Question

我做了一个简单的例子来更好地理解MVVM模式。这是一个示例解决方案的链接，因为它很难解释整个问题： http://www.2shared.com/file/jOOAnacd/MVVMTestMyCopy.html

Employee模型（包含Age属性）和EmployeeViewModel，其中包含Employee个对象，并在以下代码中更改其Age属性：< / p>

public int Age
{
    get { return _employee.Age; }
    set
    {
        if (value == _employee.Age)
            return;
        _employee.Age = value;
        NotifyPropertyChanged("Age");
    }
}

EmployeeViewModel继承自ViewModelBase类，带有标准的INotifyPropertyCHanged代码：

if (PropertyChanged != null)
{
    PropertyChanged(this, new PropertyChangedEventArgs(p));
}

我正在尝试使用ICommand改变员工的年龄：

public void Increase()
{
    this.SelectedEmployee.Age++;
    NotifyPropertyChanged("Age");
}

属性已更改，但绑定的TextBLock不会更改其值。我查看并看到NotifyPropertyChanged被调用，但PropertyChanged是null。我还确保我的应用中只有一个PeopleViewModel。那么，为什么PropertyChanged是null？

编辑：以下是ViewModelBase的完整代码：

public class ViewModelBase
{

    public String DisplayName { get; set; }

    #region INotifyPropertyChanged Members

    public event PropertyChangedEventHandler PropertyChanged;

    protected void NotifyPropertyChanged(string p)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(p));
        }
    }


    #endregion

}

PeopleViewModel个ObservalbleCollectio n包含EmployeeViewModels并设置为DataContext。属性值已更改，但如果不重新加载对象，则不会显示更改。

以下是显示绑定的PeopleViewer.xaml：

<UserControl x:Class="MVVMTestMyCopy.View.PeopleViewer"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:vm="clr-namespace:MVVMTestMyCopy.ViewModel"
             mc:Ignorable="d" 
             d:DesignHeight="316" d:DesignWidth="410">
    <UserControl.Resources>
        <vm:PeopleViewModel x:Key="viewModel"/>
    </UserControl.Resources>
    <Grid DataContext="{Binding Source={StaticResource viewModel}}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="150" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <ListBox  ItemsSource="{Binding People}"
                  Grid.Column="0"
                  Margin="5,5,4,5"
                  SelectedItem="{Binding SelectedEmployee, Mode=TwoWay}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <TextBlock Margin="2"
                                   Text="{Binding FirstName}" />
                        <TextBlock Margin="2"
                                   Text="{Binding LastName}" />
                        <TextBlock Margin="0 2"
                                   Text="[" />
                        <TextBlock Margin="2"
                                   Text="{Binding Age}" />
                        <TextBlock Margin="0 2"
                                   Text="]" />
                    </StackPanel>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
        <Grid Grid.Column="1">
            <Grid.RowDefinitions>
                <RowDefinition Height="0.75*" />
                <RowDefinition Height="0.25*" />
            </Grid.RowDefinitions>
            <Grid x:Name="EmployeeDetails"
                  Grid.Row="0"
                  DataContext="{Binding SelectedEmployee}"
                  Margin="5">
                <Grid.RowDefinitions>
                    <RowDefinition Height="1*" />
                    <RowDefinition Height="1*" />
                    <RowDefinition Height="1*" />
                </Grid.RowDefinitions>
                <TextBlock Text="{Binding FirstName}"
                           VerticalAlignment="Center"
                           HorizontalAlignment="Center"
                           Grid.Column="0"/>
                <TextBlock Text="{Binding LastName}"
                           VerticalAlignment="Center"
                           HorizontalAlignment="Center"
                           Grid.Row="1" />
                <TextBlock Text="{Binding Age}"
                           VerticalAlignment="Center"
                           HorizontalAlignment="Center"
                           Grid.Row="2" />

            </Grid>
            <StackPanel Orientation="Vertical"
                        HorizontalAlignment="Center"
                        Grid.Row="1">
                <Button x:Name="button"
                        Content="-"
                        Width="32"
                        Height="32"
                        Command="{Binding DecreaseCommand}">
                </Button>
                <Button x:Name="button1"
                        Content="+"
                        Width="32"
                        Height="32"
                        Command="{Binding IncreaseCommand}">
                </Button>
            </StackPanel>
        </Grid>
    </Grid>
</UserControl>

Answer 1

在您的项目中，您实际上并未在视图模型上实现INotifyPropertyChanged。你有：

public class ViewModelBase

但这应该是：

public class ViewModelBase : INotifyPropertyChanged

因为您没有实现INotifyPropertyChange，所以WPF绑定系统将无法为您的PropertyChanged事件添加处理程序。

Answer 2

在ViewModelBase上实现INotifyPropertyChanged接口。 http://msdn.microsoft.com/en-us/library/system.componentmodel.inotifypropertychanged.aspx 您已定义了PropertyChanged事件，但它是重要的接口。

Answer 3

我使用MVVM-Light代替您的BaseViewModel实现检查了应用程序，并且它按预期工作。

我建议使用MVVM-Light，因为Messaging，Disposing和Blendability等其他功能。

您可以使用NuGet轻松下载并安装它。

如果你想要实现INotifyPropertyChange，下面是代码：

public class MainViewModel: INotifyPropertyChanged
{        
    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void RaisePropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = this.PropertyChanged;
        if (handler != null)
        {
            var e = new PropertyChangedEventArgs(propertyName);
            handler(this, e);
        }
    } 
}

PropertyMhanged在ViewModelBase中始终为null

3 个答案: