[英]ViewModels Property after PropertyChanged event is always Null
[英]PropertyChanged event null after setting DataContext
我正在将视图的构造函数中视图的 DataContext 设置为我的 ViewModel 的实例,只是标准的东西。 此后不久,一个UPDATE_RECENT_DOCUMENTS_LIST事件从我的 ViewModel 正确捕获的事件聚合器中触发。 更改了一个属性并调用了 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,它会引发CREATE_PROJECT事件,并且同一个 ViewModel 正在接收事件,但现在,PropertyChanged 事件不再为空,并且一切都按预期进行。
在设置 DataContext 之后,在它注册到 PropertyChanged 事件之前,是否需要经过特定的时间? 是否有我可以等待的事件以确保 PropertyChanged 事件不为空?
此外,在集成 Prism 并使用非常方便的 EventAggregator 之后,我并没有使用标准的 .NET 事件遇到这个问题。
我在 View 和 ViewModel 后面展示了我的代码,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;
}
}
}
}
您也可以尝试在 UserControl 下设置 Grid 或 Element 的 DataContext。 对我来说它奏效了。
示例(如果您使用 DependencyProperty 则不起作用):
背后的代码:
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):
背后的代码:
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>
您必须在XAML
命名您的UserControl
并在绑定中使用它。 类似于以下代码:
<UserControl x:Name="uc" >
.
.
.
<TextBox Text="{Binding UserName, Mode=TwoWay, ElementName=uc}"/>
其中uc
是您的UserControl
的名称,并且还尝试在加载 UserControl 时设置 DataContext 。
希望这有帮助。
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