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CHAPTER 10 DATA B INDING
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For example, this XAML defines a Save button: <Button Command="ApplicationCommands.Save" CommandTarget="{Binding Source={StaticResource Project}, Path=CommandManager, BindsDirectlyToSource=True}" HorizontalAlignment="Left" IsDefault="True">Save</Button> Notice how the CommandTarget property is specified. The Source is a CslaDataProvider control, but the binding path indicates that the command should be routed to the data provider control s CommandManager property, which is an instance of CslaDataProviderCommandManager. In CslaDataProviderCommandManager is a SaveCommand() method that is invoked by the WPF commanding infrastructure when the Button control is clicked: Private Shared Sub SaveCommand(ByVal target As Object, _ ByVal e As ExecutedRoutedEventArgs) Dim ctl = TryCast(target, CslaDataProviderCommandManager) If ctl IsNot Nothing AndAlso ctl.Provider IsNot Nothing Then ctl.Provider.Save() End If End Sub The CslaDataProviderCommandManager has a Provider property, which is a reference to its parent CslaDataProvider control. You can see how this code delegates the call to its parent: ctl.Provider.Save() Finally, in CslaDataProvider, the Save() method does the actual work of saving the business object by calling the object s Save() method: result = savable.Save() It is important to notice that the Save() method returns a new instance of the business object. I ll discuss the details in 15, but what this means here is that the new object must be provided to WPF data binding so the UI can update properly. To do this, data binding must first be cleared and then provided with the new object: ' clear previous object MyBase.OnQueryFinished(Nothing, exceptionResult, Nothing, Nothing) ' return result to base class MyBase.OnQueryFinished(result, Nothing, Nothing, Nothing) The first OnQueryFinished() call passes Nothing for the object value. This is necessary, because if the business object is returned before clearing the value, data binding will think it is the original object, and it won t update the reference. This is much like the process of clearing and resetting a BindingSource in Windows Forms for the same reason. The Undo (cancel), New (add new item), and Remove (remove item) commands are implemented using the same technique. The CslaDataProvider control reduces the amount of UI code required to create a data-oriented WPF form. It can create or retrieve an object so the user can interact with the data. The control allows the UI developer to use commanding to add or remove items from a collection object. Finally, the control can be used to save or cancel the changes made by the user, all entirely through XAML.
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C HAPTE R 10 DA TA BINDIN G
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The PropertyStatus Control
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WPF doesn t have an exact equivalent to the Windows Forms ErrorProvider control or the Web Forms validation controls. As I discussed earlier and illustrated in Figures 10-3 and 10-4, some basic support for IDataErrorInfo is built into WPF data binding. It is possible to use XAML styling to display validation errors from the business object to the user using that technique. However, that approach still doesn t allow you to easily display all three severities of validation errors that I will cover in 11: error, warning, and information. WPF also doesn t provide a simple way to indicate that an asynchronous validation rule is currently running for a property that is data bound to a UI control. In 12, I ll discuss the authorization support built into CSLA .NET, which includes the ability to specify whether a user is allowed to read or write to specific properties. Ideally, a UI control bound to a property would enable or disable (or even become hidden) based on whether the user was allowed to edit or see the property value. There is no support for such a concept in WPF. The PropertyStatus control in the Csla.Wpf namespace is designed to address these shortcomings. This control s default appearance is similar to the Windows Forms ErrorProvider control, although as shown in Figure 10-6, it displays all three rule severities. Of course, the control is fully stylable through XAML, so you can change the visual display as needed.
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Figure 10-6. PropertyStatus displaying broken validation rules The PropertyStatus control is typically used in a form along with a TextBox control or other data bound control. The PropertyStatus control not only has a display of its own for validation errors, but it can also control the display of the associated UI control for authorization purposes (disabling or hiding the control as necessary). Here s an example of using PropertyStatus: <StackPanel Orientation="Horizontal"> <TextBox x:Name="NameTextBox" Text="{Binding Path=Name}" /> <csla:PropertyStatus Source="{Binding}" Property="Name" Target="{Binding ElementName=NameTextBox}"/> </StackPanel>
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