设置 DataContext 后 PropertyChanged 事件为 null

我将视图构造函数中视图的 DataContext 设置为 ViewModel 的实例，只是标准的东西。此后不久，一个UPDATE_RECENT_DOCUMENTS_LIST事件从事件聚合器触发，我的 ViewModel 正确捕获了该事件。更改属性并调用 onPropertyChanged 方法，但由于 PropertyChanged 事件为空而失败。

我接下来要做的就是对 UI 执行一个操作，该操作会引发创建项目事件和相同的 ViewModel 正在接收事件，但现在 PropertyChanged 事件不再为 null 并且一切都按预期工作。

设置 DataContext 之后，在注册到 PropertyChanged 事件之前是否必须经过特定的时间？是否有一个我可以等待的事件来确保 PropertyChanged 事件不为空？

另外，在集成 Prism 并使用非常方便的 EventAggregator 之后，我在使用标准 .NET 事件时没有遇到此问题。

我将在 View 和 ViewModel 后面显示我的代码，为简洁起见，省略了 View XAML。

ToolBarView.xaml.cs：

namespace ToolBarModule
{   

public partial class ToolBarView : UserControl
    {                           
        public ToolBarView(ToolBarViewModel toolBarViewModel)
        {
            InitializeComponent();             
            this.DataContext = toolBarViewModel;                                
        }        
    }
}

工具栏视图模型.cs

namespace ToolBarModule
{

public class ToolBarViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    private ToolBarCommands baseCommands;
    private IEventAggregator eventAggregator;

    private KickStartEvent kickStartEvent;
    private SubscriptionToken subscriptionToken;

    private ObservableCollection<IDocumentReference> recentDocuments = new ObservableCollection<IDocumentReference>();

    private ActionCommand newTest;
    private ActionCommand openTest;
    private ActionCommand saveTest;        
    private ActionCommand exitApplication;

    public ToolBarViewModel(){}

    public ToolBarViewModel(IEventAggregator eventAggregator)
    {
        this.eventAggregator = eventAggregator;
        baseCommands = new ToolBarCommands(eventAggregator);
        kickStartEvent = eventAggregator.GetEvent<KickStartEvent>();
        subscriptionToken = kickStartEvent.Subscribe(kickStartEventHandler, ThreadOption.UIThread, true, toolBarEventHandlerFilter);            
    }

    public ICommand NewTest
    {
        get
        {
            if (newTest == null)
            {
                newTest = new ActionCommand(baseCommands.NewTestAction);
            }
            return newTest;
        }
    }

    public ICommand OpenTest
    {
        get
        {
            if (openTest == null)
            {
                openTest = new ActionCommand(baseCommands.OpenTestAction);
            }
            return openTest;
        }
    }

    public ICommand SaveTest
    {
        get
        {
            if (saveTest == null)
            {
                saveTest = new ActionCommand(baseCommands.SaveTestAction);
            }
            return saveTest;
        }
    }


    public ICommand ExitApplication
    {
        get
        {
            if (exitApplication == null)
            {
                exitApplication = new ActionCommand(baseCommands.ExitApplicationAction);
            }
            return exitApplication;
        }
    }

    public ObservableCollection<IDocumentReference> RecentDocuments
    {
        get
        {
            return recentDocuments;
        }

        set
        {
            recentDocuments = value;
            onPropertyChanged("RecentDocuments");
        }
    }

    private void onPropertyChanged(string propertyChanged)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this,new PropertyChangedEventArgs(propertyChanged));
        }

    }

    private void kickStartEventHandler(KickStartEventsArgs e)
    {
        switch (e.EventType)
        {
            case KickStartEventsArgs.KickStartEventType.CREATE_PROJECT:
                onPropertyChanged("RecentDocuments");
            break;                


            case KickStartEventsArgs.KickStartEventType.UPDATE_RECENT_DOCUMENTS_LIST:
            RecentDocuments.Clear();

            foreach (IDocumentReference recentDocs in e.KickStartTestList)
            {
                RecentDocuments.Add(recentDocs);
            }
            onPropertyChanged("RecentDocuments");
            break;
        }
    }
}

}

您还可以尝试在 UserControl 下设置 Grid 或 Element 的 DataContext。对我来说它有效。

例子 （Doesn't如果您使用 DependencyProperty 则有效）：

背后代码：

public MyUserControl()
{
     InitializeComponent();
     this.DataContext = new { LabelText = "Hello World!" };
}

XAML

<UserControl x:Class="CoolProject.ViewModel.MyUserControl"
         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" 
         mc:Ignorable="d" 
         d:DesignHeight="450" d:DesignWidth="800">
<Label x:Name="myLabel" Content="{Binding LabelText}"/>

示例2（我的工作代码):

背后代码：

public MyUserControl()
{
     InitializeComponent();
     this.myGrid.DataContext = new { LabelText = "Hello World!" };
}

XAML

<UserControl x:Class="CoolProject.ViewModel.MyUserControl"
         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" 
         mc:Ignorable="d" 
         d:DesignHeight="450" d:DesignWidth="800">

<Grid x:Name="myGrid">
    <Label x:Name="myLabel" Content="{Binding LabelText}"/>
</Grid>