Building Custom Forms - ASP.NET Dynamic Data - page 340

Updating Data Control When FieldValue Changes
Template's FieldValue property can also change because of something outside of the field
template, perhaps another custom field template. You could imagine a custom ZipCode
field template that in response to the user entering a valid U.S. zip code could look up the
name of the city it represents and change the value displayed by the general-purpose
Text_Edit field template for the City field.
The field template ( Text_Edit ) can detect when the FieldValue property changes by
overriding the OnFieldValueChanged method, as shown in Listing 10.18. Notice how
this overridden method sets the Text property of the TextBox control using the
FieldValueEditString property of the template. As discussed in Chapter 3, “Field
Templates,” this property returns the FieldValue formatted appropriately for Edit mode.
This property is used in the template markup to provide the initial value for the Text
property at the time of data binding, and the overridden OnFieldValueChanged method
needs to be consistent with this logic.
Taking Advantage of the Extended Field Templates
With the new version of the general-purpose Text_Edit field template, you can now
modify the custom Region_Edit field template shown back in Listing 10.15 so that it no
longer depends on the Country data control being a TextBox . In the next example, its
OnLoad method was modified to initialize the AutoPostBack property of the template and
assign its FieldValueChanged event:
private UnleashedFieldTemplate countryFieldTemplate;
protected override void OnLoad(EventArgs e)
{
base.OnLoad(e);
this.countryFieldTemplate = this.FindOtherFieldTemplate(“Country”);
this.countryFieldTemplate.AutoPostBack = true;
this.countryFieldTemplate.FieldValueChanged +=
CountryFieldTemplate_FieldValueChanged;
}
The FieldValueChanged event handler, which is shown next, replaces the TextChanged
event handler from the previous version of the Region_Edit field template, which had to
extract the current field value directly from the TextBox data control. Now that the
FieldValueEditString property of the Country field template became available during
post-backs, this method uses it to update the list of regions in the drop-down list. Notice
that this is the same way the drop-down list is initially populated in the OnDataBinding
method when the page is generated for the first time:
private void CountryFieldTemplate_FieldValueChanged(
object sender, EventArgs e)
{
string country = this.countryFieldTemplate.FieldValueEditString;
this.PopulateRegionList(country);
}