Data Binding and Silverlight List Controls

CHAPTER ■■■ Data Binding and Silverlight List Controls The previous chapter focused on the form controls contained in Silverlight In this chapter, you will look at two controls that are made to display lists of data: the ListBox and DataGrid These controls are typically bound to data through a technique known as data binding, which I’ll explore first Data Binding Through data binding, UI elements (called targets) are “bound” to data from a data source (called the source), as illustrated in Figure 5-1 When the data sources change, the UI elements bound to those data sources update automatically to reflect the changes The data can come from different types of sources, and the target can be just about any UI element, including standard Silverlight controls Figure 5-1 Data binding in Silverlight 105 CHAPTER ■ DATA BINDING AND SILVERLIGHT LIST CONTROLS Data binding simplifies application development Since changes are reflected automatically, you not need to manually update the UI elements Also, by using data binding, you are able to separate the UI from the data in your application, which allows for a cleaner UI and easier maintenance The Binding Class Data binding in Silverlight is accomplished by using the Binding class The Binding class has two components—the source and target—and a property that defines the way the two are bound, called the binding mode The source is the data that is to be bound, the target is a property of the control that the data is to be bound to, and the mode defines how the data is passed between the source and the target (one-way, one-time, or two-way) You’ll see how this works in the upcoming exercise To define the binding of a control’s property, you use XAML markup extensions, such as {Binding } For example, to bind the Text property of a TextBox to a data source’s FirstName element, you would use the following XAML: Try It Out: Simple Data Binding in Silverlight To help explain data binding in Silverlight, let’s build a very simple application The application will include a Book object that contains two properties: Title and ISBN These properties will be bound to two TextBox controls Figure 5-2 shows the end result of the example Figure 5-2 Simple data binding example 106 CHAPTER ■ DATA BINDING AND SILVERLIGHT LIST CONTROLS Create a new Silverlight application in Visual Studio 2008 Name the project BasicDataBinding, and allow Visual Studio to create a Web Site project to host your application Edit the MainPage.xaml file to define two columns and six grid rows Place a TextBlock in each row in column and a TextBox in each row in column Also add some margins and some alignment assignments to improve the layout The code for the page follows: Next, edit the code behind, MainPage.xaml.cs Add a Loaded event handler for the application, which will fire when the application is loaded by the client This is accomplished with the following source code: public partial class MainPage : UserControl { public MainPage() { InitializeComponent(); this.Loaded += new RoutedEventHandler(Page_Loaded); } void Page_Loaded(object sender, RoutedEventArgs e) { } } Now you need to add a class to define a Book object Below the MainPage class, add the following class definition: namespace BasicDataBinding { public partial class MainPage : UserControl { public MainPage() { InitializeComponent(); this.Loaded += new RoutedEventHandler(Page_Loaded); } void Page_Loaded(object sender, RoutedEventArgs e) { } } public class Book 108 CHAPTER ■ DATA BINDING AND SILVERLIGHT LIST CONTROLS { public string Title { get; set; } public string ISBN { get; set; } } } Now that you have Book defined, you need to create an instance of Book and set it to the LayoutRoot’s DataContext, as follows: void Page_Loaded(object sender, RoutedEventArgs e) { Book b = new Book() { Title = "Beginning Silverlight 3: From Novice to Professional", ISBN = "978-1590599525" }; this.LayoutRoot.DataContext = b; } When you set up binding definitions for different controls, the controls not know where they are going to get their data The DataContext property sets the data context for a control that is participating in data binding The DataContext property can be set directly on the control If a given control does not have a DataContext property specified, it will look to its parent for its data context The nice thing about this model is that if you look above in the XAML for the page, you will see little indication of where the controls are getting their data This provides an extreme level of code separation, allowing designers to design XAML UIs and developers to work alongside the designers, defining the specifics of how the controls are bound to their data sources At this point, you can go ahead and start debugging the application If all goes well, you will see the four text boxes populated with the data from the Book’s instance (see Figure 5-2) With the application running, change the book title in the first text box to just “Beginning Silverlight,” by removing the “From Novice to Professional.” You might expect that, since the third text box is bound to the same data, it will automatically update to reflect this change However, a couple of things need to be done to get this type of two-way binding to work One problem is that, currently, the Book class does not support notifying bound clients of changes to its properties In other words, when a property changes in Book, the class will not notify the TextBox instances that are bound to the class of the change You could take care of this by creating a change event for each property This is far from ideal; fortunately, there is an interface that a class can implement that handles this for you This interface is known as INotifyPropertyChanged Let’s use it Modify the Book class definition to inherit from INotifyPropertyChanged Notice that