Wait, what about the Edit button in Visual Basic .NET

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Most MVVM toolkits include their own good command implementations based on ICommand. Some MVVM toolkits also include a robust messaging structure that may be used in place of events, and in some cases in place of commands and behaviors. Many even include specialized messages used to requesting that the View display the dialog UI. That messaging system for invoking a dialog is something missing in this implementation. Rather than show you an approach that will likely never be used by anyone, I recommend you use the approach recommended by the toolkit you re using. Okay, if you re really curious, here s how I would ve done it. The command would call an EditSelectedEmployee method on the ViewModel. That method would check to see whether SelectedEmployee was null. If not, it would raise an event named ShowEmployeeEditDialog with a custom EventArgs class that included the selected employee as a property. That event would be caught in the code-behind, and the code-behind would show the dialog.
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Structuring and testing with the MVVM/ViewModel pattern
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Why not do that all from the ViewModel The ViewModel shouldn t be in the business of showing dialogs or message boxes of any type. Instead, it should message the UI layer saying it needs some UI to be displayed. In this way, not only is the ViewModel potentially agnostic of Silverlight/WPF/other technology, it remains testable because the event handler in the test code could directly manipulate the values rather than show the dialog. This also allows the code-behind to keep its affinity with the View, being presentation-layer code rather than other logic.
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Commands are the traditional and still most common way of performing this functionality. They have deep support in WPF and decent support in Silverlight. A new approach to accomplishing this has recently been introduced by the Expression Blend team. This approach eschews commands and instead uses designer-friendly behaviors.
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16.4.2 Using the CallMethodAction behavior Introduced with Expression Blend 4, the CallMethodAction behavior provides an easy and designer-friendly way to wire any event from any control to a method. In some ways, it s an alternative to using ICommand and may even seem redundant. But many applications will use both approaches due to the usefulness of ICommand with buttons and menus and CallMethodAction s support for other controls, and events other than Click. You can either install Expression Blend 4 or download the Blend 4 SDK. In either case, add a project reference to the Blend SDK assembly from your main MvvmApplication project. The main assembly you want is Microsoft.Expression.Interactions.dll. You ll also need System.Windows.Interactivity.dll to support that. The behavior approach doesn t give you everything the command approach does specifically, it lacks the ability to enable or disable the button but it provides support for controls other than buttons a key limitation of the command approach. Listing 16.16 shows how to use the CallMethodAction behavior to create the link between the More Vacation! button and the ViewModel method that implements that behavior.
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Listing 16.16 Using the CallMethodAction behavior instead of the command
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<Button Height="23" Width="101" Margin="0,55,12,0" Content="More Vacation!" HorizontalAlignment="Right" VerticalAlignment="Top" x:Name="AddMoreVacation"> <i:Interaction.Triggers> <i:EventTrigger EventName="Click"> <ei:CallMethodAction MethodName="AddVacationBonusToSelectedEmployee" TargetObject="{Binding}" /> </i:EventTrigger> </i:Interaction.Triggers>
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Behavior
</Button>
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Better separation from the UI
The EventTrigger B responds to the firing of the click event. The action taken is the CallMethodAction, which is responsible for calling the method on the current object in the data context: in this case, the ViewModel. It s a simple and elegant solution that works with just about any event and any parameterless function. In support of this, the following two namespaces were added to the top of the XAML file:
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity" xmlns:ei="http://schemas.microsoft.com/expression/2010/interactions"
Those namespaces are required for the EventTrigger and CallMethodAction to be visible to XAML. Both are implemented inside the Blend SDK DLL you added as a reference. Despite the limitations (a potential performance hit due to use of reflection, and the inability to set the IsEnabled property automatically), the CallMethodAction behavior is a good low-code-overhead approach to wiring up method calls. And remember, unlike ICommand, the CallMethodAction is supported on just about any event on any control. Commands and behaviors are a great way to help separate the View from the ViewModel, keeping the contract at just a binding statement or name of a method. But they ve done nothing to fix the tight coupling problem you have between the database, the ViewModel, and the UI. For that, you ll turn to creating View-specific entities or ViewModels.
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