Friday, 10 June 2016

Xamarin.Forms Behaviors: FadeAction

Previously, I mentioned that I’d added a number of new actions to the Behaviors Library for Xamarin.Forms, to support animations. The library has the notion of behaviors and actions. A behavior is attached to a control and listens for something to happen, such as an event firing. When the “something” happens, it triggers one or more actions, such as invoking a method or command. Actions are invoked by behaviors and executed on a selected control.

In this blog post, I’ll demonstrate using the EventHandlerBehavior and FadeAction classes to run a fade animation when an event occurs.

Fading a VisualElement when an Event Fires

The EventHandlerBehavior class listens for a specified event to occur, and executes one or more actions in response. It requires you to set an EventName property to the event that you want the behavior to listen to, and an Actions property to one or more actions that should be executed in response to the event firing. Note that the Actions property of the EventHandlerBehavior instance is set indirectly by creating the FadeAction instance as a child of the EventHandlerBehavior instance.

The FadeAction class performs a fade animation when invoked, and allows the following optional properties to be set:

  • TargetObject – an object that is the VisualElement on which to run the animation. If this value is omitted the object the behavior is attached to will be set as the target on which to run the animation.
  • FinalOpacity – a double that specifies the value of the Opacity property to fade to. If this value is omitted the default value of 1 is used.
  • Duration – an int that represents the length of the animation in milliseconds. If this value is omitted the default value of 250ms is used.
  • EasingFunction – an EasingFunction that specifies any velocity changes in the animation. If this value is omitted the default value of the linear easing function is used.
  • Await – a boolean that represents whether the animation should be awaited, or whether it should be allowed to complete in the background. If this value is omitted the default value of false is used.

The following code example shows an example of using the EventHandlerBehavior and FadeAction classes to fade in an Image control when a Button is clicked:

<Image x:Name="image" Source="monkey.png" Opacity="0" VerticalOptions="CenterAndExpand" /> <Button Text="Run Animation"> <Button.Behaviors> <behaviors:EventHandlerBehavior EventName="Clicked"> <behaviors:FadeAction TargetObject="{x:Reference image}" FinalOpacity="1.0" Duration="2000" Await="true" /> </behaviors:EventHandlerBehavior> </Button.Behaviors> </Button>

When the Button.Clicked event fires, the FadeAction fades in the Image instance over 2 seconds (2000 milliseconds), and the animation is awaited. This means that if there were subsequent animations defined, they would only execute after the fade animation has completed,

The advantage of using the FadeAction is that it’s possible to invoke animations through XAML, rather than having to use C#. In addition, when combined with behaviors, a fade animation can easily be invoked from XAML when a behavior occurs, such as an event firing, or a when a piece of data changes.

The sample application that this code comes from can be downloaded from GitHub.

Summary

The FadeAction class allows fade animations to be invoked through XAML when a behavior occurs, such as an event firing, or when a piece of data changes.

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