when you inherit from INotifyPropertyChanged, you need to add using System.ComponentModel Luckily, Visual Studio will help you with this, as shown in Figure 5-3 109 CHAPTER ■ DATA BINDING AND SILVERLIGHT LIST CONTROLS Figure 5-3 Visual Studio assists when you need to add the System.ComponentModel namespace Next, you can let Visual Studio some more work for you After adding the using System.ComponentModel statement, right-click INotifyPropertyChanged and choose the Explicitly implement interface INotifyPropertyChanged option, as shown in Figure 5-4 Figure 5-4 Visual Studio also assists in implementing the INotifiyPropertyChanged interface Now Visual Studio has added a new public event to your class: public class Book : INotifyPropertyChanged { public string Title { get; set; } public string ISBN { get; set; } #region INotifyPropertyChanged Members public event PropertyChangedEventHandler PropertyChanged; #endregion } Next, you need to create a convenience method that will fire the PropertyChanged event Call it FirePropertyChanged, as shown in the following code public class Book : INotifyPropertyChanged { public string Title { get; set; } public string ISBN { get; set; } #region INotifyPropertyChanged Members void FirePropertyChanged(string property) 110 CHAPTER ■ DATA BINDING AND SILVERLIGHT LIST CONTROLS { if (PropertyChanged != null) { PropertyChanged(this, new PropertyChangedEventArgs(property)); } } public event PropertyChangedEventHandler PropertyChanged; #endregion } Now you need to extend the simplified properties by adding private members and full get/set definitions to define the get and set operations, as shown in the following code The get is just like a normal get operation, where you simply return the internal member value For the set, you first set the internal member value, and then call the FirePropertyChanged method, passing it the name of the property public class Book : INotifyPropertyChanged { private string _title; private string _isbn; public string Title { get { return _title; } set { _title = value; FirePropertyChanged("Title"); } } public string ISBN { get { return _isbn; } set { _isbn = value; FirePropertyChanged("ISBN"); } } 111 CHAPTER ■ DATA BINDING AND SILVERLIGHT LIST CONTROLS #region INotifyPropertyChanged Members void FirePropertyChanged(string property) { if (PropertyChanged != null) { PropertyChanged(this, new PropertyChangedEventArgs(property)); } } public event PropertyChangedEventHandler PropertyChanged; #endregion } With this completed, your class is set up to notify bound clients of changes to the Title and ISBN properties But you still need to take one more step By default, when you bind a source to a target, the BindingMode is set to OneWay binding, which means that the source will send the data to the target, but the target will not send data changes back to the source In order to get the target to update the source, you need to implement two-way (TwoWay) binding ■ Note Earlier, I mentioned that there are three options for BindingMode The third option is OneTime binding In this mode, the values are sent to the target control property when the object is set to the DataContext However, the values of the target property are not updated when the source value changes 10 To change to two-way binding, add the Mode=TwoWay parameter when defining the {Binding} on a control, as follows: 11 Rebuild and run your application Update any of the fields, and leave the focus on the control You’ll see that the two-way binding is triggered, and the corresponding field is also updated, as shown in Figure 5-5 Figure 5-5 Two-way binding in action Congratulations! You have just created a Silverlight application that allows for two-way data binding We will now move on to look at data binding lists of data to the two list controls provided in Silverlight: DataGrid and ListBox 113 CHAPTER ■ DATA BINDING AND SILVERLIGHT LIST CONTROLS Element to Element Binding In addition to binding to data, elements can be bound directly to other elements, which can significantly improve the readability and efficiency of your code The syntax for binding to an element is very similar to binding to a data item, the only difference is that in the binding an ElementName is specified, which is very much like setting the ItemsSource to the Element As an example, if you wanted to bind the IsEnabled property of a control to a checkbox’s IsChecked property Assuming the checkbox is named EnableButton, the binding syntax would be the following IsEnabled="{Binding IsChecked, Mode=OneWay, ElementName=EnableButton}" Notice that the binding is the same as it would be when binding to a data source, except that we have added the ElementName=EnableButton Let’s try this out in an exercise Try It Out: Element to Element Binding To help explain element to element binding in Silverlight, let’s build a very simple application The application will include a button and a checkbox When the checkbox is checked, the button is enabled, when the checkbox is unchecked, the button is disabled Let’s get started Create a new Silverlight application in Visual Studio 2008 Name the project Ch5_ElementBinding, and allow Visual Studio to create a Web Site project to host your application Edit the MainPage.xaml file to add a StackPanel to the root Grid Place a ToggleButton and CheckBox named EnableButton within that StackPanel so the ToggleButton appears above the CheckBox Add a margin of 20 on the StackPanel and on the ToggleButton and CheckBox to add some spacing between the controls The code for the page follows: Next, we need to bind the ToggleButton’s IsEnabled property to the CheckBox’s IsChecked property We will this with one way binding as described earlier in this chapter, and we will set the ElementName to EnableButton, which is the name we gave our CheckBox The updated source code should now look like the following