1
我一直在使用來自ApuntasNotas的非常好的示例代碼來了解更多有關如何有效使用MVVM Light Toolkit的信息。在MVVM中使用命令
在代碼中，在一個實例中，作者似乎利用背後的代碼來設置DataContext來處理單擊事件，這讓我感到困惑。
在XAML，在文本菜單cm一個菜單項的EditNote_Click事件處理程序中的代碼隱藏的處理：
<Window x:Class="ApuntaNotas.MainWindow" Icon="Icons/app_48.ico"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Apunta Notas"
Height="480"
x:Name="Ventana"
Width="640"
Background="Beige"
DataContext="{Binding Main, Source={StaticResource Locator}}">
.
.
.
<ScrollViewer VerticalScrollBarVisibility="Auto">
<ItemsControl Background="Beige" Padding="15" Tag="Hello" x:Name="IC"
ItemsSource="{Binding Notes}">
<ItemsControl.LayoutTransform>
<ScaleTransform ScaleX="{Binding Value, ElementName=zoomSlider}" ScaleY="{Binding Value, ElementName=zoomSlider}" />
</ItemsControl.LayoutTransform>
<ItemsControl.ContextMenu>
<ContextMenu Name="icCM">
<MenuItem Header="{Binding Source={StaticResource LocStrings}, Path=DeleteAllNotes}" Command="{Binding DeleteAllNotesCommand}" />
</ContextMenu>
</ItemsControl.ContextMenu>
<ItemsControl.ItemTemplate>
<DataTemplate>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="auto" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Grid.ContextMenu>
<ContextMenu Name="cm">
<MenuItem Header="{Binding Source={StaticResource LocStrings}, Path=Edit}" Click="EditNote_Click"/>
<MenuItem Header="{Binding Source={StaticResource LocStrings}, Path=Delete}" Click="DeleteNote_Click" />
<Separator />
<ComboBox Loaded="CmbNoteCategory_Loaded" SelectionChanged="CmbNoteCategory_SelectionChanged">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Name}" />
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
</ContextMenu>
.
.
.
在代碼隱藏中，EditNote_Click處理程序的定義如下：
private void EditNote_Click(object sender, RoutedEventArgs e)
{
var menuItem = e.Source as MenuItem;
if (menuItem != null)
ViewModel.EditNoteCommand.Execute(menuItem.DataContext as Model.Note);
}
EditNoteCommand具有以下特徵：
public RelayCommand<Note> EditNoteCommand { get; private set; }
我的問題是，爲什麼作者沒有將EditNoteCommand命令（已寫入且可用）與MenuItem的Command屬性鏈接到XAML中？
例如，我試圖替換下列內容，它編譯了，但生成了一個異常（如下所示）。我懷疑我的方法是合理的，但我錯過了將DataContext或其他內容傳遞給命令代碼的內容。我重置的DataContext綁定到Main便於命令結合：
<MenuItem Header="{Binding Source={StaticResource LocStrings}, Path=Edit}" DataContext="{Binding Main, Source={StaticResource Locator}}" Command="{Binding EditNoteCommand}"/>
這種嘗試產生內EditNote參照other以下異常 - 一個通過EditNoteCommand調用的方法：
EditNote：
private void EditNote(Note other)
{
ActualNote = other;
SelectedCategory = other.Category;
}
例外：
System.NullReferenceException was unhandled Message=Object reference not set to an instance of an object. Source=ApuntaNotas StackTrace:
at ApuntaNotas.ViewModel.MainViewModel.EditNote(Note other) in C:\Documents and Settings\wcatlan\My Documents\Visual Studio 2010\Projects\ApuntaNotas\trunk\ApuntaNotas\ViewModel\MainViewModel.cs:line 171
at GalaSoft.MvvmLight.Command.RelayCommand`1.Execute(Object parameter)
at MS.Internal.Commands.CommandHelpers.CriticalExecuteCommandSource(ICommandSource commandSource, Boolean userInitiated)
at System.Windows.Controls.MenuItem.InvokeClickAfterRender(Object arg)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
at System.Windows.Threading.DispatcherOperation.InvokeImpl()
at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
at System.Threading.ExecutionContext.runTryCode(Object userData)
at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Windows.Threading.DispatcherOperation.Invoke()
at System.Windows.Threading.Dispatcher.ProcessQueue()
at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
at System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
at System.Windows.Threading.Dispatcher.Run()
at System.Windows.Application.RunDispatcher(Object ignore)
at System.Windows.Application.RunInternal(Window window)
at System.Windows.Application.Run(Window window)
at System.Windows.Application.Run()
at ApuntaNotas.App.Main() in C:\Documents and Settings\wcatlan\My Documents\Visual Studio 2010\Projects\ApuntaNotas\trunk\ApuntaNotas\obj\Debug\App.g.cs:line 0
at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart() InnerException:
我們可能沒」 t需要完整的堆棧跟蹤。 :) – 2011-02-23 13:01:03
我看到完整的堆棧跟蹤沒有問題。 – Robaticus 2011-02-23 13:25:16