我正在使用“事件到命令”行为here，在ListView的ItemTapped事件中实现命令。我将EventToCommandBehavior和BehaviorBase类复制到了我的项目中。
这是我的观点
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:d="http://xamarin.com/schemas/2014/forms/design"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:AppVentas.Behaviors"
mc:Ignorable="d"
x:Class="AppVentas.Views.UsuariosView">
<ContentPage.Content>
<ListView HasUnevenRows="True"
ItemsSource="{Binding Usuarios}"
ItemTapped="ListView_ItemTapped">
<ListView.Behaviors>
<local:EventToCommandBehavior
EventName="ItemTapped"
Command="{Binding OpenChatCommand}"/>
</ListView.Behaviors>
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<Label Text="{Binding Nombre}"/>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</ContentPage.Content>
还有我的ViewModel
public class UsuariosViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public ObservableCollection<Usuarios> Usuarios { get; set; }
public ICommand OpenChatCommand { get; set; }
private UsuarioController usuarioController = new UsuarioController();
public UsuariosViewModel()
{
Usuarios = new ObservableCollection<Usuarios>(usuarioController.Get().Where(i => i.Token == null));
OpenChatCommand = new Command<Usuarios>(OpenChat);
}
void OpenChat(Usuarios usuario)
{
//trying to do something
}
}
问题是OpenChatCommand永远不会执行，EventToCommandBehavior类的方法OnEvent会执行，但是Command.Execute（resolvedParameter）行；只是什么也没做。
如果正在使用，我正在使用PropertyChanged.Fody包。
感谢您的帮助。
答案 0 :(得分：0)
假设您的bindingcontext正确设置为UsuariosViewModel的实例，那么我在这里看到的问题是您没有传递命令参数。您的命令接受Usuarios，但是您需要通过EventToCommandBehavior上的CommandParameter属性将其传递给用户。
我还注意到您已经定义了ListView_ItemTapped，但是我不确定您在该方法中正在做什么。不需要使该命令起作用，但是也许您正在将其用于其他用途。
答案 1 :(得分：0)
从Reusable EventToCommandBehavior开始，我建议您可以在代码中使用Converter，我创建的代码很简单，您可以看一下：
<ContentPage
x:Class="Listviewsample.MainPage"
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:converter="clr-namespace:Listviewsample"
xmlns:d="http://xamarin.com/schemas/2014/forms/design"
xmlns:local="clr-namespace:Listviewsample.Behaviors"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">
<ContentPage.Resources>
<converter:TappedItemConverter x:Key="converter1" />
</ContentPage.Resources>
<StackLayout>
<!-- Place new controls here -->
<ListView
x:Name="listview1"
HasUnevenRows="True"
ItemsSource="{Binding images}">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<StackLayout>
<Label Text="{Binding title}" />
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
<ListView.Behaviors>
<local:EventToCommandBehavior
Command="{Binding command1}"
Converter="{StaticResource converter1}"
EventName="ItemTapped" />
</ListView.Behaviors>
</ListView>
</StackLayout>
public partial class MainPage : ContentPage
{
public ObservableCollection<imagemodel> images { get; set; }
public Command command1 { get; set; }
public MainPage()
{
InitializeComponent();
images = new ObservableCollection<imagemodel>()
{
new imagemodel(){title="image 1"},
new imagemodel(){title="image 2"},
new imagemodel(){title="image 3"}
};
command1 = new Command<imagemodel>(commandpage);
this.BindingContext = this;
}
private void commandpage(imagemodel m)
{
Console.WriteLine("the image model title is {0}",m.title.ToString());
}
}
public class imagemodel
{
public string title { get; set; }
}
BehaviorBase.cs：
public class BehaviorBase<T> : Behavior<T> where T : BindableObject
{
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;
}
void OnBindingContextChanged(object sender, EventArgs e)
{
OnBindingContextChanged();
}
protected override void OnBindingContextChanged()
{
base.OnBindingContextChanged();
BindingContext = AssociatedObject.BindingContext;
}
}
EventToCommandBehavior.cs：
public class EventToCommandBehavior : BehaviorBase<View>
{
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);
}
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);
}
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;
}
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);
}
}
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);
}
}
Converter.cs，行为的Command属性是绑定到关联的ViewModel的command1属性的数据，而Converter属性设置为TappedItemConverter实例，该实例从ItemTappedEventArgs返回ListView的Tapped项。 / p>
public class TappedItemConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
var eventArgs = value as ItemTappedEventArgs;
return eventArgs.Item;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
这是github上的示例，您可以看一下：