Question

以下是来自我的XAML视图的onload事件触发器。

<interactivity:Interaction.Behaviors>
    <core:EventTriggerBehavior EventName="Loaded">
        <core:EventTriggerBehavior.Actions>
            <core:InvokeCommandAction Command="{Binding LoadedCommand}" />
        </core:EventTriggerBehavior.Actions>
    </core:EventTriggerBehavior>
</interactivity:Interaction.Behaviors>
<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <Grid.RowDefinitions>
        <RowDefinition Height="*" />
        <RowDefinition Height="100" />
    </Grid.RowDefinitions>
    <GridView ItemsSource="{Binding Appointments}" IsItemClickEnabled="True">
        <GridView.ItemTemplate>
            <DataTemplate x:DataType="vm:Appointment">
                <StackPanel Orientation="Vertical" HorizontalAlignment="Center">
                    <StackPanel Margin="20,20,0,0">
                        <TextBlock FontSize="18" Text="{x:Bind Name}" VerticalAlignment="Bottom"></TextBlock>
                        <TextBlock FontSize="10" Text="{x:Bind Category }" VerticalAlignment="Bottom"></TextBlock>
                    </StackPanel>
                </StackPanel>
            </DataTemplate>
        </GridView.ItemTemplate>
    </GridView>
    <TextBlock Grid.Row="1" Name="ResultTextBlock" FontSize="24" Foreground="Red" FontWeight="Bold" />
</Grid>

这里的命令实现。

 public class DelegateCommand : ICommand
{
    private readonly Predicate<object> _canExecute;
    private readonly Action<object> _execute;

    public event EventHandler CanExecuteChanged;

    public DelegateCommand(Action<object> execute)
               : this(execute, null)
    {
    }

    public DelegateCommand(Action<object> execute,
               Predicate<object> canExecute)
    {
        _execute = execute;
        _canExecute = canExecute;
    }


    public void RaiseCanExecuteChanged()
    {
        if (CanExecuteChanged != null)
        {
            CanExecuteChanged(this, EventArgs.Empty);
        }
    }

    public bool CanExecute(object parameter)
    {
        if (_canExecute == null)
        {
            return true;
        }

        return _canExecute(parameter);
    }

    public void Execute(object parameter)
    {
        _execute(parameter);
    }

}

命令未执行。感谢您的帮助。

我尝试了像这样的viewmodel.View模型有Onload Command，它调用异步方法ExecuteLoadedCommandAsync来给出结果

 public class AppointmentViewModel : ViewModelbase
{       
    public List<Appointment> Appointments { get; set; }

    public DelegateCommand LoadedCommand { get; }

    public AppointmentViewModel()
    {
        LoadedCommand = new DelegateCommand(async (param) => await ExecuteLoadedCommandAsync());
    }

    public async Task ExecuteLoadedCommandAsync()
    {
        Appointments = await AppointmentService.GetAppointments();
    }      
}

仍然没有用。

Answer 1

我测试了上面的代码片段。实际上class A{ public: void foo(int a, int b); protected: virtual void foo(int a) = 0; }; class B : public A{ public: using A::foo; protected: void foo(int a); }; void f() { B b; b.foo(1,2); // OK b.foo(3); // error: ‘virtual void B::foo(int)’ is protected within this context }已经执行了。 ExecuteLoadedCommandAsync()的命令没有错。在InvokeCommandAction方法中添加断点以测试此问题。

但是ExecuteLoadedCommandAsync()记录无法加载到视图中。这是因为当视图模型new实例化时，Appointments的值为null，并且在执行Appointments事件之后设置该值。因此，对于动态更改的属性，您需要为loaded属性实现INotifyPropertyChanged，如下所示：

Appointments

XAML视图的代码隐藏：

public class AppointmentViewModel : INotifyPropertyChanged
{
   //public List<Appointment> Appointments { get; set; }
   private List<Appointment> appointments;
   public List<Appointment> Appointments
   {
       get
       {
           return appointments;
       }
       set
       {
           if (appointments != value)
           {
               appointments = value;
               OnPropertyChanged("Appointments");
           }
       }
   }

   public DelegateCommand LoadedCommand { get; }

   public AppointmentViewModel()
   {        
       LoadedCommand = new DelegateCommand((param) => ExecuteLoadedCommand());
   }

   public void ExecuteLoadedCommand()
   {
       Appointments = GetAppointments();
   }

   public List<Appointment> GetAppointments()
   {
       List<Appointment> allappointments = new List<Appointment>();
       allappointments.Add(new Appointment() { Name = "name1", Category = "category1" });
       allappointments.Add(new Appointment() { Name = "name2", Category = "category4" });
       allappointments.Add(new Appointment() { Name = "name3", Category = "category3" });
       return allappointments;
   }
   #region INotifyPropertyChanged
   public event PropertyChangedEventHandler PropertyChanged;

   private void OnPropertyChanged(string propertyName)
   {
       if (PropertyChanged != null)
       {
           PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
       }
   }
   #endregion  
}

更多详情请参阅Data binding in depth。

不调用Interaction.Behaviors，EventTriggerBehavior命令

1 个答案: