PropertyMhanged在ViewModelBase中始终为null

Question

我做了一个简单的例子来更好地理解MVVM模式.以下是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

有Employee模型(带Age属性)EmployeeViewModel,它包含Employee对象并Age在以下代码中更改其属性:

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

EmployeeViewModel是从ViewModelBase具有标准INotifyPropertyCHanged代码的类继承的:

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

我正在尝试使用ICommand来改变员工的年龄:

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

属性已更改,但绑定的TextBLock不会更改其值.我查了一下,发现NotifyPropertyChanged被调用,但是PropertyChanged是null.我还确保PeopleViewModel我的应用程序中只有一个.那么,为什么是PropertyChanged就是null？

编辑:这是完整的代码ViewModelBase:

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

}

有PeopleViewModel包含ObservalbleCollection,其中EmployeeViewModels和设置DataContext.属性的值已更改,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.