INotifyPropertyChanged 事件处理程序始终为空

Question

我使用 .NETFramework,Version=v4.6.1

我有一个窗口，MainWindow。这是 XAML：

<Window x:Class="VexLibrary.DesktopClient.Views.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"


        xmlns:local="clr-namespace:VexLibrary.DesktopClient.Views"

        Title="MainWindow" Height="600" Width="800">
    <Grid>
        <StackPanel>
            <Grid Style="{StaticResource TitleBar}">
                <Border Style="{StaticResource TitleBarBorder}">
                    <DockPanel>
                        <StackPanel DockPanel.Dock="Left" Orientation="Horizontal">
                            <TextBlock Style="{StaticResource TitleBarIcon}" Text="&#xE10F;" />
                            <Label Style="{StaticResource TitleBarTitle}" Content="{Binding Path=CurrentPageTitle, UpdateSourceTrigger=PropertyChanged}" ></Label>
                        </StackPanel>
                        <StackPanel DockPanel.Dock="Right" Orientation="Horizontal" HorizontalAlignment="Right">
                            <Label Style="{StaticResource TitleBarTime}">12:05 AM</Label>
                            <StackPanel Orientation="Horizontal">
                                <Label Style="{StaticResource TitleBarUsername}">Hassan</Label>
                                <Button>
                                    <TextBlock Style="{StaticResource TitleBarIcon}" Text="&#xE7E8;" />
                                </Button>
                            </StackPanel>
                        </StackPanel>
                    </DockPanel>
                </Border>
            </Grid>
            <Frame Width="700" Height="507" Source="Pages/Dashboard.xaml" />
        </StackPanel>
    </Grid>
</Window>

注意： <Label Style="{StaticResource TitleBarTitle}" Content="{Binding Path=CurrentPageTitle, UpdateSourceTrigger=PropertyChanged}" ></Label>

MainWindow.xaml.cs constructor:中的DataContext设置如下

this.DataContext = new MainViewModel();

在<Frame> 中，加载了Page Dashboard.xaml。

页面Dashboard.xaml有出处：

<Page x:Class="VexLibrary.DesktopClient.Views.Pages.Dashboard"
      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:local="clr-namespace:VexLibrary.DesktopClient.Views.Pages"
      mc:Ignorable="d" 
      d:DesignHeight="460" d:DesignWidth="690"
      Title="Page1">

    <Grid Width="690" Height="460" HorizontalAlignment="Center" VerticalAlignment="Center">
        <!-- Members, Users, Books -->
        <!-- Returns, Subscriptions, Statistics -->
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="1*" />
            <ColumnDefinition Width="1*" />
            <ColumnDefinition Width="1*" />
        </Grid.ColumnDefinitions>

        <Grid.RowDefinitions>
            <RowDefinition Height="1*" />
            <RowDefinition Height="1*" />
        </Grid.RowDefinitions>

        <Button Style="{StaticResource MenuButton}" Grid.Column="0" Grid.Row="0">&#xE125;</Button>
        <Button Style="{StaticResource MenuButton}" Grid.Column="0" Grid.Row="1">&#xE845;</Button>
        <Button Style="{StaticResource MenuButton}" Grid.Column="1" Grid.Row="0">&#xE13D;</Button>
        <Button Style="{StaticResource MenuButton}" Grid.Column="1" Grid.Row="1">&#xE821;</Button>
        <Button Style="{StaticResource MenuButton}" Grid.Column="2" Grid.Row="0">&#xE8F1;</Button>
        <Button Style="{StaticResource MenuButton}" Grid.Column="2" Grid.Row="1" Command="{Binding ViewStatistics}">&#xEA37;</Button>
    </Grid>
</Page>

在Dashboard.xaml.cs constructor 中，我定义了这样的DataContext：DataContext = new DashboardViewModel();

DashboardViewModel.cs源代码是这样的（省略命名空间）

namespace VexLibrary.DesktopClient.ViewModels
{
    class DashboardViewModel : ViewModel
    {
        private MainViewModel parentViewModel;

        public DashboardViewModel()
        {
            this.parentViewModel = new MainViewModel();
        }

        public ICommand ViewStatistics
        {
            get
            {
                return new ActionCommand(p => this.parentViewModel.LoadPage("Statistics"));
            }
        }
    }
}

现在，在这段代码中，注意 Button 和 Command：

<Button Style="{StaticResource MenuButton}" Grid.Column="2" Grid.Row="1" Command="{Binding ViewStatistics}"></Button>

