Triggers

Triggers allow you to set properties in response to certain conditions or events.

Property Triggers

A property trigger sets a property in response to a change in the value of another property.

Here is an example of using a property trigger to change the background color and text color of an Entry element when it gets focused:

using FmgLib:mauiMarkup;

public class PropertyTriggerPage : ContentPage
{
    public PropertyTriggerPage()
    {
	    this
        .Resources(
	        new ResourceDictionary
	        {
	            new Style<Entry>
	            {
	                Entry.BackgroundColorProperty.Set(Colors.Black),
	                Entry.TextColorProperty.Set(Colors.White),

	                new Trigger(typeof(Entry))
	                    .Property(Entry.IsFocusedProperty)
	                    .Value(true)
	                    .Setters(
	                        new Setters<Entry>(e => e
	                            .BackgroundColor(Colors.Yellow)
	                            .TextColor(Colors.Black))
	                    ),
	            }
	        }
        )
        .Content(
			new StackLayout()
			.Children(
				new Entry().Text("Enter name"),
	            new Entry().Text("Enter password"),
	            new Entry().Text("Enter address")
			)
		);
    }
}

Data Triggers

A data trigger sets a property in response to a change in the value of a data source.

Here is an example of using a data trigger to disable a button if the text length of an Entry element is zero:

this.Content(new StackLayout()
	.Children(
	    new Entry().Assign(out var entry).Text("Enter text..."),
	    new Button()
		    .Text("Save")
	        .Triggers(
	            new DataTrigger(typeof(Button))
	                .Binding(e => e.Path("Text.Length").Source(entry))
	                .Value(0)
	                .Setters(new Setters<Entry>(e => e.IsEnabled(false)))
	        ),
	)
)

Event Triggers

An event trigger sets a property in response to an event.

Here is an example of using an event trigger to validate the input of an Entry element as a number:

this
.Content(new StackLayout()
	.Children(
	    new Entry()
		    .Assign(out var entry)
		    .Text("Enter text...")
	        .Triggers(
	            new EventTrigger()
	                .Event("TextChanged")
	                .Actions(new NumericValidationTriggerAction())
	        )
	)
)

And here is the definition of the NumericValidationTriggerAction class:

public class NumericValidationTriggerAction : TriggerAction<Entry>
{
    protected override void Invoke(Entry entry)
    {
        double result;
        bool isValid = Double.TryParse(entry.Text, out result);
        entry.TextColor = isValid ? Colors.Black : Colors.Red;
    }
}

And here is the definition of the NumericValidationTriggerAction class:

public class NumericValidationTriggerAction : TriggerAction<Entry>
{
    protected override void Invoke(Entry entry)
    {
        double result;
        bool isValid = Double.TryParse(entry.Text, out result);
        entry.TextColor = isValid ? Colors.Black : Colors.Red;
    }
}