.net qr code generator The CslaDataProvider Control in VB.NET

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The CslaDataProvider Control
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WPF has a data control concept similar to the data source control concept from ASP.NET 2.0. In WPF, these data controls are called data providers, and they allow declarative data access from your XAML code or your code-behind. Data provider controls are powerful, because they abstract the concept of data access within a WPF form, and they can support additional behaviors such as providing asynchronous access to data. As with ASP.NET, WPF provides an ObjectDataProvider control that might, at first glance, appear to be a good way to work with CSLA .NET style business objects. Unfortunately, the ObjectDataProvider control has some of the same limitations as the ASP.NET ObjectDataSource control: It requires a Public constructor. It has no way to call a Shared (or any other type of) factory method. CSLA .NET style business objects have non-Public constructors, and factory methods are used to create or retrieve the object. Additionally, CSLA .NET objects intrinsically support n-level undo and persistence, and the ObjectDataProvider has no knowledge of those capabilities either. What s needed is a data provider control that understands how to call Shared factory methods and how to manage the object s lifetime: interacting with n-level undo and CSLA .NET style object persistence. The CslaDataProvider is a WPF data provider control that understands how to interact with CSLA .NET business objects. This control can not only create or retrieve a business object, but it can also manage the object s entire lifetime through to saving (inserting, updating or deleting) the object into the database or canceling any changes made to the object by the user.
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Finally, like many other data provider controls, CslaDataProvider supports asynchronous loading of the object by implementing an IsAsynchronous property. This can be a powerful feature in many cases, because it tells the control to create or retrieve the object on a background thread, and the UI updates automatically when the process is complete.
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Declaring the Class
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The .NET Framework includes a base class that makes it relatively easy to create a data provider control, and CslaDataProvider inherits from that class: Public Class CslaDataProvider Inherits DataSourceProvider End Class In the simplest case, a subclass of DataSourceProvider needs only to override the BeginQuery() method, where it creates or retrieves the object requested by data binding. The CslaDataProvider control does that, and quite a bit more.
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The BeginQuery Method
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A subclass of DataSourceProvider needs to override the BeginQuery() method, which data binding invokes when it needs the control to create or retrieve an object. This can happen in the following cases: When the WPF form is loaded and IsInitialLoadEnabled is True (the default) When a property of the data provider control is changed (via data binding or code) The BeginQuery() method must honor some properties from the base class. First, it must support the concept of deferred refresh, which allows the UI code to set many properties of the data provider control and have the query run only once after they ve all been set. The IsRefreshDeferred property on the base class controls this. Second, it must support the IsInitialLoadEnabled property. If this property is False, then the first time BeginQuery() is invoked, it must return without doing any work. Finally, the CslaDataProvider control supports an IsAsynchronous property, and if that is True, then the query is run on a background thread. Here s the code: Protected Overrides Sub BeginQuery() If Me.IsRefreshDeferred Then Return End If If _firstRun Then _firstRun = False If Not IsInitialLoadEnabled Then Return End If End If Dim request = New QueryRequest() request.ObjectType = _objectType request.FactoryMethod = _factoryMethod request.FactoryParameters = _factoryParameters request.ManageObjectLifetime = _manageLifetime If IsAsynchronous Then System.Threading.ThreadPool.QueueUserWorkItem(AddressOf DoQuery, request) Else DoQuery(request) End If End Sub
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