Event in Visual Basic .NET

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SelectObject InsertObject UpdateObject DeleteObject
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Requests that the UI provide a reference to the business object that is the data source Requests that the UI insert a new business object based on the data from the form Requests that the UI update a business object with the data from the form, based on the key value provided Requests that the UI delete the business object based on the key value provided
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.NET These four events are directly analogous to the four method names required by the ASP ObjectDataSource. Rather than using reflection to invoke a set of methods, I opted to raise events, as I feel that this is an easier programming model. With the ObjectDataSource, the UI developer
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CHAPTER 5 s COMPLETING THE FRAMEWORK
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must implement four methods (or defer to those in an ADO.NET TableAdapter); while with CslaDataSource, the developer simply handles these four events. There is a custom EventArgs object for each of the events: SelectObjectArgs, InsertObjectArgs, UpdateObjectArgs, and DeleteObjectArgs, respectively. Each one provides properties that are used within the event handler.
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Handling the Events in a Page
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For instance, a SelectObject event handler may look like this: Protected Sub CustomerDataSource_SelectObject( _ ByVal sender As Object, ByVal e As Csla.Web.SelectObjectArgs) _ Handles CustomerDataSource.SelectObject e.BusinessObject = Customer.NewCustomer() End Sub Notice that SelectObjectArgs defines a BusinessObject property, which must be set to the object that is to be used as a data source. A typical UpdateObject event handler is a bit different: Protected Sub CustomerDataSource_UpdateObject( _ ByVal sender As Object, ByVal e As Csla.Web.UpdateObjectArgs) _ Handles CustomerDataSource.UpdateObject Dim obj As Customer = Customer.GetCustomer(e.Keys("Id").ToString) Csla.Data.DataMapper.Map(e.Values, obj) obj.Save() e.RowsAffected = 1 End Sub The e.Keys value is a name/value list of key values. The Id key value is used to load the customer object from the database, and then the DataMapper class is used to map the values from e.Values (another name/value list) to the properties of the object. If you wanted to implement a type of field-level concurrency, the e.OldValues list contains the values from the data source when the form was loaded. InsertObjectArgs exposes only e.Values, while DeleteObjectArgs exposes only e.Keys. The usage of both these events is comparable to UpdateObject: Protected Sub CustomerDataSource_InsertObject( _ ByVal sender As Object, ByVal e As Csla.Web.InsertObjectArgs) _ Handles CustomerDataSource.InsertObject Dim obj As Customer = Customer.NewCustomer() Csla.Data.DataMapper.Map(e.Values, obj) obj.Save() e.RowsAffected = 1 End Sub Protected Sub CustomerDataSource_DeleteObject( _ ByVal sender As Object, ByVal e As Csla.Web.DeleteObjectArgs) _ Handles CustomerDataSource.DeleteObject Customer.DeleteCustomer(e.Keys("Id").ToString) e.RowsAffected = 1 End Sub
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All the custom EventArgs objects except SelectObjectArgs include a RowsAffected property that the event handler should set to indicate how many rows of data were affected by the operation. In 10, you ll see complete examples of these event handlers, including exception handling and the use of Session to reduce the number of hits on the database.
Event Declaration
Each event is declared in CslaDataSource, along with a method to raise that event. For instance, here s the SelectObject event and related method: Public Event SelectObject As EventHandler(Of SelectObjectArgs) Friend Sub OnSelectObject(ByVal e As SelectObjectArgs) RaiseEvent SelectObject(Me, e) End Sub EventHandler(Of T) is a generic template in the .NET Framework that simplifies the declaration of event handler type events. Notice that the OnSelectObject() method is Friend in scope. It will only be called by CslaDataSourceView, and shouldn t be visible to UI or business developers.
CslaDataSourceView
The real work of data binding at runtime is handled by the CslaDataSourceView. ASP.NET gets a reference to this object through CslaDataSource, but it is this object that actually interacts with the data source. The object maintains a reference to the CslaDataSource control that created it, along with the assembly name and type name of the business object to which it is bound: Public Class CslaDataSourceView Inherits DataSourceView Private mOwner As CslaDataSource Private mTypeName As String Private mTypeAssemblyName As String The class inherits from System.Web.UI.DataSourceView, and so is required (at a minimum) to override the ExecuteSelect() method. It also overrides ExecuteInsert(), ExecuteUpdate(), and ExecuteDelete(). ASP .NET calls these methods to trigger the appropriate operation at the appropriate time. For instance, when a control s data binding requires data, ASP .NET invokes the ExecuteSelect() method, and when data is to be inserted, it invokes the ExecuteInsert() method.
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