MSDN Magazine - September 2008 - (Page 33)

object implements INotifyPropertyChanged and if the source’s property setters raise the PropertyChanged event. In the previous example, OneWay binding is used for the CategoryName since it is displayed in a TextBlock and cannot be edited. TwoWay binding is used for the UnitsInStock since it is presented in a TextBox which can be edited. When the user edits the value of UnitsInStock in the TextBox and when the TextBox loses focus, the change is sent back to the source object. OneTime binding is optimal for presenting read-only information that will never change while presented. OneWay binding is great for presenting read-only information that may change at some point while being presented. To illustrate, if OneTime binding was used instead of OneWay on the CategoryName TextBlock, when the Cancel button is clicked and the product list is refreshed the TextBlock using the OneTime binding mode would not be updated. Finally, TwoWay binding is the best choice when the user must be able to change the data in a control and have the change reflected in the data source For example, the product class has a UnitPrice property that sets the value of the product’s price and calls the base class’s PropertyChangedHandler method: [DataMember] public decimal UnitPrice { get { return _unitPrice; } set { _unitPrice = value; base.PropertyChangedHandler(this, "UnitPrice"); } } This method, in turn, raises the PropertyChanged event. This event is then sent to any OneWay or TwoWay bindings, which causes the bindings to update the target controls with the new value. Every property’s setter in the Product class uses this pattern. Change Events One of the keys to having OneWay and TwoWay binding notify the target when the source has changed is to implement the INotifyPropertyChanged interface. The Product class implements this interface, which consists of a single event, PropertyChanged. The PropertyChanged event accepts as parameters the sender and the name of the changed property. Silverlight data binding listens for these events from data sources. If the class does not both implement this interface and raise the PropertyChanged event in the property setters, then the target will never receive updates. The PropertyChangedEventHandler is implemented in the Product class through the following line of code: public override event PropertyChangedEventHandler PropertyChanged; oneTime binding is optimal for presenting static, read-only information. The property change notifications can be demonstrated through the sample application by selecting a product from the ListBox and editing the unit price in the TextBox. Since the tbUnitPrice TextBox uses TwoWay binding, this causes the new price to be sent to the source. Once the source is updated, the PropertyChanged event is raised. This causes any bindings that are listening for the event to update the target values. Since the ListBox implements OneWay binding, its value for the price will automatically update itself to the new price value. In this case, the event is overridden rather than simply defined because the Product class inherits from a custom EntityBase class which implements the INotifyPropertyChanged interface. The EntityBase class, shown here, implements the PropertyChangedEventHandler virtually so it can be overridden in the derived classes such as Product or Category: [DataContract] public abstract class EntityBase : INotifyPropertyChanged { protected void PropertyChangedHandler(EntityBase sender, string propertyName) { if (PropertyChanged != null) PropertyChanged(sender, new PropertyChangedEventArgs( propertyName)); } public virtual event PropertyChangedEventHandler PropertyChanged; } Wrap-Up As you can see, Silverlight 2 data-binding features are simple to implement using the declarative binding syntax shown in the sample application. The bindings listen for the PropertyChanged event so they can update their targets. In this column, I showed how easy it is to implement binding through Silverlight, how to communicate with WCF services to interact with business objects and a database, and how to define a ClientAccessPolicy.xml file to allow and restrict remote communications.  John PaPa (johnpapa.net) is a Senior Consultant with ASPSOFT (aspsoft.com) The EntityBase class also defines a PropertyChangedHandler method as a convenience to derived classes which, in turn, raises the PropertyChanged event. This is a simple refactoring to put the logic of raising the event into a single place and reduce code. and a baseball fan who spends summer nights rooting for the Yankees with his family. John, a C# MVP and INETA speaker, has authored several books and is now working on his latest, titled Data Access with Silverlight 2, to be published by O’Reilly in December 2008. He often speaks at conferences such as DevConnections and VSLive. msdnmagazine.com September 2008 33 http://johnpapa.net http://aspsoft.com http://msdnmagazine.com

Table of Contents Feed for the Digital Edition of MSDN Magazine - September 2008

MSDN Magazine - September 2008 Contents Toolbox CLR Inside Out Data Points Advanced Basics Office Space Cutting Edge Hierarchy ID New Features for Microsoft SQL Server 2008 Prism Data Services Advanced WPF Test Run Security Briefs Foundations { End Bracket }

MSDN Magazine - September 2008

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