RelayCommand Params &绑定

Question

视图：

使用WPF(MVVM)玩基本计算器。对于第一个num，我有一个TextBox，对于第二个num有一个TextBox，对于结果有一个TextBlock，有一个按钮来执行AddCommand并返回结果。什么是正确的XAML语法将这些控件绑定到正确的数据。

模型：

public class Operation : INotifyPropertyChanged
{
    private double _result;
    public Operation()
    {
        _result = 0;
    }

    public double Result
    {
        get { return _result; }
        set
        {
            if (value != _result)
            {
                _result = value;
                RaisePropertyChanged("Result");
            }
        }
    }

    public double DoAdd(double first, double second)
    {
        _result = first + second;
        return _result;
    }
}

ViewModel:

public class CalcViewModel
{
    private Operation _operation;
    public RelayCommand AddCommand { get; set; }

    public CalcViewModel()
    {
        _operation = new Operation();

        // This is not correct, how to define the AddCommand here so it takes two params
        // The first and second nums to work with.
        AddCommand = new RelayCommand(first, second => ExecuteAddCommand(first, second));
    }

    private void ExecuteAddCommand(double first, double second)
    {

        // How to bind this returned double to the TextBlock in View
        _oepration.DoAdd(first, second);
    }
}

根据Vlad的请求编辑新版本的代码

模型：

public class Operation
    {
        private double _result;

        public Operation()
        {
            _result = 0;
        }

        public double Result
        {
            get { return _result; }
        }

        public void PerformAdd(double leftNum, double rightNum)
        {
            _result = leftNum + rightNum;
        }
    }

ViewModel:

 public class CalcViewModel
    {
        private Operation _operation;
        public double LeftNumber { get; set; }
        public double RightNumber { get; set; }
        public double Result { get; set; }

        public RelayCommand AddCommand { get; set; }

        public CalcViewModel()
        {
            AddCommand = new RelayCommand(a => ExecuteAddCommand());
            _operation = new Operation();
        }

        private void ExecuteAddCommand()
        {
            _operation.PerformAdd(LeftNumber, RightNumber);
            Result = _operation.Result;
        }

视图XAML:

<TextBox Text="{Binding LeftNumber}" />
<TextBox Text="{Binding RightNumber}" />
<TextBox Text="{Binding Result}" />
<Button Content="Add" Command="{Binding AddCommand}" />

视图代码背后：

public partial class CalcUserControl : UserControl
{
    CalcViewModel vm;

    public CalcUserControl()
    {
        InitializeComponent();
        vm = new CalcViewModel();
        this.DataContext = vm;
    }
}

我尝试了所有的绑定模式，但没有任何结果。我在这里还有一个问题，在这种情况下，默认的绑定模式是什么？

我甚至认为这与计算的数据类型有关，所以我从double到int，但仍然不起作用。

Answer 1

好吧，让我们看看能做什么。

1)模型。模型不需要任何花哨的东西。我会保持简单，让它只返回值，而不是使用NotifyPropertyChanged。毕竟，这是个模特。

public class BinaryOperation
{
    double _l, _r, _result = 0.0;
    public Operation(double l, double r)
    {
        _l = l; _r = r;
    }

    public double Result
    {
        get { return _result; }
    }

    public PerformAdd()
    {
        _result = _l + _r;
    }
}

2) ViewModel。在这里，您的RelayCommand并不需要任何参数。但是您需要在VM中存储操作数的值，这样您的视图就可以绑定到它们，而不是在命令中发送它们。记住，业务逻辑不属于视图，视图只是盲目地绑定到VM！因此，您需要3 DPs (左加载项，右加载项，结果)在您的VM。

3)当命令到达时，只需从VM获取加载项，请模型执行操作，检索结果并将其分配给VM的结果DP。(现在，您的模型操作是快速的，所以您不需要以异步的方式完成它。但也许在将来.)

4)视图。您需要使用窗口/UserControl来绑定到VM的属性。它将是一件简单的事情，如：

<TextBox Text="{Binding LeftAddend}"/>
<TextBox Text="{Binding RightAddend}"/>
<TextBox Text="{Binding Result}"/>
<Button Command="{Binding AddCommand}">Add</Button>

(不要忘记将DataContext设置为正确的。)

编辑：

VM类必须是一个依赖对象！属性应该定义为依赖项属性。就像这样：

public class CalcViewModel : DependencyObject
{
    private Operation _operation;

    public double LeftNumber
    {
        get { return (double)GetValue(LeftNumberProperty); }
        set { SetValue(LeftNumberProperty, value); }
    }

    public static readonly DependencyProperty LeftNumberProperty = 
        DependencyProperty.Register("LeftNumber", typeof(double), typeof(CalcViewModel));

    public double RightNumber
    {
        get { return (double)GetValue(RightNumberProperty); }
        set { SetValue(RightNumberProperty, value); }
    }

    public static readonly DependencyProperty RightNumberProperty = 
        DependencyProperty.Register("RightNumber", typeof(double), typeof(CalcViewModel));

    public double Result
    {
        get { return (double)GetValue(ResultProperty); }
        set { SetValue(ResultProperty, value); }
    }

    public static readonly DependencyProperty ResultProperty = 
        DependencyProperty.Register("Result", typeof(double), typeof(CalcViewModel));

    public RelayCommand AddCommand { get; set; }

    public CalcViewModel()
    {
        AddCommand = new RelayCommand(a => ExecuteAddCommand());
        _operation = new Operation();
    }

    private void ExecuteAddCommand()
    {
        _operation.PerformAdd(LeftNumber, RightNumber);
        Result = _operation.Result;
    }
}

或者，如果您想使用INotifyPropertyChanged，并且您正在使用.NET 4.5

public class CalcViewModel : INotifyPropertyChanged
{
    private Operation _operation;

    public event PropertyChangedEventHandler PropertyChanged;

    void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }

    double _leftNumber;
    public double LeftNumber
    {
        get { return _leftNumber; }
        set
        {
            if (value == _leftNumber) return;
            _leftNumber = value;
            NotifyPropertyChanged();
        }
    }

    double _rightNumber;
    public double RightNumber
    {
        get { return _rightNumber; }
        set
        {
            if (value == _rightNumber) return;
            _rightNumber = value;
            NotifyPropertyChanged();
        }
    }

    double _result;
    public double Result
    {
        get { return _result; }
        set
        {
            if (value == _result) return;
            _result = value;
            NotifyPropertyChanged();
        }
    }

    public RelayCommand AddCommand { get; set; }

    public CalcViewModel()
    {
        AddCommand = new RelayCommand(a => ExecuteAddCommand());
        _operation = new Operation();
    }

    private void ExecuteAddCommand()
    {
        _operation.PerformAdd(LeftNumber, RightNumber);
        Result = _operation.Result;
    }
}

较早的.NET版本也是如此：

    void NotifyPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }

    double _leftNumber;
    public double LeftNumber
    {
        get { return _leftNumber; }
        set
        {
            if (value == _leftNumber) return;
            _leftNumber = value;
            NotifyPropertyChanged("LeftNumber");
        }
    }

等。

Answer 2

谢谢大家，特别是弗拉德。只是一个小小的错误，y已经在class CalcViewModel : DependencyObject上声明了属性class CalcViewModel : DependencyObject两次。

它现在运作良好:)