SILVERLIGHT CONCEPTS in VB.NET

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SILVERLIGHT CONCEPTS
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public static readonly DependencyProperty WidthProperty; By convention, dependency properties end with the word Property, and this is adhered to throughout Silverlight. Notice that it is marked public while this is also a convention, there is no compelling reason to not expose it publicly. The dependency property should be just as visible as the .NET property wrapper. The .NET property provides a shortcut, hiding the fact that there is an underlying dependency property, since it wraps the calls to GetValue and SetValue. public double Width { get { return (double) this.GetValue(WidthProperty); } set { base.SetValue(WidthProperty, value); } } Simply declaring the dependency property is not enough it must be registered with the dependency property system using the DependencyProperty.Register static method. The Register method takes the following parameters: public static DependencyProperty Register( string name, Type propertyType, Type ownerType, PropertyMetadata typeMetadata Although you won t do much with it for now, let s create a new dependency property named TextSize in the MainPage.xaml.cs file. Add the following code to the class: public static readonly DependencyProperty TextSizeProperty = DependencyProperty.Register("TextSize", typeof(double), typeof(MainPage), new PropertyMetadata(new PropertyChangedCallback(onTextSizeChanged))); public double TextSize { get { return ((double)this.GetValue(TextSizeProperty)); } set { this.SetValue(TextSizeProperty, value); } } The name of the dependency property (passed as the first parameter to Register) does not need to have Property appended to it this convention holds only for the actual field name in the class. Now you have a new dependency property that can be used for data binding or any of the other various sources that can modify dependency property values. There is one other useful aspect of dependency properties: property change notifications. This ability to capture property changes is vital for validating a property value at the last possible moment. This is useful for scenarios such as a progress bar, where there is a clear minimum and maximum value, and values greater than or less than these values should be constrained to their respective endpoints.
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SILVERLIGHT CONCEPTS
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The final parameter to the Register method is where you specify a handler for the property change notification. Here s a handler for constraining TextSizeProperty to no larger than 36: private static void onTextSizeChanged(DependencyObject source, DependencyPropertyChangedEventArgs e) { if (((double)source.GetValue(e.Property)) > 36.0) { source.SetValue(e.Property, 36.0); } }
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Note A callback for property changes is the perfect place to validate and constrain dependency property values. It is also a great place to hold logic for modifying dependent properties, so when one changes, it affects other dependency property values of the DependencyObject that contains the properties.
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The first parameter is the instance of DependencyObject this is what you use to retrieve and set the value for the property. The Property member of the DependencyPropertyChangedEventArgs for this handler is then used as a parameter to GetValue and SetValue. If you try setting the value of the TextSize property to higher than 36 and then display its value, you will see it goes no higher than 36.
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Attached Properties
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An attached property is a special type of dependency property. Attached properties provide a way to assign values to properties on objects that do not actually have the property the attached property values are generally used by parent objects in the element hierarchy. You have already seen several attached properties. Let s look again at the XAML code used to create header text for the login screen: <TextBlock HorizontalAlignment="Center" Text="Please enter your information" Grid.Column="0" Grid.Row="0" Grid.ColumnSpan="2"/> The Grid class defines several attached properties, including Column, Row, and ColumnSpan, which are used by the TextBlock object. If you look up the TextBlock object on MSDN, you won t find anything close to Grid.Row or Grid.Column properties. This is because Column, Row, and ColumnSpan are defined as attached properties on the Grid class. The Grid class defines a total of four attached properties: Column, Row, ColumnSpan, and RowSpan. The dotted syntax is used to specify the class that does provide these dependency properties. By using this syntax, it is possible to attach arbitrary properties to objects that do not have them. The attached properties for the Grid layout control provide a way for child elements to specify where they should be located in the grid. You can identify the attached properties by looking for an Attached Properties section in the MSDN documentation for a particular class. If you attempt to use a random dependency property as an attached property, the parser will throw an exception. Registering an attached property is accomplished in a similar fashion to normal dependency properties but uses RegisterAttached instead of Register.
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