doc's Attached behavior vs doc's attached property? - xamarin

Question

I want to apply some validation on the entry input, I went to the docs page of the attached behaviors
and did this:
public enum TextType { Email, Phone, }
public static class Validator
{
public static readonly BindableProperty TextTypeProperty = BindableProperty.CreateAttached(
"TextType", typeof(TextType), typeof(Validator), TextType.Email, propertyChanged: ValidateText);
public static TextType GetTextType(BindableObject view)
{
return (TextType)view.GetValue(TextTypeProperty);
}
public static void SetTextType(BindableObject view, TextType textType)
{
view.SetValue(TextTypeProperty, textType);
}
private static void ValidateText(BindableObject bindable, object oldValue, object newValue)
{
var entry = bindable as Entry;
entry.TextChanged += Entry_TextChanged;
}
private static void Entry_TextChanged(object sender, TextChangedEventArgs e)
{
var entry = sender as Entry;
bool isValid = false;
switch (GetTextType(sender as Entry))
{
case TextType.Email:
isValid = e.NewTextValue.Contains("#");
break;
case TextType.Phone:
isValid = Regex.IsMatch(e.NewTextValue, #"^\d+$");
break;
default:
break;
}
if (isValid)
entry.TextColor = Color.Default;
else
entry.TextColor = Color.Red;
}
}
in XAML:
<Entry beh:Validator.TextType="Email" Placeholder="Validate Email"/>
but it doesn't work, the setter nor the propertyChanged call back are never called,
also what is the difference between this "Attached behavior" and the attached property, the two pages are pretty identical

Answer 1

what does the logic have to do with the propertyChanged method is not called?
From this MSDN Attached Behaviors,you can see that An attached property can define a propertyChanged delegate that will be executed when the value of the property changes.
According to your code, you set TextType=TextType.Email firstly, then you also set
<Entry beh:Validator.TextType="Email" Placeholder="Validate Email"/>
Validator.TextType="Email", the attached property doesn't change, so PropertyChanged method is not call.
You can modify your code like this, then you will find the propertychanged will be called.
<Entry beh:Validator.TextType="Phone" Placeholder="Validate Email"/>

Answer 2

Comparing to the sample you ignore most of the logic from OnAttachBehaviorChanged in your ValidateText and it is quite necessary:
static void OnAttachBehaviorChanged (BindableObject view, object oldValue, object newValue)
{
var entry = view as Entry;
if (entry == null) {
return;
}
bool attachBehavior = (bool)newValue;
if (attachBehavior) {
entry.TextChanged += OnEntryTextChanged;
} else {
entry.TextChanged -= OnEntryTextChanged;
}
}
Considering the differences, attached properties are something that is applied to a specific control, you can apply behavior technically to any control though probably it will work only on some of them but it may work on multiple control types properly.