[Xamarin] 将任意事件绑定到命令—EventToCommand

虽然在使用MVVM模式开发的时候，一般都使用Command来绑定事件处理程序，但并不是所有的事件都可以绑定到Command的，这样一来就造成了代码中事件和命令混用的情况

通过附加Behavior可以完成将事件转化为命令的功能

首先定义一个Behavior基类

public class BehaviorBase<T> : Behavior<T> where T : BindableObject
{
    /// <summary>
    /// 附加对象
    /// </summary>
    public T AssociatedObject { get; private set; }

    protected override void OnAttachedTo(T bindable)
    {
        base.OnAttachedTo(bindable);
        AssociatedObject = bindable;

        if (bindable.BindingContext != null)
        {
            BindingContext = bindable.BindingContext;
        }

        bindable.BindingContextChanged += OnBindingContextChanged;
    }

    protected override void OnDetachingFrom(T bindable)
    {
        base.OnDetachingFrom(bindable);
        bindable.BindingContextChanged -= OnBindingContextChanged;
        AssociatedObject = null;
    }

    private void OnBindingContextChanged(object sender, EventArgs e)
    {
        OnBindingContextChanged();
    }

    protected override void OnBindingContextChanged()
    {
        base.OnBindingContextChanged();
        BindingContext = AssociatedObject.BindingContext;
    }
}

接下来实现EventToCommandBehavior

public class EventToCommandBehavior : BehaviorBase<View>
    {
        private Delegate eventHandler;

        public static readonly BindableProperty EventNameProperty = BindableProperty.Create("EventName", typeof(string), typeof(EventToCommandBehavior), null, propertyChanged: OnEventNameChanged);
        public static readonly BindableProperty CommandProperty = BindableProperty.Create("Command", typeof(ICommand), typeof(EventToCommandBehavior), null);
        public static readonly BindableProperty CommandParameterProperty = BindableProperty.Create("CommandParameter", typeof(object), typeof(EventToCommandBehavior), null);
        public static readonly BindableProperty InputConverterProperty = BindableProperty.Create("Converter", typeof(IValueConverter), typeof(EventToCommandBehavior), null);

        public string EventName
        {
            get { return (string)GetValue(EventNameProperty); }
            set { SetValue(EventNameProperty, value); }
        }

        public ICommand Command
        {
            get { return (ICommand)GetValue(CommandProperty); }
            set { SetValue(CommandProperty, value); }
        }

        public object CommandParameter
        {
            get { return GetValue(CommandParameterProperty); }
            set { SetValue(CommandParameterProperty, value); }
        }

        public IValueConverter Converter
        {
            get { return (IValueConverter)GetValue(InputConverterProperty); }
            set { SetValue(InputConverterProperty, value); }
        }

        protected override void OnAttachedTo(View bindable)
        {
            base.OnAttachedTo(bindable);
            RegisterEvent(EventName);
        }

        protected override void OnDetachingFrom(View bindable)
        {
            DeregisterEvent(EventName);
            base.OnDetachingFrom(bindable);
        }

        private void RegisterEvent(string name)
        {
            if (string.IsNullOrWhiteSpace(name))
            {
                return;
            }

            EventInfo eventInfo = AssociatedObject.GetType().GetRuntimeEvent(name);
            if (eventInfo == null)
            {
                throw new ArgumentException(string.Format("EventToCommandBehavior: Can't register the '{0}' event.", EventName));
            }
            MethodInfo methodInfo = typeof(EventToCommandBehavior).GetTypeInfo().GetDeclaredMethod("OnEvent");
            eventHandler = methodInfo.CreateDelegate(eventInfo.EventHandlerType, this);
            eventInfo.AddEventHandler(AssociatedObject, eventHandler);
        }

        private void DeregisterEvent(string name)
        {
            if (string.IsNullOrWhiteSpace(name))
            {
                return;
            }

            if (eventHandler == null)
            {
                return;
            }
            EventInfo eventInfo = AssociatedObject.GetType().GetRuntimeEvent(name);
            if (eventInfo == null)
            {
                throw new ArgumentException(string.Format("EventToCommandBehavior: Can't de-register the '{0}' event.", EventName));
            }
            eventInfo.RemoveEventHandler(AssociatedObject, eventHandler);
            eventHandler = null;
        }

        /// <summary>
        /// 事件处理程序，使用委托调用
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="eventArgs"></param>
        private void OnEvent(object sender, object eventArgs)
        {
            if (Command == null)
            {
                return;
            }

            object resolvedParameter;
            if (CommandParameter != null)
            {
                resolvedParameter = CommandParameter;
            }
            else if (Converter != null)
            {
                resolvedParameter = Converter.Convert(eventArgs, typeof(object), null, null);
            }
            else
            {
                resolvedParameter = eventArgs;
            }

            if (Command.CanExecute(resolvedParameter))
            {
                Command.Execute(resolvedParameter);
            }
        }

        private static void OnEventNameChanged(BindableObject bindable, object oldValue, object newValue)
        {
            var behavior = (EventToCommandBehavior)bindable;
            if (behavior.AssociatedObject == null)
            {
                return;
            }

            string oldEventName = (string)oldValue;
            string newEventName = (string)newValue;

            behavior.DeregisterEvent(oldEventName);
            behavior.RegisterEvent(newEventName);
        }
    }

在xaml中使用

<CheckBox x:Name="SingleCheckBox" Grid.Column="0" IsChecked="{Binding IsChoose}">
    <CheckBox.Behaviors>
        <local:EventToCommandBehavior 
            EventName="CheckedChanged" 
            Command="{Binding Source = {x:Reference ShoppingCar},Path=BindingContext.SelectSingle}" 
            CommandParameter="{Binding Source={x:Reference SingleCheckBox}, Path=IsChecked}">
        </local:EventToCommandBehavior>
    </CheckBox.Behaviors>
</CheckBox>