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In the example shown in Figure 8-5 (which binds TextBox.Text to TextBlock.FontSize), there s another quirk. As you change the displayed font size by typing in the text box, nothing happens. The change is not applied until you tab to another control. This behavior is different from the behavior you see with the slider control. With that control, the new font size is applied as you drag the slider thumb there s no need to tab away. To understand this difference, you need to take a closer look at the binding expressions used by these two controls. When you use OneWay or TwoWay binding, the changed value is propagated from the source to the target immediately. In the case of the slider, there s a one-way binding expression in the TextBlock. Thus, changes in the Slider.Value property are immediately applied to the TextBlock.FontSize property. The same behavior takes place in the text box example changes to the source (which is TextBlock.FontSize) affect the target (TextBox.Text) immediately. However, changes that flow in the reverse direction from the target to the source don t necessarily happen immediately. Instead, their behavior is governed by the Binding.UpdateSourceTrigger property, which takes one of the values listed in Table 8-2. When the text is taken from the text box and used to update the TextBlock.FontSize property, you re witnessing an example of a target-to-source update that uses the UpdateSourceTrigger.LostFocus behavior.

The source is updated immediately when the target property changes. The source is updated when the target property changes and the target loses focus. The source is not updated unless you call the BindingExpression.UpdateSource() method. The updating behavior is determined by the metadata of the target property (technically, its FrameworkPropertyMetadata.DefaultUpdateSourceTrigger property). For most properties, the default behavior is PropertyChanged, although the TextBox.Text property has a default behavior of LostFocus.

Tip If the CardSpace identity selector is displayed but then you are shown a dialog box that says Incoming

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Remember that the values in Table 8-2 have no effect over how the target is updated. They simply control how the source is updated in a TwoWay or OneWayToSource binding. With this knowledge, you can improve the text box example so that changes are applied to the font size as the user types in the text box. Here s how: <TextBox Text="{Binding ElementName=txtSampleText, Path=FontSize, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Name="txtFontSize"></TextBox>

Tip The default behavior of the TextBox.Text property is LostFocus, simply because the text in a text box will

change repeatedly as the user types, causing multiple refreshes. Depending on how the source control updates itself, the PropertyChanged update mode can make the application feel more sluggish. Additionally, it might cause the source object to refresh itself before an edit is complete, which can cause problems for validation.

policy failed validation, there s a good chance there s a spelling or capitalization issued in the ic:informationCard element.

For complete control over when the source object is updated, you can choose the UpdateSourceTrigger.Explicit mode. If you use this approach in the text box example, nothing happens when the text box loses focus. Instead, it s up to your code to manually trigger the update. For example, you could add an Apply button that calls the BindingExpression.UpdateSource() method, triggering an immediate refresh and updating the font size. Of course, before you can call BindingExpression.UpdateSource(), you need a way to get a BindingExpression object. A BindingExpression object is a slim package that wraps together two things: the Binding object you ve already learned about (provided through the BindingExpression.ParentBinding property) and the object that s being bound from the source (BindingExpression.DataItem). In addition, the BindingExpression object provides two methods for triggering an immediate update for one part of the binding: UpdateSource() and UpdateTarget(). To get a BindingExpression object, you use the GetBindingExpression() method, which every element inherits from the base FrameworkElement class, and pass in the target property that has the binding. Here s an example that changes the font size in the TextBlock based on the current text in the text box: // Get the binding that's applied to the text box. BindingExpression binding = txtFontSize.GetBindingExpression(TextBox.TextProperty); // Update the linked source (the TextBlock). binding.UpdateSource();

In .NET, events are first-class citizens: they are represented by delegates and declared with the event keyword (see Listing 5 1). Listing 5 1. Declaring a Plain Event in .NET delegate void MyEventHandler(); public event MyEventHandler EventOccurred; The code in Listing 5 1 declares a delegate that both accepts and returns no values and then declares an event that requires a receiver that matches that delegate. Events can be subscribed to using the += operator, and unsubscribed with the -= operator, as in Listing 5 2. Listing 5 2. Subscribing and Unsubscribing to an Event MyEventHandler eventHandler = new MyEventHandler(this.Handler); EventOccurred += eventHandler; EventOccurred -= eventHandler; The event can then be fired by calling it as if it were a method. However, if there are no subscribers to the event it will be null, so this must be tested for first, as shown in Listing 5 3. Listing 5 3. Firing an Event if(EventOccurred != null) { EventOccurred(); }

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