April 21, 2019

How to add Effects in Xamarin Forms

Effects allow the changing of platform specific properties on a native control without the need for a Custom Renderer. They are typically used for changing styling of a property when the property has not been exposed via the Xamarin Forms Control.

  • Effects are designed to be reusable
  • Need a custom renderer if you want to override methods
  • Need a custom renderer if you want to replace the control used on the platform

Documentation: Introduction to Effects

Example Use Case: Changing the color of the placeholder text in an Entry control.

First on the native platform, in this example iOS, we create a Platform Effect.

[assembly:ResolutionGroupName ("Xamarin")]

[assembly:ExportEffect (typeof(PlaceholderEffect), “PlaceholderEffect”)]

namespace Mobile.iOS { public class PlaceholderEffect : PlatformEffect<UIView, UITextField> { protected override void OnAttached() { Control.AttributedPlaceholder = new NSAttributedString(Control.Placeholder, null, UIColor.Red); } protected override void OnDetached() { } protected override void OnElementPropertyChanged(PropertyChangedEventArgs args) { base.OnElementPropertyChanged(args); // You can do effects only when certain properties change here. } } }

Next if we are in a PCL we need to add a RoutingEffect so XAML in the PCL can reference the platform specific implementation.

public class PlaceholderEffect : RoutingEffect
    public PlaceholderEffect () : base ("Xamarin.PlaceholderEffect")

Now we can add the effect to the control.

// Add this to your page attributes (change Mobile.Effects to whatever full namespace your placeholder effect is in)

<Entry Text="{Binding EntryField}">
        <local:PlaceholderEffect />

https://docs.microsoft.com – https://xamarinhelp.com – https://blog.xamarin.com/

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