設置DataContext後PropertyChanged事件爲空

Question

我將視圖的構造函數中View的DataContext設置爲ViewModel的實例，只是標準的東西。此後不久，UPDATE_RECENT_DOCUMENTS_LIST事件從我的ViewModel正確捕獲的Event Aggregator觸發。屬性發生了變化，onPropertyChanged方法被調用，但是由於PropertyChanged事件爲null，它失敗了。

我做的非常接下來的事情就是它提出了一個CREATE_PROJECT事件和相同的視圖模型被接收事件，除了現在的UI操作，PropertyChanged事件不再爲空，一切都按預期工作。

在註冊到PropertyChanged事件之前設置DataContext之後是否需要經過特定的時間？是否有可以等待的事件確保PropertyChanged事件不爲null？

另外，在集成Prism和使用非常方便的EventAggregator之後，我沒有遇到使用標準.NET事件的這個問題。

爲了簡潔起見，我在View和ViewModel後面顯示了我的代碼，省略了View XAML。

ToolBarView.xaml.cs：

namespace ToolBarModule 
{ 

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

ToolBarViewModel.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; 
     } 
    } 
} 

}

Answer 1

你有你的名字在UserControl和XAML在結合使用。喜歡的東西下面的代碼：

<UserControl x:Name="uc" > 
. 
. 
. 
<TextBox Text="{Binding UserName, Mode=TwoWay, ElementName=uc}"/> 

哪裏uc是您UserControl的名稱，並且還嘗試設置的DataContext時，用戶控件加載。

希望得到這個幫助。