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7. With the application running, change the book title in the first text box to just
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Beginning Silverlight 2, 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.
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8. 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.
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Figure 5-3. Visual Studio assists when you need to add the System.ComponentModel
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namespace.
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CHAPTER 5 DATA BINDING AND SILVERLIGHT LIST CONTROLS
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9. Next, you can let Visual Studio do 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
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Figure 5-4.
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Figure 5-4. Visual Studio also assists in implementing the INotifiyPropertyChanged
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interface.
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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 } 10. 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) { if (PropertyChanged != null)
CHAPTER 5 DATA BINDING AND SILVERLIGHT LIST CONTROLS
{ PropertyChanged(this, new PropertyChangedEventArgs(property)); } } public event PropertyChangedEventHandler PropertyChanged; #endregion } 11. 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
CHAPTER 5 DATA BINDING AND SILVERLIGHT LIST CONTROLS
{ _isbn = value; FirePropertyChanged("ISBN"); } } #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.
12. To change to two-way binding, add the Mode=TwoWay parameter when defining the {Binding} on a control, as follows: <TextBlock Text="Book Title" VerticalAlignment="Center" Margin="5" />
CHAPTER 5 DATA BINDING AND SILVERLIGHT LIST CONTROLS
<TextBlock Text="ISBN-13" VerticalAlignment="Center" Margin="5" Grid.Row="1" /> <TextBox Text="{Binding Title, Mode=TwoWay}" Height="24" Margin="5" Grid.Column="1" /> <TextBox Text="{Binding ISBN, Mode=TwoWay }" Height="24" Margin="5" Grid.Column="1" Grid.Row="1" /> <TextBlock Text="Book Title" VerticalAlignment="Center" Margin="5" Grid.Row="2" /> <TextBlock Text="ISBN-13" VerticalAlignment="Center" Margin="5" Grid.Row="3" /> <TextBox Text="{Binding Title, Mode=TwoWay }" Height="24" Margin="5" Grid.Column="1" Grid.Row="2" /> <TextBox Text="{Binding ISBN, Mode=TwoWay }" Height="24" Margin="5" Grid.Column="1" Grid.Row="3" /> 13. 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.
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