Xamarin.Forms Trigger Example

NumericValidationTrigger.cs

// Visual elements can react to events or property changes.
// Triggers can be thought of for "conditional styles".
// Types: Trigger, EventTrigger, DataTrigger, MultiTrigger

//EXAMPLE: hooking trigger to a property
//You have a trigger that fires when a property changes

//create the trigger
var trigger = new Trigger(typeof(Entry));
trigger.Property = Entry.IsFocusedProperty; // hooked to the "IsFocused" property!
trigger.Value = true; //if it's true...
trigger.Setters.Add(new Setter {Property = Entry.ScaleProperty, Value = 1.5});

//add the trigger to an element
var entry = new Entry{ Placeholder = "Enter name" };
entry.Triggers.Add(trigger);


//EXAMPLE: hooking trigger to an event
//Event triggers hook to events, instead of properties.

// You can add this to the OnTextChangedEvent of an Entry to turn text red if it's not numeric
public class NumericValidationTrigger : TriggerAction<Entry>
{
 protected override void Invoke(Entry entry)
 {
  double result;
  bool isValid = Double.TryParse(entry.Text, out result);
  entry.TextColor = isValid ? Color.Default : Color.Red;
 }
}

//DataTriggers can bind to OTHER controls, which is awesome for enabling buttons based on other controls' state.