Supported versions: 2.0.0-pre1 or higher Supported platforms: WPF, Xamarin.Forms
Trigger is a Xaml concept. Triggers apply values or invoke actions depending on conditions. There are 3 types
Property-triggers apply values if a dependency-property of a control matches a given value.
I.e. if you want to gray-out a button if it is not enabled, you can define a property-trigger as followed:
Button {
@Property IsEnabled false {
BackgroundColor: #dddddd;
}
}
Data-Triggers apply values if a value of a property on the binding-context (aka. data-context) matches a given value.
I.e. if you want to want to make the background of a Entry red if the validation fails, define a data-trigger as followed:
Entry {
@Data IsValid false {
BackgroundColor: Red;
}
}
Event-Triggers invoke trigger-actions if an event is raised.
In Xamarin.Forms, trigger-actions can be defined by subclassing TriggerAction<>.
In WPF only predefined trigger-actions are available, like BeginStoryboard.
Parameters are provided as a block and written like normal style-definition blocks. It follows the pattern <property-name>: <property-value>.
Button {
@Event Clicked {
ToColorTriggerAction: {
To: Red;
Duration 5000;
}
}
}
ToColorTriggerAction is a custom TriggerAction - see ToColorTriggerAction
Property- and data-triggers support enter/exit actions.
Enter actions are executed if the condition on the trigger is first fulfilled. Exit actions on the other hand are executed if a previous matching condition is no longer true.
Let's take the previous data-trigger and write it with enter/exit actions:
Entry {
@Data IsValid false {
@Enter: {
ToColorTriggerAction: {
To: Red;
Duration 1000;
}
}
@Exit: {
ToColorTriggerAction: {
To: Green;
Duration 1000;
}
}
}
}
Here the ToColorTriggerAction code used in this page:
public class ToColorTriggerAction : TriggerAction<VisualElement>
{
public static Task<bool> ColorTo(VisualElement self, Color fromColor, Color toColor, Action<Color> callback, uint length = 250, Easing easing = null)
{
Func<double, Color> transform = (t) =>
Color.FromRgba(fromColor.R + t * (toColor.R - fromColor.R),
fromColor.G + t * (toColor.G - fromColor.G),
fromColor.B + t * (toColor.B - fromColor.B),
fromColor.A + t * (toColor.A - fromColor.A));
return ColorAnimation(self, "ToColorTriggerAction", transform, callback, length, easing);
}
static Task<bool> ColorAnimation(VisualElement element, string name, Func<double, Color> transform, Action<Color> callback, uint length, Easing easing)
{
easing = easing ?? Easing.Linear;
var taskCompletionSource = new TaskCompletionSource<bool>();
if (element.AnimationIsRunning("ToColorTriggerAction"))
{
element.AbortAnimation("ToColorTriggerAction");
}
element.Animate<Color>(name, transform, callback, 16, length, easing, (v, c) => taskCompletionSource.SetResult(c));
return taskCompletionSource.Task;
}
public Color To { get; set; }
public uint Duration { get; set; }
protected override void Invoke(VisualElement entry)
{
ColorTo(entry, entry.BackgroundColor, To, c => entry.BackgroundColor = c, Duration, Easing.Linear);
}
}
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