在调用在MainWindowVieModel构造函数中创建并存储在Report类中的StudentReportViewModel的另一个实例。

Reports = new List<Report>
{
    new Report{ Name = "Students Report", ViewModel = new StudentsReportViewModel()},
    new Report{ Name = "Marks Report", ViewModel = new MarksReportViewModel()}
};

您需要删除这些实例之一，以便UserControl的DataContext绑定到您要从中生成报告消息的同一视图模型实例。我建议从StudentsReport.xaml删除此代码：

<UserControl.DataContext>
    <vm:StudentsReportViewModel/>
</UserControl.DataContext>

动态ViewModel的绑定值在WPF中为空

1 个答案: