Setting Entry Behaviors using Style attribute - xaml

Question

I have defined my style as such:
<ContentView.Resources>
<ResourceDictionary>
<Style TargetType="Entry" x:Key="IntegralEntryBehavior">
<Setter Property="Behaviors" Value="valid:EntryIntegerValidationBehavior"/>
</Style>
</ResourceDictionary>
</ContentView.Resources>
And multiple similar Entries:
<StackLayout Grid.Column="0" Grid.Row="0">
<Entry Style="{StaticResource IntegralEntryBehavior}"/>
</StackLayout>
If I define Entry behavior like this, I get an error, that Entry.Behaviors property is readonly, but it's possible to define behavior without using Style attribute inside Entry as such:
<Entry.Behaviors>
<valid:EntryIntegerValidationBehavior/>
</Entry.Behaviors>
What is the difference between these approaches and why does only the second one work? Is it possible to modify the first approach to make it work? I'm looking for a shorter way to define this behavior for each entry than the second option.

Answer 1

You can checkout the example here:
https://learn.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/behaviors/creating#consuming-a-xamarinforms-behavior-with-a-style
Basically, add an attached property to your behavior and then set the style setter's property to that attached property. The attached property handles adding itself to the Entry that you attach it to.
public class EntryIntegerValidationBehavior : Behavior<Entry>
{
public static readonly BindableProperty AttachBehaviorProperty =
BindableProperty.CreateAttached ("AttachBehavior", typeof(bool), typeof(EntryIntegerValidationBehavior), false, propertyChanged: OnAttachBehaviorChanged);
public static bool GetAttachBehavior (BindableObject view)
{
return (bool)view.GetValue (AttachBehaviorProperty);
}
public static void SetAttachBehavior (BindableObject view, bool value)
{
view.SetValue (AttachBehaviorProperty, value);
}
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.Behaviors.Add (new EntryIntegerValidationBehavior ());
} else {
var toRemove = entry.Behaviors.FirstOrDefault (b => b is EntryIntegerValidationBehavior);
if (toRemove != null) {
entry.Behaviors.Remove (toRemove);
}
}
}
// Actual behavior code here
}
Finally edit your style to look like this:
<Style TargetType="Entry" x:Key="IntegralEntryBehavior">
<Setter Property="valid:EntryIntegerValidationBehavior.AttachBehavior" Value="true"/>
</Style>