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Private Sub Control_BindingComplete(ByVal sender As Object, _ ByVal e As BindingCompleteEventArgs) Select Case e.BindingCompleteState Case BindingCompleteState.Exception If BindingError IsNot Nothing Then RaiseEvent BindingError(Me, _ New BindingErrorEventArgs(e.Binding, e.Exception)) End If Case Else If (e.BindingCompleteContext = _ BindingCompleteContext.DataSourceUpdate) AndAlso _ TypeOf e.Binding.DataSource Is BindingSource AndAlso _ GetReadValuesOnChange(DirectCast(e.Binding.DataSource, _ BindingSource)) Then e.Binding.ReadValue() End If End Select End Sub The BindingComplete event includes a BindingCompleteEventArgs parameter, and that parameter includes a property indicating whether the binding process completed with an exception or normally. In the case of an exception, the BindingSourceRefresh control raises its own BindingError event, so the UI developer can be informed that an exception occurred and can take steps. By default, exceptions during binding are silently swallowed by data binding, and the only place you ll see them is in the output window of the debugger in Visual Studio. By raising this event when an error occurs, the control enables the UI developer to more easily detect and troubleshoot data binding issues. The normal case, however, is that the binding succeeds. In that case, the e parameter includes a reference to the currently active Binding object. It is this Binding object that isn t refreshed automatically when data binding gets a PropertyChanged event from the underlying data source. By calling its ReadValue() method, this code forces data binding to read the value from the data source and update the current control s display as well. The BindingSourceRefresh control should be used to force data refreshes for all BindingSource controls bound to detail forms. It isn t necessary when only complex controls such as a GridView or ListBox are bound to theobject.
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The CslaActionExtender Control and Associated Components
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When an object is bound to a BindingSource control, the BindingSource control assumes it has full and exclusive control over the object. This means that when an object is data bound, the only interaction with the object should be through the BindingSource control. This includes doing things like saving the object, which is a concept the BindingSource doesn t understand. So, how do you save an object You must first unbind it; you need to disconnect it from the BindingSource. This is also true for any explicit calls to BeginEdit(), CancelEdit(), or ApplyEdit(). I discuss these n-level undo methods in 13. The UI commonly calls these methods when it includes a top-level Cancel button on the form. Unfortunately, unbinding an object from a BindingSource control is not as easy as setting the DataSource property to Nothing. Although that breaks the reference from the control to the object, it doesn t end any current edit session where the BindingSource has called BeginEdit() through the IEditableObject interface. The next thing you might consider is simply calling EndEdit() or CancelEdit() on the BindingSource before setting the value to Nothing.
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CHAPTER 10 DATA B INDING
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bindingSource.EndEdit() bindingSource.DataSource = Nothing However, this won t work either, because as soon as you call EndEdit(), the BindingSource immediately calls BeginEdit(). You need to disconnect the object first and then end the edit session. This helper method demonstrates the process: Protected Sub UnbindBindingSource(ByVal source As BindingSource, _ ByVal apply As Boolean, ByVal isRoot As Boolean) Dim current = TryCast(source.Current, System.ComponentModel.IEditableObject) If isRoot Then source.DataSource = Nothing End If If current IsNot Nothing Then If apply Then current.EndEdit() Else current.CancelEdit() End If End If End Sub Although you could include this code in each form or create a base form from which you inherit, that s complexity that would be nice to avoid. The complexity is even worse if you have a master-detail display in your form. In that case, you must remember to unbind all child BindingSource controls first and then unbind the root (master) BindingSource last. To avoid all this complexity, CSLA .NET includes the CslaActionExtender control. This control extends any control that implements IbuttonControl, so the control automatically understands how to unbind the objects before interacting with them. Like the other controls I ve discussed so far, this is an extender control, so it adds extra behaviors to existing controls in this case, button-style controls. Behind the scenes, CslaActionExtender uses BindingSourceHelper and BindingSourceNode objects to do the actual work. This is important, because you can use BindingSourceHelper and BindingSourceNode directly to get the same behavior if you re not using a button-style control. For example, if you implement your Save and Cancel buttons on a ToolBar, you ll be unable to use CslaActionExtender, but you ll still be able to use BindingSourceHelper and BindingSourceNode to simplify your UI code.
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