如何在MVVM Light框架中使用RelayCommand

Question

我刚刚开始学习MVVM Light框架,但我找不到任何关于如何使用RelayCommand的简单示例.出于学习的目的,我只想在我的视图中有一个按钮,当点击时显示一个hello world world消息框,并且每隔一分钟启用一次(基本上如果DateTime.Now.Minute%2 == 0) .

按钮XAML如何显示以及如何在ViewModel中定义RelayCommand HelloWorld？

谢谢你的帮助!!

Answer 1

RelayCommand其目的是实现ICommandButton控件所需的接口,并将调用传递给ViewModel中通常位于它们旁边的其他函数.

例如,您将拥有一个ViewModel类,如:

class HelloWorldViewModel : ViewModelBase
{
    public RelayCommand DisplayMessageCommand { get; private set; }

    private DispatchTimer _timer;

    public HelloWorldViewModel()
    {
        this.DisplayMessageCommand = new RelayCommand(this.DisplayMessage, CanDisplayMessage);

        // Create a timer to go off once a minute to call RaiseCanExecuteChanged
        _timer = new DispatchTimer();
        _timer = dispatcherTimer.Tick += OnTimerTick;
        _timer.Interval = new Timespan(0, 1, 0);
        _timer.Start();
    }

    private void OnTimerTick(object sender, EventArgs e)
    {
        this.DisplayMessageCommand.RaiseCanExecuteChanged();
    }

    public bool CanDisplayMessage()
    {
        return DateTime.Now.Minute % 2 == 0;
    }

    public void DisplayMessage()
    {
        //TODO: Do code here to display your message to the user
    }
}

在您的控件中,您DataContext可以在后面的代码中或在XAML中直接通过a设置DataContext={StaticResource ...}

然后,您的按钮将绑定到ViewModel中的命令,就像这样

<Button Content='Push me' Command='{Binding DisplayMessageCommand}' />

单击Button时,它使用DisplayMessageCommand和调用Execute()此对象,该对象RelayCommand只转发到DisplayMessage方法上.

在DispatchTimer一次熄灭一分钟,电话RaiseCanExecuteChanged().这允许绑定到命令的按钮重新检查命令是否仍然有效.否则,您可以仅单击该按钮以查明该命令当前不可用.

Answer 2

或者用lambda

    private RelayCommand<anyobject> _AddCmd;
    public ICommand AddPoint
    {
        get
        {
            return _AddCmd ??
                (
                _AddCmd = new RelayCommand
                    (
                        (obj) =>
                        {
                            ViewModelWF.ZeroPoints.Add(new WM.Point(0, 0));
                        }
                    )
                );
        }
    }

    private RelayCommand _DeleteCmd;
    public ICommand DeletePoint
    {
        get
        {
            return _DeleteCmd ??
                (
                _DeleteCmd = new RelayCommand
                    (
                        () =>
                        {
                            int idx = wpfZeroPoints.SelectedIndex;
                        },
                        () =>
                        {
                            return wpfZeroPoints.SelectedIndex <= 0;
                        }
                    )
                );
        }
    }