.net qr code generator DATA B INDING in Visual Basic .NET

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CHAPTER 10 DATA B INDING
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The Source and Property properties must be set so the control has access to the business object property. The Target property must be set for the authorization support to work, and this is optional. If you don t want automatic disabling of the UI control based on the business object s authorization rules, then don t set the Target property. Perhaps the most interesting part of the PropertyStatus control s implementation is the fact that it supports visual styling. Its default appearance is defined in the Generic.xaml file contained in the \Themes folder. WPF defines the name of the folder and file; you must use these names when building controls. In Generic.xaml, you ll find a Style element for the PropertyStatus control: <Style TargetType="{x:Type csla:PropertyStatus}"> I won t go through all the XAML in the style, because it is quite extensive and key parts were created using Expression Blend. However, it s important to understand that this style references two ControlTemplate elements that are also defined in Generic.xaml: DefaultPopupTemplate and BrokenRuleTemplate. These are constituent parts of the overall control UI, and they allow a UI designer to override just parts of the visual display. For example, a designer could replace DefaultPopupTemplate to change the look and feel of the popup that is shown when the user hovers his mouse over an error icon. The PropertyStatus style also uses a number of other named elements such as some StoryBoard elements and a Grid element. The names of these elements, as defined by x:Key properties, are important for linking the elements to the code that implements the control itself. In the PropertyStatus control code, attributes are applied to the class to indicate that the control uses a series of control templates. <TemplatePart(Name := "root", Type := GetType(FrameworkElement))> _ <TemplatePart(Name := "popup", Type := GetType(Popup))> _ <TemplatePart(Name := "errorImage", Type := GetType(FrameworkElement))> _ <TemplatePart(Name := "warningImage", Type := GetType(FrameworkElement))> _ <TemplatePart(Name := "informationImage", Type := GetType(FrameworkElement))> _ <TemplatePart(Name := "busy", Type := GetType(BusyAnimation))> _ <TemplatePart(Name := "Valid", Type := GetType(Storyboard))> _ <TemplatePart(Name := "Error", Type := GetType(Storyboard))> _ <TemplatePart(Name := "Warning", Type := GetType(Storyboard))> _ <TemplatePart(Name := "Information", Type := GetType(Storyboard))> _ Public Class PropertyStatus Inherits ContentControl End Class The control also defines a set of dependency properties, such as the Target property discussed earlier. A dependency property is a special way of declaring properties so they are fully available to data binding in WPF and WF. Public Shared ReadOnly TargetProperty As DependencyProperty = _ DependencyProperty.Register( _ "Target", _ GetType(DependencyObject), _ GetType(PropertyStatus)) Public Property Target() As DependencyObject Get Return DirectCast(GetValue(TargetProperty), DependencyObject) End Get Set(ByVal value As DependencyObject) SetValue(TargetProperty, value) End Set End Property
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C HAPTE R 10 DA TA BINDIN G
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As you can see, the declaration of a dependency property is not all that different from a CSLA .NET property with a managed backing field, as discussed in 7. In fact, I drew inspiration from the dependency property syntax when designing the managed backing field syntax for CSLA .NET. WPF uses the DependencyProperty field to provide metadata about the dependency property, and it uses that metadata to support data binding. The use of dependency properties is required to create bindable properties in any WPF UI component such as a control. It is not required when creating data sources such as a business object. The Source property is important, because it provides the control with access to the underlying data source object. When this property changes, the control must disconnect from any previous data source and connect to the new data source. This is required because the PropertyStatus control listens for events from the data source object. The SetSource() method coordinates this process. Private Sub SetSource(ByVal oldSource As Object, ByVal newSource As Object) DetachSource(oldSource) AttachSource(newSource) Dim bb As BusinessBase = TryCast(newSource, BusinessBase) If bb IsNot Nothing AndAlso Not String.IsNullOrEmpty([Property]) Then IsBusy = bb.IsPropertyBusy([Property]) End If UpdateState() End Sub The UpdateState() method causes an immediate check of the validation status for the bound property from the data source object. This way, the UI is updated immediately to reflect the status of the new data source object. I ll cover the UpdateState() method later in this section of the chapter. The DetachSource() and AttachSource() methods unhook and hook the event handlers for the data source object s BusyChanged and PropertyChanged events. For example, here s the AttachSource() method: Private Sub AttachSource(ByVal source As Object) Dim busy As INotifyBusy = TryCast(source, INotifyBusy) If busy IsNot Nothing Then AddHandler busy.BusyChanged, AddressOf source_BusyChanged End If Dim changed As INotifyPropertyChanged = TryCast(source, INotifyPropertyChanged) If changed IsNot Nothing Then AddHandler changed.PropertyChanged, AddressOf source_PropertyChanged End If End Sub The BusyChanged event is used to turn on and off a busy animation. The result is that the user has a visual indication that the business object property is currently executing an asynchronous validation rule. The default visual appearance in this case is to display a BusyAnimation control, which I ll discuss later in this chapter. The PropertyChanged event is used to trigger a check of the source object s authorization and validation rules through a call to the UpdateState() method. The UpdateState() method uses the validation and authorization concepts discussed in s 11 and 12 to ask the business object whether the property has any associated broken validation rules. Dim allRules As New List(Of BrokenRule)() For Each r In businessObject.BrokenRulesCollection If r.[Property] = [Property] Then allRules.Add(r) End If Next
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