成功调用Command，父LoadPage方法正确执行。父视图模型如下所示：

namespace VexLibrary.DesktopClient.ViewModels
{
    public class MainViewModel : ViewModel
    {
        private string currentPageTitle;

        public string CurrentPageTitle
        {
            get
            {
                return this.currentPageTitle;
            }
            set
            {
                currentPageTitle = value;
                NotifyPropertyChanged();
            }
        }

        public void LoadPage(string pageName)
        {
            this.CurrentPageTitle = pageName;
            Console.WriteLine(CurrentPageTitle);
        }
    }
}

CurrentPageTitle 已成功更新。但是，它不会在视图中更新。

父视图模型继承ViewModel，基本上有这样的代码：

namespace VexLibrary.Windows
{
    public abstract class ViewModel : ObservableObject, IDataErrorInfo
    {
        public string this[string columnName]
        {
            get
            {
                return OnValidate(columnName);
            }
        }

        [Obsolete]
        public string Error
        {
            get
            {
                throw new NotImplementedException();
            }
        }

        protected virtual string OnValidate(string propertyName)
        {
            var context = new ValidationContext(this)
            {
                MemberName = propertyName
            };

            var results = new Collection<ValidationResult>();
            bool isValid = Validator.TryValidateObject(this, context, results, true);

            if (!isValid)
            {

                ValidationResult result = results.SingleOrDefault(p =>
                                                                  p.MemberNames.Any(memberName =>
                                                                                    memberName == propertyName));

                return result == null ? null : result.ErrorMessage;
            }

            return null;
        }
    }
}

ObservableObject.cs:

namespace VexLibrary.Windows
{
    public class ObservableObject : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        // [CallerMemberName] automatically resolves the property name for us.
        protected void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
        {
            PropertyChangedEventHandler handler = PropertyChanged;

            Console.WriteLine(handler == null);
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }
}

调试后发现，调用了NotifyPropertyChanged，但handler始终为null。我该如何解决？这不会更新 MainWindow.xaml 中的文本。我测试看看属性值是不是变了，是的，在MainViewModel.cs中改了

另外，我测试了标签本身是否可见。为此，我给了变量一个值，它正确显示，但没有更新。

Answer 1

DashboardViewModel 正在实例化 MainViewModel 的新实例，而不是使用分配给 MainWindow 的 DataContext 的实例（因此也是视图绑定到的实例）。

要使您的代码正常工作，您需要将 MainViewModel 的正确实例传递给 DashboardViewModel，因为该实例将具有属性更改事件的处理程序。

编辑：根据下面的评论，您应该按如下方式实例化您的子视图模型：

namespace VexLibrary.DesktopClient.ViewModels
{
    public class MainViewModel : ViewModel
    {
        private ViewModel _currentViewModel;

        public MainViewModel()
        {
            _currentViewModel = new DashboardViewModel(this);
        }

        public ViewModel CurrentViewModel
        {
            get { return _currentViewModel; }
            private set
            {
                _currentViewModel = value;
                OnPropertyChanged();
            }
        }
    }
}

然后您可以修改您的 Xaml，以便框架从 CurrentViewModel 属性获取它的数据上下文，如下所示：

<Window x:Class="VexLibrary.DesktopClient.Views.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:VexLibrary.DesktopClient.Views"
        Title="MainWindow" Height="600" Width="800">
    <Grid>
        <StackPanel>
            <Grid Style="{StaticResource TitleBar}">
                <Border Style="{StaticResource TitleBarBorder}">
                    <DockPanel>
                        <StackPanel DockPanel.Dock="Left" Orientation="Horizontal">
                            <TextBlock Style="{StaticResource TitleBarIcon}" Text="&#xE10F;" />
                            <Label Style="{StaticResource TitleBarTitle}" Content="{Binding Path=CurrentPageTitle, UpdateSourceTrigger=PropertyChanged}" ></Label>
                        </StackPanel>
                        <StackPanel DockPanel.Dock="Right" Orientation="Horizontal" HorizontalAlignment="Right">
                            <Label Style="{StaticResource TitleBarTime}">12:05 AM</Label>
                            <StackPanel Orientation="Horizontal">
                                <Label Style="{StaticResource TitleBarUsername}">Hassan</Label>
                                <Button>
                                    <TextBlock Style="{StaticResource TitleBarIcon}" Text="&#xE7E8;" />
                                </Button>
                            </StackPanel>
                        </StackPanel>
                    </DockPanel>
                </Border>
            </Grid>
            <Frame Width="700" Height="507" Source="Pages/Dashboard.xaml" DataContext="{Binding CurrentViewModel}"/>
        </StackPanel>
    </Grid>
</Window>

然后将需要使用某种形式的视图位置/导航来更改框架以显示正确的视图。一些 MVVM 框架（例如，CaliburnMicro）可以为您做到这一点。

同样，为了使这段代码可测试，子视图模型的实例化应该委托给注入到 MainViewModel 中的工厂类。

希望对你有帮助。