Xamarin.Forms 自定义 TapGestureRecognizer 附加属性

While creating Xamarin.Forms applications, definitely you are going to need TapGestureRecognizer often. Implementing it in XAML many times may end up with a lot of unnecessary code. Let’s take a look at that simple clickable Image:

 <Image Source="img.png">
   <Image.GestureRecognizers>
     <TapGestureRecognizer Command="{Binding CloseCommand}" CommandParamter="{Binding .}" />
   </Image.GestureRecognizers>
 </Image>

This is a lot of lines, especially when you have to add many clickable controls. However, we can do it better and put everything into a single line using our custom attached property:

XHTML

xmlns:ex="clr-namespace:MyApp.Extensions;assembly=MyApp"
...
<Image Source="img.png" ex:Gestures.TapCommand="{ex:Command CloseCommand, Parameter={Binding .}}"/>

xmlns:ex="clr-namespace:MyApp.Extensions;assembly=MyApp"
...
<Image Source="img.png" ex:Gestures.TapCommand="{ex:Command CloseCommand, Parameter={Binding .}}"/>

Implementation

 using System;
 using Xamarin.Forms;
 using Xamarin.Forms.Xaml;
 using System.Windows.Input;

 namespace MyApp.Extensions
 {
     [ContentProperty("Command")]
     public class CommandExtension : BindableObject, IMarkupExtension
     {
         public string Command { get; set; }

         // Binding source for Command
         public object Source { get; set; }

         // CommandParameter
         public object Parameter { get; set; }

         public object ProvideValue(IServiceProvider serviceProvider)
         {
             return this;
         }
     }
 }

 using Xamarin.Forms;
 using System.Linq;

 namespace MyApp.Extensions
 {
     public class Gestures
     {
         #region TapCommand

         public static readonly BindableProperty TapCommandProperty =
             BindableProperty.CreateAttached<Gestures, CommandExtension>(
               x => (CommandExtension)x.GetValue(Gestures.TapCommandProperty),
               null, BindingMode.OneWay, propertyChanged: TapCommandChanged);

         #endregion

         private static void TapCommandChanged(BindableObject source, CommandExtension oldVal,
                                               CommandExtension newVal)
         {
             var tapGesture = new TapGestureRecognizer();

             // Set command
             var commandBinding = new Binding(newVal.Command, source: newVal.Source);
             tapGesture.SetBinding(TapGestureRecognizer.CommandProperty, commandBinding);

             // Set command parameter
             if (newVal.Parameter is Binding)
             {
                 tapGesture.SetBinding(TapGestureRecognizer.CommandParameterProperty,
                                       newVal.Parameter as Binding);
             }
             else
             {
                 tapGesture.CommandParameter = newVal.Parameter;
             }

             // Remove old TapGestureRecognizer
             var view = source as View;
             var toRemove = view.GestureRecognizers.OfType<TapGestureRecognizer>().FirstOrDefault();
             view.GestureRecognizers.Remove(toRemove);

             // Add new one
             view.GestureRecognizers.Add(tapGesture);
         }
     }
 }