Code Simplified – Viral Sarvaiya

Code Simplified – Viral Sarvaiya, Web Developer Friends, dot net Developer, Sql Server Developer

Simple Data Binding in Silverlight

Posted by Viral Sarvaiya on October 1, 2010

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.

1.  Create a new Silverlight application in Visual Studio 2010. Name the project BasicDataBinding, and allow Visual Studio to create a Web application project to host your application.

2. Edit the MainPage.xaml file to define two columns and six grid rows. Place a TextBlock in each row in column 1 and a TextBox in each row in column 2. Also add some margins and some alignment assignments to improve the layout.

The code for the page follows:


<Grid x:Name="LayoutRoot" Background="White">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
<RowDefinition />
<RowDefinition />
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<TextBlock Text="Book Title"
VerticalAlignment="Center"
Margin="5" />
<TextBlock Text="ISBN-13"
VerticalAlignment="Center"
Margin="5"
Grid.Row="1" />
<TextBox Text="{Binding Title}"
Height="24"
Margin="5"
Grid.Column="1" />
<TextBox Text="{Binding ISBN}"
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}"
Height="24"
Margin="5"
Grid.Column="1" Grid.Row="2" />
<TextBox Text="{Binding ISBN}"
Height="24"
Margin="5"
Grid.Column="1" Grid.Row="3" />

</Grid>

3. 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
{
public string Title { get; set; }
public string ISBN { get; set; }
}
}

4. 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 4: From Novice to Professional",
ISBN = "978-1430229889"
};
this.LayoutRoot.DataContext = b;
}

When you set up binding definitions for different controls, the controls do 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.

5. 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

6.  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.

7.  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.

Next, you can let Visual Studio do some more work for you. After adding the using System.ComponentModel statement, right-click INotifyPropertyChanged and select Implement Interface ➤ Implement Interface from the pop-up menu

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; }
public event PropertyChangedEventHandler PropertyChanged;
}

8. 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; }
public event PropertyChangedEventHandler PropertyChanged;
void FirePropertyChanged(string property)
{
if (PropertyChanged != null)
{
PropertyChanged(this,
new PropertyChangedEventArgs(property));
}
}
}

9. 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");
}
}
public event PropertyChangedEventHandler PropertyChanged;
void FirePropertyChanged(string property)
{
if (PropertyChanged != null)
{
PropertyChanged(this,
new PropertyChangedEventArgs(property));
}
}
}

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.

10. 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" />
<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" />

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,

Congratulations! You have just created a Silverlight application that allows for two-way data binding.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

 
%d bloggers like this: