Building Custom Forms - ASP.NET Dynamic Data - page 337

FieldValueChanged Event
The OnFieldValueChanged method is responsible for invoking the FieldValueChanged
event handlers, if any have been assigned, when the FieldValue property changes.
Following the standard .NET event implementation pattern, this method is declared as
virtual. This allows the descendants to handle the event by overriding the method, which
is more efficient than assigning an event handler.
NOTE
The FieldValueChanged event explicitly defines the add and remove blocks to take
advantage of the Events collection inherited from the ASP.NET Control class. This
collection is optimized for storing infrequently assigned event handlers and allows you
to store event handlers in a hash table instead taking space for each event in every
instance of the control. The FieldValueChangedEvent object is used as a hash key to
store and retrieve the FieldValueChanged event handlers, if any.
AutoPostBack Property
The AutoPostBack property of the field template encapsulates the AutoPostBack property
of the data entry controls. Consumers, such as the custom Region field template, typically
set it when they need to receive immediate notifications about field value changes.
Unfortunately, ASP.NET controls do not have a common base class that would allow
setting their AutoPostBack property generically. The individual field templates are
responsible for initializing their data controls based on this property.
Extending Existing Field Templates
To take advantage of the better encapsulation offered by the new UnleashedFieldTemplate
base class, you need to implement the following changes in each of the existing field
templates:
. Change the field template to inherit from the UnleashedFieldTemplate class.
. Set data control's AutoPostBack property value with the value from the
AutoPostBack property of the field template.
. Raise the FieldValueChanged event when data control's value changes.
. Update data control's value when FieldValue changes.
With the exception of the last one, these tasks are what the original version of the
Region_Edit field template had hard-coded. By implementing these changes, you are
encapsulating this logic inside of each general-purpose field template and allowing the
custom field template to access the information without taking dependencies on the inter-
nal implementation details of other templates. Listings 10.17 and 10.18 show the markup
and code-behind of the Text_Edit field template that has already been extended. The
changes (indicated by bold font) are discussed through the remainder of this section.