.net qr code generator The BindingSourceNode Class in VB.NET

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The BindingSourceNode Class
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The BindingSourceNode class is designed to wrap a BindingSource component, which sits on your Windows form. The name of this class is because it represents a node in what can be a tree of binding sources. When binding a form to a business object that is a parent to one or more child objects or collections, the form requires a BindingSource component for each object or collection. This hierarchical set of BindingSource components is represented by a set of BindingSourceNode objects. The BindingSourceNode class has a recursive design. Not only does it contain a property to hold the corresponding BindingSource component, but it also contains a property that contains a list of other BindingSourceNode objects. Each instance also holds a reference to its parent. It is through this design that a tree of BindingSource components can be represented like this: Private _Source As BindingSource Private _Children As List(Of BindingSourceNode) Private _Parent As BindingSourceNode
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C HAPTE R 10 DA TA BINDIN G
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These member variables are exposed through public properties. The BindingSourceNode class also provides methods to assist you in any of the following tasks that involve binding or unbinding: Bind Unbind SetEvents ResetBindings These methods wrap standard functionality that you need to perform in the case of saving a bound business object or invoking n-level undo functionality on a bound business object. As I mentioned earlier, when saving or undoing a business object that is participating in Windows Forms data binding, you must first properly unbind the object before acting upon it. The methods exposed by the BindingSourceNode class help you by performing this functionality for you and, more importantly, by taking into account any child BindingSource components that it may contain. Since the BindingSourceNode class uses a recursive design, the helper methods can be called from within any level of the tree, but more often than not, you will be addressing the top-level node. In fact, to avoid any potential n-level undo parent-to-child mismatches, this is the recommended practice. Because the act of saving or invoking an undo operation involves more than one of the helper operations exposed by the BindingSourceNode object, additional methods are typically used from the Windows form. Public Sub Apply() SetEvents(False) Unbind(False) End Sub Public Sub Cancel(ByVal businessObject As Object) SetEvents(False) Unbind(True) Bind(businessObject) End Sub Public Sub Close() SetEvents(False) Unbind(True) End Sub
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The BindingSourceHelper Class
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The tree of BindingSourceNode objects is built using the BindingSourceHelper component. The BindingSourceHelper component exposes a Shared method called InitializeBindingSourceTree, which returns an instance of BindingSourceNode. This method builds the entire tree of BindingSource components on your form. The InitializeBindingSourceTree() method accepts two arguments. Public Shared Function InitializeBindingSourceTree( _ ByVal container As IContainer, _ ByVal rootSource As BindingSource) As BindingSourceNode End Function The two arguments correspond to the form s container property and the BindingSource object on your form that binds to your root object. You can find the form s container property in the designer partial class that gets created along with a form and modified dynamically every time you drop controls or components onto the form. The components you place on a form that sit in the component tray get added to the container property, so it is this property that the InitializeBindingSourceTree() method needs.
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CHAPTER 10 DATA B INDING
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The functionality contained in BindingSourceHelper could have resided in BindingSourceNode as a Shared method, but it made more sense to separate these two classes out in the interest of maintaining a strict separation of the node class from the class that creates the actual tree. The design also allows for future enhancements to be added to the BindingSourceHelper class while maintaining a clean BindingSourceNode class. Using these classes is quite simple and can save you lots of confusing code. In a conventional Windows Forms binding situation, you set up your BindingSource components declaratively and then set the DataSource property of the root BindingSource component to the top-level business object. This step is taken care of automatically if you re using the new components, in which case you obtain an instance of a BindingSourceNode class by calling the InitializeBindingSourceTree() method from the BindingSourceHelper component. After this, you simply call the Bind method of the new variable and send your top-level business object into its only argument. Private _bindingTree As BindingSourceNode = Nothing Private Sub BindUI() _bindingTree = BindingSourceHelper.InitializeBindingSourceTree( _ Me.Container, orderBindingSource) _bindingTree.Bind(order) End Sub You can now use the instance of the BindingSourceNode class, depicted in the previous code by the _bindingTree field, to unbind or rebind to assist you while performing save or undo operations. You use the BindingSourceNode class s Apply() method just before you save your business object. After this, you can incorporate the aid of the object s Bind() method. Use the Cancel method to invoke an undo operation; it rebinds the object afterward automatically. I should reiterate that the BindingSourceNode object contains the entire tree of BindingSource components as it corresponds to the parent-child object hierarchy you have designed into the business object that you will be binding. All actions performed on BindingSourceNode object propagate properly to the child nodes and in fact do so in reverse order, so the lowest child gets hit first and the root gets hit last. Should you have two distinct object hierarchies represented on a Windows form, thus having two root BindingSource components, you would need two separate instances of the BindingSourceNode class. And remember, all interaction with your business object should now take place through these two classes. The following is a sample of a form s code that uses all the functionality of these two components in a toolbar scenario, with a root object representing an order with some order detail children: Public Partial Class OrderMaint2 Inherits Form Public Sub New() InitializeComponent() End Sub Public Sub New(ByVal orderId As Guid) InitializeComponent() _order = Order.GetOrderWithDetail(orderId) BindUI() End Sub Private _order As Order = Nothing Private _bindingTree As BindingSourceNode = Nothing
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