C＃和WPF中的命令

Question

我有非常简单的代码，但是它不起作用。

我有一个命令：

public class SetYesCommand : ICommand , INotifyPropertyChanged
    {
        public event EventHandler CanExecuteChanged;
        public event PropertyChangedEventHandler PropertyChanged;

        ViewModelBase view = new ViewModelBase();

        public bool CanExecute(object parameter)
        {
            return true;
        }

        public void Execute(object parameter)
        {
            view.Sname = "Yes";
            MessageBox.Show("Command works");
            onpropertychanged("Sname");
        }

        private void onpropertychanged(string propertyname)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyname));
            }
        }
    }
}

这是我的基本视图模型类：

 public class ViewModelBase : INotifyPropertyChanged
    {
        private  string _s;
        public  string Sname {
            get { return _s; }
            set { _s = value;
                onPropertyChanged("Sname");
            }

        }

        private void onPropertyChanged(string propertyname)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyname));
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }
}

这是我的XAML代码：

<Window x:Class="Test.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Test"
        mc:Ignorable="d"
        xmlns:viewmodel="clr-namespace:Test.ViewModel"
         xmlns:commands="clr-namespace:Test.ViewModel.Commands"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <viewmodel:ViewModelBase  x:Key="base" ></viewmodel:ViewModelBase>
        <viewmodel:Converters  x:Key="convert" ></viewmodel:Converters>
        <commands:SetYesCommand x:Key="setyes"/>
    </Window.Resources>
    <StackPanel>
        <Button Width="100" Height="30" Command="{StaticResource setyes}"/>
        <TextBox DataContext="{StaticResource base}" Text="{Binding Sname , Mode=TwoWay}"/>
        <CheckBox DataContext="{StaticResource base}" IsChecked="{Binding Sname , Mode=TwoWay , Converter={StaticResource convert}}"  />
        <TextBox />
    </StackPanel>
</Window>

如您所见，我将用户界面中的两个元素绑定到视图模型基类中的字符串属性，并且在用户界面中定义了按钮的命令。 该命令起作用的原因是，如您所见，我使用一个消息框对其进行了检查，但是UI中其他两个元素（文本框和复选框）的值未更改。

Answer 1

您正在SetYesCommand内创建ViewModelBase的新实例，因此view.Sname =“ Yes”不会更改绑定到元素的视图模型的Sname属性。

这正在工作：

<Window x:Class="Test.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Test"
        mc:Ignorable="d"
        xmlns:viewmodel="clr-namespace:Test.ViewModel"
        xmlns:commands="clr-namespace:Test.ViewModel.Commands"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <viewmodel:ViewModelBase  x:Key="base" ></viewmodel:ViewModelBase>
        <viewmodel:Converters  x:Key="convert" ></viewmodel:Converters>
    </Window.Resources>
    <StackPanel>
        <Button Width="100" Height="30" Command="{Binding Source={StaticResource base},Path=YesCommand}"/>
        <TextBox DataContext="{StaticResource base}" Text="{Binding Source={StaticResource base},Path=Sname , Mode=TwoWay}"/>
        <CheckBox DataContext="{StaticResource base}" IsChecked="{Binding Sname , Mode=TwoWay , Converter={StaticResource convert}}"  />
        <TextBox />
    </StackPanel>
</Window>

public class ViewModelBase : INotifyPropertyChanged
    {
        private string _s;

        private Commands.SetYesCommand _yesCommand;

        public ViewModelBase()
        {
            _yesCommand = new SetYesCommand(this);
        }

        public string Sname
        {
            get { return _s; }
            set
            {
                _s = value;
                onPropertyChanged("Sname");
            }

        }

        public SetYesCommand YesCommand => _yesCommand;

        private void onPropertyChanged(string propertyname)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyname));
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }

public class SetYesCommand : ICommand, INotifyPropertyChanged
{
    public event EventHandler CanExecuteChanged;
    public event PropertyChangedEventHandler PropertyChanged;

    private ViewModelBase view;

    public SetYesCommand(ViewModelBase view)
    {
        this.view = view;
    }

    public bool CanExecute(object parameter)
    {
        return true;
    }

    public void Execute(object parameter)
    {
        view.Sname = "Yes";
        MessageBox.Show("Command works");
        onpropertychanged("Sname");
    }

    private void onpropertychanged(string propertyname)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyname));
        }
    }
}

Answer 2

您正在命令中创建ViewModelBase的新实例。 视图模型应该创建命令，而不是相反。

ICommand接口的典型实现通常接受要执行的Action<object>和决定是否执行命令的Predicate<object> ：

public class SetYesCommand : ICommand
{
    public event EventHandler CanExecuteChanged;
    private readonly Action<object> _execute;
    private readonly Predicate<object> _canExecute;

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

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

    public void Execute(object parameter)
    {
        if (_execute != null)
            _execute(parameter);
    }
}

然后在视图模型中实现该逻辑：

public class ViewModelBase : INotifyPropertyChanged
{
    private string _s;
    public string Sname
    {
        get { return _s; }
        set
        {
            _s = value;
            onPropertyChanged("Sname");
        }

    }

    public ViewModelBase()
    {
        TheCommand = new SetYesCommand(OnCommandExecuted, null);
    }

    private void OnCommandExecuted(object parameter)
    {
        Sname = "Yes";
        MessageBox.Show("Command works");
    }

    public ICommand TheCommand { get; private set; }

    private void onPropertyChanged(string propertyname)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyname));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

在视图中，您绑定到视图模型的command属性，该属性返回ICommand实现的实例：

<Button Width="100" Height="30" DataContext="{StaticResource base}" Command="{Binding TheCommand}"/>

Answer 3

您似乎对命令和视图模型之间的关系感到困惑。

命令对象应该是viewmodel的属性，而不是相反。

从基本的通用ICommand实现开始

public class BasicCommand: ICommand
{
    private readonly Action _execute;

    public BasicCommand(Action execute)
    {
        _execute = execute;
    }

    public bool CanExecute(object parameter)
    {
        return true;
    }

    public void Execute(object parameter)
    {
        _execute?.Invoke();
    }

    public event EventHandler CanExecuteChanged;
}

比在您的viewmodel中，将命令定义为如下所示的属性

public class MyViewModel: ViewModelBase
{
    public MyViewModel()
    {
        DoSomethingCommand = new BasicCommamd(()=>OnDoSomething());
    }

    public ICommand DoSomethingCommand {get;}

    private void OnDoSomething()
    {
        // put whatever you want the command to do here
    }
}

现在，您可以将Window的DataContext设置为MyViewModel的实例，并将按钮的command属性绑定到DoSomethingCommand。