How to sublass TriggerAction?

You can install Microsoft.Xaml.Behaviors.Wpf in NuGet and create Enter as below:

 public class Enter : TriggerAction<DependencyObject>  
    {  
        public Enter()  
        {  
        }  
  
        protected override void Invoke(object parameter)  
        {  
            
        }  
    }  

But it may be impossible to use it like EnterActions = { new Enter() }. From the Microsoft Document TriggerBase.EnterActions Property, the EnterActions gets (no sets) a collection of TriggerAction objects to apply when the trigger object becomes active. This property does not apply to the EventTrigger class.
And when I assign value to EnterActions like ExitActions = { new Exit() } , the error shows that Property or indexer 'TriggerBase.EnterActions' cannot be assgined to --- it is read only. It may be impossibe to implement with ExitActions =....

You could add BeginStoryboard to EnterActions, I provide you a workaround as below:

void setTemplate()  
        {  
            var border = new FrameworkElementFactory(typeof(Border));  
            var path = new FrameworkElementFactory(typeof(Path)) { Name = "thePath" };  
            path.SetValue(Path.StretchProperty, Stretch.Uniform);  
            path.SetValue(Path.FillProperty, Brushes.Gray);  
            path.SetValue(Path.DataProperty, Geometry.Parse(icon));  
            border.SetValue(Border.BackgroundProperty, Brushes.Transparent);  
            border.AppendChild(path);  
  
            Trigger trigger = new Trigger()  
            {  
                Property = IsMouseOverProperty,  
                Value = true,  
                Setters = { new Setter() { Property = Path.FillProperty, Value = Brushes.Blue, TargetName = "thePath" } },  
            };  
  
            var enterAnimation = new ColorAnimation()  
            {  
                From = Colors.Blue,  
                To = Colors.LightPink,  
                Duration = new TimeSpan(0,0,1),  
                AutoReverse = true  
            };  
  
            var exitAnimation = new ColorAnimation()  
            {  
                From = Colors.LightPink,  
                To = Colors.Blue,  
                Duration = new TimeSpan(0,0,1),  
                AutoReverse=true  
            };  
  
            var propertyChain = new[]  
            {  
               Shape.FillProperty,  
               SolidColorBrush.ColorProperty  
            };  
            var propertyPath = new PropertyPath("(0).(1)", propertyChain);  
  
            Storyboard.SetTargetProperty(enterAnimation, propertyPath);  
            Storyboard.SetTargetProperty(exitAnimation, propertyPath);  
            Storyboard.SetTargetName(enterAnimation, "thePath");  
            Storyboard.SetTargetName(exitAnimation, "thePath");   
  
            var storyboard = new Storyboard();  
  
            storyboard.Children.Add(enterAnimation);  
            storyboard.Children.Add(exitAnimation);  
  
            var action = new BeginStoryboard();  
            action.Storyboard = storyboard;  
  
            trigger.EnterActions.Add(action);  
  
            Template = new ControlTemplate(typeof(RepeatButton))  
            {  
                VisualTree = border,  
                Triggers =  
                {  
                    trigger  
                }  
            };  
        }  

The result like:

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