Question

正在使用mvvm模式的Windows Phone 8项目。我有一个LongListSelector作为我在视图上的控件之一，它绑定到一个Answers列表。当用户从列表中选择答案时，必须将参数发送到视图模型以验证所选答案是否正确。我遇到的问题是指定什么作为命令参数，我已经尝试了selectedItem但它向视图模型发送了null。任何帮助表示赞赏。

这是我的xaml

        <StackPanel Orientation="Vertical" >
            <StackPanel Orientation="Horizontal" Background="WhiteSmoke">
                <Image Height="100" Width="100" Source="{Binding CurrentQuestion.Graphic}" />
                <TextBlock Text="{Binding CurrentQuestion.Statement}" FontSize="30" Foreground="Black" />
            </StackPanel>
            <StackPanel>
                <phone:LongListSelector  
                    ItemsSource="{Binding CurrentQuestion.Answers}"
                    SelectionChanged="LongListSelector_SelectionChanged"
                    x:Name="Answers"

                    >                              

                    <phone:LongListSelector.ItemTemplate>
                        <DataTemplate>
                            <StackPanel Margin="0,10,0,17" Background="{Binding  Status, Converter={StaticResource StatusConverter}}" >
                                <Button 
                                    Content="{Binding Text}"
                                    Height="100"
                                    Width="auto"
                                    Foreground="Black"
                                    FontSize="18"
                                    Command="{Binding ElementName=Answers, Path=DataContext.ProcessQuestionCommand}"
                                    CommandParameter="{Binding  Path=ItemTemplate.Selecteditem}"

                                    />
                            </StackPanel>
                        </DataTemplate>
                    </phone:LongListSelector.ItemTemplate>
                </phone:LongListSelector>
            </StackPanel>

        </StackPanel>
    </Grid>

这是我的viewmodel的一部分

 public ICommand ProcessQuestionCommand 
    {
        get
        {
            if (_processQustionCommand==null)
            {
                _processQustionCommand = new RelayCommand<int>(ProcessQuestion, CanProcessQuestion);
            }
            return _processQustionCommand; 
        }
    }

public ICommand ProcessQuestionCommand 
    {
        get
        {
            if (_processQustionCommand==null)
            {
                _processQustionCommand = new RelayCommand<int>(ProcessQuestion, CanProcessQuestion);
            }
            return _processQustionCommand; 
        }
    }

    private bool CanProcessQuestion(int answerId)
    {
        //int answerId = Convert.ToInt32(answerIdStr);
        if (answerId> CurrentQuestion.Answers.Count()-1)
        {
            return false;
        }

        if (CurrentQuestion.Answers[answerId].Status!=AnswerStatus.UNANSWERED)
        {
            return false;
        }

        return true;
    }

    private void ProcessQuestion(int answerId)
    {
        //int answerId = Convert.ToInt32(answerIdStr);

        int correctAns = _currentQuestion.GetRightAnswer();
        if (answerId != correctAns)
        {
            _currentQuestion.Answers[answerId].Status = AnswerStatus.INCORRECT;
        }
        else
        {
            _currentQuestion.Answers[answerId].Status = AnswerStatus.CORRECT;

            if (Test.Count > _questionCount)
            {
                _questionCount++;
                CurrentQuestion = Test[_questionCount];
            }
            else
            {
                //tell view to finish up and move screens
                // show a popup alerting the user that the practise has ended
            }
        }
    }

我的命令类

 public class RelayCommand<T>:ICommand
  {
   private readonly Action<T> _executeAction;
   private readonly Func<T, bool> _canExecuteMethod;

   public RelayCommand(Action<T> executeAction)
       :this(executeAction,null)
   {
   }

   public RelayCommand(Action<T> executeAction, Func<T,bool> canExecuteMethod)
   {
       if (executeAction==null)
       {
           throw new ArgumentNullException("executeAction");
       }
       this._executeAction = executeAction;
       this._canExecuteMethod = canExecuteMethod;
   }

   public bool CanExecute(T parameter)
   {
       if (_canExecuteMethod!= null)
       {
           return _canExecuteMethod(parameter);
       }
       return true;
   }

   public event EventHandler CanExecuteChanged;

   public void Execute(T parameter)
   {
       if (_executeAction!= null)
       {
           _executeAction(parameter); 
       }
   }

   public bool CanExecute(object parameter)
   {

       return CanExecute((T)parameter);
      // return true;

   }

   public void Execute(object parameter)
   {

       Execute((T)parameter);
      // _executeAction((T)parameter);
   }

}

Answer 1

你不能将CommandParameter绑定到afaik，那么为什么不通过将SelectedItem作为参数传递而完全避免这个问题，而是将它绑定到ViewModel上的SelectedAnswer属性，然后直接在ProcessQuestion命令逻辑中访问它

CommandParameter绑定在longlistselector控件上

1 个答案: