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C HAPTE R 10 DA TA BINDIN G
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Private _order As Order = Nothing Private Sub BindUI() cslaActionExtender1.ResetActionBehaviors(_order) End Sub Private Sub cslaActionExtender1_SetForNew(ByVal sender As Object, _ ByVal e As CslaActionEventArgs) _order = Order.NewOrder() BindUI() End Sub Private Sub cslaActionExtender1_ErrorEncountered(ByVal sender As Object, _ ByVal e As ErrorEncounteredEventArgs) MessageBox.Show(e.Ex.Message) End Sub End Class At this point, you should have an understanding of the support provided for Windows Forms data binding within CSLA .NET. I chose to discuss Windows Forms first, because as a general rule, if you support Windows Forms data binding, you support everything else. That used to be true, but it s becoming less true as WPF matures, so I ll talk about WPF next.
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WPF is the newest UI technology discussed in this chapter, and it is evolving rapidly. Even from .NET 3.0 to 3.5 and from 3.5 to 3.5 SP1, there have been substantial changes to the features provided by data binding in WPF. I expect the changes to continue as WPF rapidly matures toward parity with the features of Windows Forms data binding. WPF data binding is similar in some ways to Windows Forms. Both technologies are rich and interactive, providing immediate, event-driven interaction between the UI and the business objects. WPF supports some of the same data binding interfaces used by Windows Forms, and it has some new ones of its own, most of which are optional or redundant. I ll discuss supporting data binding for a single object first, and then I ll discuss data binding support for collections and lists. I ll wrap up this topic by discussing some custom controls provided by CSLA .NET to simplify the use of data binding in WPF.
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Binding to a single object in WPF is largely automatic. You can bind to nearly any object in .NET, and it just works. As with Windows Forms, however, the data binding experience is enhanced if the object implements the same interfaces listed earlier in Table 10-1. The INotifyPropertyChanged interface should be considered the bare minimum required to participate in data binding. It notifies data binding that a property of the object has changed. WPF handles the PropertyChanged event somewhat differently from Windows Forms a topic I ll explore later in this section. The WPF data grid control uses the IEditableObject interface to roll back changes to a row of data if the user presses Esc. The Windows Forms data grid relies on this same behavior.
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CHAPTER 10 DATA B INDING
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At the time of this writing, the WPF data grid control is simply a project on CodePlex (www.codeplex.com) and is not an official part of the product. However, currently, it uses IEditableObject, as described here.
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Data binding uses the IDataErrorInfo interface to change the display to visually indicate that the property to which a control is bound is invalid. Unlike Windows Forms, which uses an ErrorProvider control to do this, WPF data binding supports this concept natively, and you manage the appearance of a control with an invalid value by using an Extensible Application Markup Language (XAML) style. Because of the differences between Windows Forms and WPF, the only interface needing further discussion is INotifyPropertyChanged.
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PropertyChanged Event Handling
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When WPF data binding handles a PropertyChanged event from an object, it assumes that only that specific property has changed. Because of that assumption, data binding updates only those controls that are bound to that specific property. This is fundamentally different from Windows Forms, which updates all controls bound to the same object even if they are bound to different properties. This means that when BusinessBase raises a PropertyChanged event, it needs to act differently for WPF than for Windows Forms. In the case of Windows Forms, even if multiple properties might have changed, it is most efficient to raise PropertyChanged only for one of the properties, since they ll all get refreshed anyway. However, in WPF, it is important to raise PropertyChanged for every property that changes. This includes properties changed directly by the user, as well as properties changed indirectly through business rules or processing. This also includes properties that didn t actually change but may have become valid or invalid because some other property changed. For example, in an Invoice object, the AmountDue property might become invalid when the CreditLimit property is changed. If the object raises only PropertyChanged for CreditLimit, the UI won t display any visual cue to indicate that AmountDue is now invalid. Figure 10-3 illustrates this.
Figure 10-3. Incorrect WPF display when using the Windows Forms model In Figure 10-3, the user sets the total due to 190 and then changes the credit limit from 5,000 to 50. Obviously, the Total Due control now exceeds the credit limit and should be invalid, but no visual cue is provided for the user. The object also needs to raise a PropertyChanged event for AmountDue, so data binding refreshes that property s display. If there s a PropertyChanged event for CreditLimit and TotalDue, the visual display will look like Figure 10-4.
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